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The TTY demystified

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Beware, though: What you are about to see is not particularly elegant. In fact, the TTY subsystem — while quite functional from a user's point of view — is a twisty little mess of special cases. To understand how this came to be, we have to go back in time.

Meanwhile, however, the computers — still quite large and primitive, but able to multitask — were becoming powerful enough to be able to interact with users in realtime. When the command line eventually replaced the old batch processing model, teletypes were used as input and output devices, because they were readily available on the market.

There was a plethora of teletype models around, all slightly different, so some kind of software compatibility layer was called for. In the UNIX world, the approach was to let the operating system kernel handle all the low-level details, such as word length, baud rate, flow control, parity, control codes for rudimentary line editing and so on. Fancy cursor movements, colour output and other advanced features made possible in the late 1970s by solid state video terminals such as the VT-100, were left to the applications.

In present time, we find ourselves in a world where physical teletypes and video terminals are practically extinct. Unless you visit a museum or a hardware enthusiast, all the TTYs you're likely to see will be emulated video terminals — software simulations of the real thing. But as we shall see, the legacy from the old cast-iron beasts is still lurking beneath the surface.

A user types at a terminal (a physical teletype). This terminal is connected through a pair of wires to a UART (Universal Asynchronous Receiver and Transmitter) on the computer. The operating system contains a UART driver which manages the physical transmission of bytes, including parity checks and flow control. In a naïve system, the UART driver would then deliver the incoming bytes directly to some application process. But such an approach would lack the following essential features:

Incidentally, the kernel provides several different line disciplines. Only one of them is attached to a given serial device at a time. The default discipline, which provides line editing, is called N_TTY (drivers/char/n_tty.c, if you're feeling adventurous). Other disciplines are used for other purposes, such as managing packet switched data (ppp, IrDA, serial mice), but that is outside the scope of this article.

Together, a particular triplet of UART driver, line discipline instance and TTY driver may be referred to as a TTY device, or sometimes just TTY. A user process can affect the behaviour of any TTY device by manipulating the corresponding device file under /dev. Write permissions to the device file are required, so when a user logs in on a particular TTY, that user must become the owner of the device file. This is traditionally done by the login(1) program, which runs with root privileges.

The physical line in the previous diagram could of course be a long-distance phone line:

This does not change much, except that the system now has to handle a modem hangup situation as well.

Let's move on to a typical desktop system. This is how the Linux console works:

The TTY driver and line discipline behave just like in the previous examples, but there is no UART or physical terminal involved anymore. Instead, a video terminal (a complex state machine including a frame buffer of characters and graphical character attributes) is emulated in software, and rendered to a VGA display.

The console subsystem is somewhat rigid. Things get more flexible (and abstract) if we move the terminal emulation into userland. This is how xterm(1) and its clones work:

To facilitate moving the terminal emulation into userland, while still keeping the TTY subsystem (session management and line discipline) intact, the pseudo terminal or pty was invented. And as you may have guessed, things get even more complicated when you start running pseudo terminals inside pseudo terminals, à la screen(1) or ssh(1).

Now let's take a step back and see how all of this fits into the process model.

These attributes are used for job control.

The following example illustrates the relationship between processes, jobs and sessions:

The basic idea is that every pipeline is a job, because every process in a pipeline should be manipulated (stopped, resumed, killed) simultaneously. That's why kill(2) allows you to send signals to entire process groups. By default, fork(2) places a newly created child process in the same process group as its parent, so that e.g. a ^C from the keyboard will affect both parent and child. But the shell, as part of its session leader duties, creates a new process group every time it launches a pipeline.

The TTY driver keeps track of the foreground process group id, but only in a passive way. The session leader has to update this information explicitly when necessary. Similarly, the TTY driver keeps track of the size of the connected terminal, but this information has to be updated explicitly, by the terminal emulator or even by the user.

Now let's take a closer look at how the TTY drivers, the line disciplines and the UART drivers in the kernel communicate with the userland processes.

UNIX files, including the TTY device file, can of course be read from and written to, and further manipulated by means of the magic ioctl(2) call (the Swiss army-knife of UNIX) for which lots of TTY related operations have been defined. Still, ioctl requests have to be initiated from processes, so they can't be used when the kernel needs to communicate asynchronously with an application.

So a signal is a crude mechanism that allows the kernel to communicate asynchronously with a process. Signals in UNIX aren't clean or general; rather, each signal is unique, and must be studied individually.

As you can see, signals are numbered starting with 1. However, when they are used in bitmasks (e.g. in the output of ps s), the least significant bit corresponds to signal 1.

Suppose that you are editing a file in your (terminal based) editor of choice. The cursor is somewhere in the middle of the screen, and the editor is busy executing some processor intensive task, such as a search and replace operation on a large file. Now you press ^Z. Since the line discipline has been configured to intercept this character (^Z is a single byte, with ASCII code 26), you don't have to wait for the editor to complete its task and start reading from the TTY device. Instead, the line discipline subsystem instantly sends SIGTSTP to the foreground process group. This process group contains the editor, as well as any child processes created by it.

The editor has now been stopped. This fact is reported to the session leader using a SIGCHLD signal, which includes the id of the suspended process. When all processes in the foreground job have been suspended, the session leader reads the current configuration from the TTY device, and stores it for later retrieval. The session leader goes on to install itself as the current foreground process group for the TTY using an ioctl call. Then, it prints something like "[1]+ Stopped" to inform the user that a job was just suspended.

The same thing happens if the TTY is connected to a serial port. yes would be able to transmit data at a much higher rate than, say, 9600 baud, but if the serial port is limited to that speed, the kernel buffer soon fills up and any subsequent write(2) calls block the process (or fail with the error code EAGAIN if the process has requested non-blocking I/O).

What if I told you, that it is possible to explicitly put the TTY in a blocking state even though there is space left in the kernel buffer? That until further notice, every process trying to write(2) to the TTY automatically blocks. What would be the use of such a feature?

Suppose we're talking to some old VT-100 hardware at 9600 baud. We just sent a complex control sequence asking the terminal to scroll the display. At this point, the terminal gets so bogged down with the scrolling operation, that it's unable to receive new data at the full rate of 9600 baud. Well, physically, the terminal UART still runs at 9600 baud, but there won't be enough buffer space in the terminal to keep a backlog of received characters. This is when it would be a good time to put the TTY in a blocking state. But how do we do that from the terminal?

We have already seen that a TTY device may be configured to give certain data bytes a special treatment. In the default configuration, for instance, a received ^C byte won't be handed off to the application through read(2), but will instead cause a SIGINT to be delivered to the foreground job. In a similar way, it is possible to configure the TTY to react on a stop flow byte and a start flow byte. These are typically ^S (ASCII code 19) and ^Q (ASCII code 17) respectively. Old hardware terminals transmit these bytes automatically, and expect the operating system to regulate its flow of data accordingly. This is called flow control, and it's the reason why your xterm sometimes appears to lock up when you accidentally press ^S.

There's an important difference here: Writing to a TTY which is stopped due to flow control, or due to lack of kernel buffer space, will block your process, whereas writing to a TTY from a background job will cause a SIGTTOU to suspend the entire process group. I don't know why the designers of UNIX had to go all the way to invent SIGTTOU and SIGTTIN instead of relying on blocking I/O, but my best guess is that the TTY driver, being in charge of job control, was designed to monitor and manipulate whole jobs; never the individual processes within them.

To find out what the controlling TTY for your shell is called, you could refer to the ps l listing as described earlier, or you could simply run the tty(1) command.

Some of these settings refer to UART parameters, some affect the line discipline and some are for job control. All mixed up in a bucket for monsieur. Let's have a look at the first line:

Occasionally, you may come across a UNIX system where the backspace key doesn't work. This happens when the terminal emulator transmits a backspace code (either ASCII 8 or ASCII 127) which doesn't match the erase setting in the TTY device. To remedy the problem, one usually types stty erase ^H (for ASCII 8) or stty erase ^? (for ASCII 127). But please remember that many terminal applications use readline, which puts the line discipline in raw mode. Those applications aren't affected.

As you type, your terminal emulator transmits information to the kernel. Usually, the kernel echoes the same information back to the terminal emulator, allowing you to see what you type. Without character echoing, you can't see what you type, but we're in cooked mode so the line editing facilities are still working. Once you press enter, the line discipline will transmit the edit buffer to cat, which will reveal what your wrote.

You will get your prompt back, but after five seconds, the background job transmits hello, world to the terminal, in the middle of whatever you were typing.

I hope this article has provided you with enough information to get to terms with TTY drivers and line disciplines, and how they are related to terminals, line editing and job control. Further details can be found in the various man pages I've mentioned, as well as in the glibc manual (info libc, "Job Control").

Finally, while I don't have enough time to answer all the questions I get, I do welcome feedback on this and other pages on the site. Thanks for reading!

Discuss this page

Disclaimer: I am not responsible for what people (other than myself) write in the forums. Please report any abuse, such as insults, slander, spam and illegal material, and I will take appropriate actions. Don't feed the trolls.

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Anonymous

Sun 24-Aug-2008 21:36

Very good and informative article for a complex topic

the tty system really gets demystified here.

Only a small correction:

Your statement "icanon switches between raw and cooked mode" is not completely true.

'stty -icanon' still interprets control characters such as Ctrl-C whereas 'stty raw' disables even this and is the real raw mode.

Greetings,

-Andreas.

lft

Linus Åkesson

Fri 29-Aug-2008 16:42

Very good and informative article for a complex topic

the tty system really gets demystified here.

Only a small correction:

Your statement "icanon switches between raw and cooked mode" is not completely true.

'stty -icanon' still interprets control characters such as Ctrl-C whereas 'stty raw' disables even this and is the real raw mode.

Thanks!

Yes, you're quite right. I've fixed it.

Anonymous

Wed 26-Nov-2008 07:13

I have been trying to chew through documents in every unix book possible to explain how the TTY system really works. Your article has been more informative than all of these books. Without a programmers knowledge of how a unix kernel works, it's quite difficult for a System Administrator to understand concepts that are so crucial to their jobs. Thank you for your writing such a great article. Your help is truly appreciated.

Anonymous

Wed 10-Dec-2008 12:27

I really admire you!! I'm not use to post, but this article was by far the best I ever read about tty, i read many articles and even books but nothing so clear like this.

Keep doing things like this please...

Excelente!

Muchas Gracias

Fede Tula

Anonymous

Sat 20-Dec-2008 05:20

Good article. A few years ago, after some experimentation, I wrote up some TTY stuff for myself, because I couldn't find any good documentation. For example, I never understood why process group leaders (and consequently also session leaders) are prevented from calling setsid(2). (See the manual for what setsid(2) does.) I think I can explain it now.

The process P is a pg leader if P.PID = P.PGID. In the example of the article, "Job" means process group, and ls (103) is a process group leader:

ls.PID = 103

ls.PGID = 103

ls.SID = 101

ls.CTTY = /dev/pts/0

Suppose we allow ls to call setsid(2). This would have the following consequences:

ls.PID = 103 # unchanged

ls.PGID = 103 # set to ls.PID, but in fact this is no change!

ls.SID = 103 # set to ls.PID

ls.CTTY =

Now ls is session leader (ls.SID = ls.PID), and ls is process group leader (ls.PGID = ls.PID).

At this point, however, sort (104) would belong to a process group (103) whose leader's (ls's) SID (103) doesn't match sort's SID (101)!

sort.PID = 104

sort.PGID = 103

sort.SID = 101

the pg leader for pg 103 is ls (103):

ls.PID = ls.PGID = 103 = sort.PGID

however

ls.SID = 103 != 101 = sort.SID

We have two processes in the same process group belonging to different sessions!

ls is prevented from calling setsid() because as current process leader its PGID doesn't change when it is set to its PID, while its SID changes. Thus it would leave the session while staying in the process group.

Sort, OTOH, can call setsid(), becuase it also leaves the process group:

sort.PID=104

sort.PGID=104 # leaves process group too

sort.SID=104

sort.CTTY=

fork(2)-ing and calling setsid(2) in the child helps, because the child gets a new PID, which will be different from any PGID of the parent (as that PGID was the PID of some process), and so when the child calls setsid(2), the child.PGID := child.PID operation will actually change the child's inherited PGID and so the child will be able to leave the process group.

Right after fork():

parent.PGID = some_old_PID

child.PID = new_PID

child.PGID = parent.PGID = some_old_PID

The child calls setsid():

child.PGID = child.PID = new_PID != some_old_PID = parent.PGID

A session leader *could* call setsid(), despite also being a process group leader, since neither its PGID nor its SID would change. However, its CTTY would be set to , and this would result in a situation where the original controlling process (= a session leader with a CTTY), for example, your shell, has no more access to the terminal!

Furthermore, there is the rule that when a controlling process dies, each session member (each process P with P.SID = SL.SID) loses access to the terminal (and possibly get a SIGHUP on the next read/write). This clearly shows the intent that no session member shall have access to the terminal when the session leader has none. This principle would be violated if the current session leader could detach from the CTTY by calling setsid(). (Or all session members would have to lose access to the CTTY, just as if the session leader died.)

Anonymous

Thu 25-Dec-2008 19:19

good job & best introductory for TTY

Anonymous

Mon 20-Apr-2009 08:28

amazing man , this is amazing !!! best tty article ever

Anonymous

Tue 2-Jun-2009 19:27

very good article!

Anonymous

Mon 15-Jun-2009 16:10

Holy Smokes dude, those were the days!

Riff

www.absolute-anonymity.us.tc

Anonymous

Mon 15-Jun-2009 19:59

I rarely comment on Blogs as I usually feel my input would not be necessary, but I must say this has been one of the best written and descriptive documents I have had the pleasure of learning from

Anonymous

Mon 15-Jun-2009 22:41

ZZZZZZZZZZZZZZZZZZZZZZZZZZ to long, jesus christ write up a summery for us lazy people

Anonymous

Tue 16-Jun-2009 00:22

I've been looking for an article like this for a very long time... thanks very much for putting this together!

Anonymous

Tue 16-Jun-2009 02:38

Great summary, and very useful. Thanks!

Anonymous

Tue 16-Jun-2009 06:57

Just freaking great!

script to make commands like less (more) adapt to changed screen size.

It even tries to exit from the command leaving the cursor on the "correct" place.

The kludge should work well with anything, that ought to be updated because of

a change in terminal window size.

CAVEATS

It's written for Unix under Mac OsX, doesn't really know if tput are implemented under Linux.

Well here we go. I'm sorry for the loss of tabs, it should have been indented.

I have used this for a year and a half and it really works.

! /bin/bash

export LESS=" -I -r -f -J -S -g -M -x 4"

-I ignore case when searching

-r "raw" do not preparate ctrl-chars,

-f force open special files (may be binary) BEWARE OF ANSISEQUENCES.

-J show status column

-S chop long lines.

-g highlight on last hit in the search.

-M Most Verbose status column...

-x 4 tabspacing = 4

-------------------------------------- the kludge starts here.................

ORIGLINES=$LINES

ESC=printf "\e"

ScreenRedraw_off=echo -n "$ESC""[8m"

ScreenRedraw_on=echo -n "$ESC""[0m"

function OkayScreen()

{

export PS1="" # Turns off the prompt to avoid cluttering..

echo -n ${ScreenRedraw_off}

CURLINES=bash -i < ~/bin/kludge.bash

^^^^^^^^^^^ NB! the path where kludge.bash should be placed.

if [ $CURLINES -gt $ORIGLINES ] ; then

TO_SKIP="$(expr "$CURLINES" '-' "$ORIGLINES")"

if [ $TO_SKIP -lt 3 ] ; then

TO_SKIP="$(expr "$TO_SKIP" '-' '2')"

else

TO_SKIP="$(expr "$TO_SKIP" '-' '1')"

tput cuu 1 #cursor up one line

echo -n ${ScreenRedraw_on}

echo -n "$" #restores prompt

echo -n ${ScreenRedraw_off}

tput cud $TO_SKIP

echo -n ${ScreenRedraw_on}

echo # activate cli correct position.

else

tput cuu 2

echo ${ScreenRedraw_on}

}

trap OkayScreen SIGWINCH

if [ -t 0 ] ; then # /* this enables syntax highlighting */

$VIMRUNTIME/macros/less.sh $@ /* After tinkering with vim */

else

/usr/bin/less $@

fi

trap '' SIGWINCH

cp ./tmp/.vimrc~

Anonymous

Tue 16-Jun-2009 07:05

Just freaking great!

Forgot the innerpart, which makes it all work ....

This is a second script called kludge.bash which I have in my ~/bin folder.

Needs to execute this to get the changed winsize in a new process since at

least bash 2.05a didn't update the LINE variable in active process in the terminal window.

! /bin/bash

### SYS Finds the number of lines in a window after window rechange - less... !#

kludge.scr - to be placed in the ~/bin folder is the inner workings of the bash script named less

PS1=""

shopt -s checkwinsize

echo $LINES

Anonymous

Tue 16-Jun-2009 23:58

Thanks for the great article!

I'd recommend adding some info about the *wide* spread myth of parent's death triggering SIGHUPs for all its children.

Recall that the related behavior only applies to session leaders, and is triggered in any of the two following cases:

1:

IF session leader exiting

..IF it has ctty

....send SIGHUP to foreground PG

..ELSE

....send SIGHUP to foreground PG at last time it had ctty

2:

IF session leader detaching (TIOCNOTTY)

..send SIGHUP to foreground PG

--JuanJo

Anonymous

Wed 17-Jun-2009 10:42

Actually, to be more precise:

1:

IF session leader exiting

..IF it has ctty

....send SIGHUP to foreground PG

..ELSE

....send SIGHUP,SIGCONT to foreground PG at last time it had ctty

2:

IF session leader detaching (TIOCNOTTY)

..send SIGHUP,SIGCONT to foreground PG

See: http://google.com/codesearch/p?hl=en&sa=N&cd=2&ct=rc#p4tPAkVsQ%5C_c/linux-2.2.26/drivers/char/tty%5C_io.c&l=537"

http://google.com/codesearch/p?hl=en&sa=N&cd=2&ct=rc#p4tPAkVsQ%5C_c/linux-2.2.26/drivers/char/tty%5C_io.c&l=537

--JuanJo AKA jjo

lft

Linus Åkesson

Sun 21-Jun-2009 12:58

A reader pointed out that VT (in VT-100) stands for "video terminal", not "virtual terminal". This has been fixed.

Anonymous

Thu 30-Jul-2009 13:07

Thanks for the article, really helped !

Anonymous

Thu 30-Jul-2009 19:09

Thankyou, I found this to be an excellent TTY primer.

Anonymous

Fri 31-Jul-2009 22:14

Bookmarked.

Good tips =)

Anonymous

Mon 10-Aug-2009 07:21

Very nice. I'm currently (trying to) write a Unix-clone mostly from scratch, and this is a good resource on how TTYs are supposed to work.

Anonymous

Sun 16-Aug-2009 15:32

Hi--how are things in Sweden?

Every once in a while I get up the ambition to complain about the width of text on a web page, and you're the lucky winner today--sorry ;-)

This could be a good article--from the looks of it, it probably is--but why is it (and so many other web pages today) so wide?

Checking one line at random, it is 130 characters wide:

echo "Meanwhile, however, the computers — still quite large and primitive, but able to multitask — were becoming powerful enough to" | wc

1 20 130

Oh, and I'm ignoring the stuff in the left hand panel / column--I simply horizontally scroll so that panel is not visible.

I have three choices if I want to read your article:

horizontally scroll on each line

set the type size very small (or zoom out), so an entire line appears on the screen, then use a magnifying glass

copy and paste the text to a file and read it in an editor--possibly deleting hard line breaks to let the text flow better.

Ideally, and I've seen it done this way, so I believe it can be done:

the text should be arranged to wrap to the width of the (reader's) window

if there are long lines of code (pre-formatted text), or wide pictures, or something like that, the other text should still wrap to the width of the reader's window, although he'll have to horizontally scroll to see the full picture or code or whatever. (This is the part I'm specifically referring to as having seen done, but I can't remember any details (like an example, or how to do it--I'll try to pay attention and find some).

Anyway, sorry for the rant--thanks for making the effort to create and disseminate pages with information like this!

Randy Kramer

Anonymous

Fri 4-Sep-2009 09:24

Wonderful article.the explanation is meticulous and elegant. Thank you.

Anonymous

Sat 12-Sep-2009 14:09

GREAT ARTICLE!!! Read articles and books on Terminal I/O and none as clear as this summary.

Well Done!

Anonymous

Wed 23-Sep-2009 14:26

Thanks for a good article

Anonymous

Sat 17-Oct-2009 08:12

Thanks for this very good article

Anonymous

Tue 3-Nov-2009 17:53

Thanks for such a wonderful article. I am a beginner in tty and your article was of great help!!

Anonymous

Thu 5-Nov-2009 19:54

nice article..thanx for sharing ur knowledge :)

Anonymous

Tue 22-Dec-2009 20:58

Thanks a lot for this precise article ! This has been very useful to me... (reading an external device on RS232 from bash...)

Anonymous

Tue 9-Feb-2010 07:34

Thank you for sharing such a great artical. -Hai

Anonymous

Thu 15-Apr-2010 09:08

Thanks a lot for this great article

Anonymous

Tue 4-May-2010 08:36

Wonderful article! I never learned much about unix process stuff (apart from little practical things like piping, detaching, killing, etc), but now I feel like I'm starting to see what's behind the magical terminals! Thank you :)

Anonymous

Wed 12-May-2010 01:00

"daemonizing" a process - detaching from the tty - would probably also fit here. -rurban

Anonymous

Thu 13-May-2010 01:17

What does TTY mean?

lft

Linus Åkesson

Thu 13-May-2010 15:33

What does TTY mean?

TeleTYpe.

Anonymous

Wed 14-Jul-2010 03:21

What Randy Kramer said.

Anonymous

Sat 24-Jul-2010 14:28

excellent article, well done!

Anonymous

Sun 25-Jul-2010 00:31

The process of daemonizing is covered elsewhere, easy to Google. It involved forking, killing the parent, then calling setsid() in the child, and optionally chdir()ing to / and closing stdin/stdout/stderr.

I am currently working on writing a toy OS, and this was very useful in its treatment of the basic structure of the TTY subsystem. Thanks.

ralph

Ralph Corderoy

Sun 25-Jul-2010 14:22

Nice article, various points...

The erase and kill characters used to be # and @, and as you were printing on paper there was no rubbing out, so you might see

$ ls @wc -l /etvc##c/passwd

42 /etc/passwd

where the @' was killing the whole line entered so far and the##' was erasing the preceding `vc'.

It's only modern shell that provide line editing, hence shell history substitutions like !!' and! existing. If /bin/sh is a plain old non-line-editing shell on your system then you can see the difference in tty settings by using `stty -a' from another terminal to capture the differences. Don't run stty(1) from, e.g., the bash shell since the shell will alter the tty settings before running stty. Here, bash has the literal next character, lnext, being undefined and turns off -icrnl, -icanon, and -echo.

The above example of # and @ was achieved by

$ sh

$ stty erase # kill @ -crterase -echok

$ ls @wc -l /etvc##c/passwd

42 /etc/passwd

$ stty sane

$ exit

where /bin/sh is dash(1) on this Ubuntu system.

"Write permissions to the device file are required, so when a user logs in on a particular TTY, that user must become the owner of the device file." I think it's read permission that's required to alter a tty's settings. It did used to be write, in the very early days, but since write(1) and mesg(1) meant users could write to one another's terminals it also meant they could alter their settings. Much fun could be had with changing erase to e' for a second and back again at random intervals whilst the user was trying to type. So it was switched to require read permission which only the owner of tty normally has. This can be seen in stdin of stty needing to be re-directed to specify the terminal, and not stdout, e.g.stty -a </dev/pts/1'.

Flow control, e.g. ^S and ^Q, existed long before the signals for job control. IIRC, it was Berkeley that added all the ^Z stuff and related signals, it wasn't Bell Labs.

The Linux kernel doesn't bother to implement all of the normal control characters. Flush is one that's missing, IIRC, which is set with stty's `eol2'. It's a shame.

Cheers,

Ralph.

P.S. There's a typo, `1970:s'.

lft

Linus Åkesson

Sun 25-Jul-2010 16:14

ralph wrote:

Nice article, various points...

Thanks! That was very interesting. It hadn't occured to me that erase/kill would be usable without interactive line editing, but it makes sense.

ralph wrote:

P.S. There's a typo, `1970:s'.

Changed to 1970s.

Anonymous

Mon 2-Aug-2010 20:22

How do you restart bash in tty0? For some reason my bash has died in all tty's (Ctrl-Alt-Fn). I does not show the login prompt anymore.

How can I restart it? I cannot reboot the machine. I can ssh into it from another machine.

lft

Linus Åkesson

Thu 5-Aug-2010 08:10

How do you restart bash in tty0? For some reason my bash has died in all tty's (Ctrl-Alt-Fn). I does not show the login prompt anymore.

How can I restart it? I cannot reboot the machine. I can ssh into it from another machine.

I don't know what's wrong in your particular case, but it's init (pid 1) that's supposed to (re-)start the login program in each terminal. You can modify init's configuration at runtime by editing /etc/inittab and then doing "kill -HUP 1". But it's probably not an error in the configuration file, so use ps(1) to investigate what processes are running in the terminals.

Anonymous

Mon 9-Aug-2010 23:41

Great article.

For another great source on TTY devices you can go to:

http://publib.boulder.ibm.com/infocenter/aix/v6r1/index.jsp?topic=/com.ibm.aix.genprogc/doc/genprogc/ttysys.htm

Enjoy,

Ori

Anonymous

Fri 13-Aug-2010 10:59

Very good article...

We want the part II were the concepts gets even closer to the Linux implementation...

just an example:

an xterm session under ssh when resized makes the TTY device to adjust the terminal size and generate SIGWINCH signal for the running app to know about the change...

but if the xterm is under a serial line when resize occurs NOTHING happens (not kernel side size update, not SIGWINCH signal)... I know under serial line xterm does not communicate the size change, well where is the the place to patch in order to solve this, from the xterm side it would be very easy to send a escape sequence telling the TTY driver the new size but this driver should be patched for catching it and react as in the ssh case....

Thanks for your out of ordinary (little or inexistant content + lot of google adds) article. I hope we can get a deeper version someday.

Pat

Anonymous

Sat 21-Aug-2010 03:00

Hi,

A very interesting article. I have just published a related article on the terminals in French : http://www.etud.insa-toulouse.fr/~mcheramy/wordpress/?p=198

(And if you don't understand french, there are few interesting links in english at the end)

http://www.etud.insa-toulouse.fr/~mcheramy/wordpress/?p=198

Thanks.

Max.

Anonymous

Fri 27-Aug-2010 12:22

How to Modify the tty driver so that control-W erases the previous word typed by the user.

Anonymous

Mon 25-Oct-2010 13:14

Nithin: thanks for the page....i got a lot of information from ur web page...

Anonymous

Wed 1-Dec-2010 14:45

Great article, very imformative please explain how to prevent UART overrun

Anonymous

Thu 9-Dec-2010 18:19

Hi there ! Would it be possible to create a raw tty which redirects input to some fifo and another tty to read from that fifo ? I know that is extrange what I'm asking...I'm building a z80 emulator and if this is possible would help me to test the input/output routines of the emulator without writting code to read/display...

Anonymous

Sun 26-Dec-2010 21:32

very helpful indeed Thanks a ton.

Anonymous

Thu 6-Jan-2011 10:46

Good article.Thanks you very much

/Renjith G

Anonymous

Wed 12-Jan-2011 13:12

this article is very good .

now i have a question.

linux use /dev/console in booting before the init called . so i write a progrm that run after kernel booted and pass my progrm with option init in boot parameter and bypass init program so the first program that run is my program.

this is my question : how i correct my program to resive signels from /dev/consoole?

Anonymous

Tue 25-Jan-2011 09:19

Awesome article! Its been very helpful in understanding TTY layer as whole.

Thanks

Ambresh

Anonymous

Wed 9-Feb-2011 17:56

Hi, this seems to be an excellent technical article, however is there any chance you can provide a broad description of TTY for a non technical audience. e.g. explain in simple language the origin, function and implications of TTY.

You may not consider this your remit, which is fair enough.

Anonymous

Wed 16-Feb-2011 07:57

Great Job, learned much

Anonymous

Wed 23-Feb-2011 20:35

Very good document, thanks!

Anonymous

Fri 25-Feb-2011 22:10

great article. I have known the term 'TTY' for a long time and never knew what the history behind it. Seeing how even current day UNIX system still make use of TTY is quite interesting to me. Now I feel more 'connected' when I press a key on my keyboard and seeing a character appear on the monitor screen. A good read.

Anonymous

Tue 1-Mar-2011 21:42

Wow! The name of the article says it all, but for real this time! Thanks!

Regards,

Newman

Anonymous

Fri 18-Mar-2011 20:18

Amazing article. Thanks so much for putting this together. I've been using Linux for 10 years, and this taught me quite a bit.

Anonymous

Mon 11-Apr-2011 18:08

ZZZZZZZZZZZZZZZZZZZZZZZZZZ to long, jesus christ write up a summery for us lazy people

if(easilyBored || notInterested){

goToAnotherSiteThen();

} else {

try {

readAndLearn();

}

catch(DontUnderstandException dde){

wiki(dde.getSubject());

}

finally {

enjoyArticle();

}

Anonymous

Wed 29-Jun-2011 12:19

Fantastic expalanation. Keep up the good work

Anonymous

Fri 1-Jul-2011 02:12

Greatly informative article. Of course, Bookmarked.

Anonymous

Mon 11-Jul-2011 14:18

Very informative article; Thanks for knowledge sharing ...

Anonymous

Wed 7-Sep-2011 13:48

Fucking awesome. Thanks a lot for the info!

Anonymous

Wed 7-Sep-2011 13:49

Fucking awesome. Thanks a lot for the info!

Anonymous

Fri 9-Sep-2011 14:25

Excelent article, I started to read it at work, now I'm printing it to read it more carefuly on my way home. Thanks for sharing all this!

Anonymous

Mon 19-Sep-2011 20:32

Thank you for sharing your knoledge so generously!

Anonymous

Tue 27-Sep-2011 06:01

thanks for the works!

Anonymous

Fri 30-Sep-2011 14:39

Hi,

I am currently working on Solaris sparc 10, where i am seeing below problem with default setting.

Shell is allowing me to type in only 256 characters. e.g.

bash-3.00$ cat

SunStudio12u1-SunOS-SPARC-tar-MLSunStudio12u1-SunOS-SPARC-tar-MLSunStudio12u1-SunOS-SPARC-tar-MLSunStudio12u1-SunOS-SPARC-tar-MLSunStudio12u1-SunOS-SPARC-tar-MLSunStudio12u1-SunOS-SPARC-tar-MLSunStudio12u1-SunOS-SPARC-tar-MLSunStudio12u1-SunOS-SPARC-tar-MLS

If i attempt to input more data nothing happens. After going through your notes it seems like i am crossing line buff limit. If yes can you please suggest how i can increase this limit

Minal Patil

Anonymous

Mon 24-Oct-2011 19:11

can anybody give exact command for sending messages via linux?

Anonymous

Wed 2-Nov-2011 02:21

Hi,

can you please suggest how i can increase this limit

http://tinyurl.com/6yql6r8

Anonymous

Mon 14-Nov-2011 20:33

From your disclaimer: “Please report any abuse, such as insults, slander, spam and illegal material”

Just a note: “slander” only applies to verbal communication. For written, use the term “libel” instead. :-D

Anonymous

Tue 22-Nov-2011 15:04

Thanks for writing the article. It was a bit heavy going at times, but it's helped me understand some of what is going on with the whole terminal thing.

Anonymous

Mon 12-Dec-2011 03:22

This was enormously informative and helpful. I've bookmarked it for future use. Thank you!

Anonymous

Wed 21-Dec-2011 03:20

I am trying to solve a mystery with certain services started at reboot or via a root crontab.

These sercvices seem to fail, unless I login to a shell that has a proper TTY setup

(eg: PuTTY of any type of ssh session)

If I start up these services with a proper tty, they work and continue to run.

My theory is that on reboot or via root crontab there is no tty.

How can I create a psuedo tty master/slave pair underwhich to run these services so they work, with me doing a manual ssh login......?

Larry Wichter

Anonymous

Thu 12-Jan-2012 14:37

Very informative article, thanks very much for sharing it with the web.

Anonymous

Wed 18-Jan-2012 00:00

Real great article thanks very much.

Anonymous

Mon 20-Feb-2012 04:35

Superb post. Thank you.

Anonymous

Mon 5-Mar-2012 17:15

Great article! I'm currently running a real Teletype ASR-33 on a serial port with Ubuntu. I have to use stty to set the proper parameters for an uppercase-only terminal, change tabs to spaces, etc. Is there a way to determine what port I'm logging into (ttyS0? S1? S2, S3 or S4?) Right now, I have to run stty on all ports and get an access denied error for any port I'm not connected to. I'd like to find out what port I'm on and only run the stty on that port. Thanks for your time!

Anonymous

Mon 2-Apr-2012 12:21

Is there a way to determine what port I'm logging into (ttyS0? S1? S2, S3 or S4?)

Sounds like a job for the tty(1) command.

Anonymous

Mon 2-Apr-2012 17:14

I think the commentary about line editing being part of the operating system to simplify programs is potentially misleading - the simplification of applications is a side effect.

If you're connected something like a VT100, the terminal handles line editing, and programs send control codes to the terminal to switch between cooked mode and raw mode. When virtual terminals were implemented, this functionality became part of the operating system to maintain compatibility with existing software.

Anonymous

Wed 11-Apr-2012 23:15

This is a really great job! Really helped with background jobs writing to stdout and going to sleep (putting 'stty -tostop;' before background command). It takes much time to generate meaningful examples - much appreciated!

Anonymous

Sat 14-Apr-2012 11:54

Thanks !!

Anonymous

Sat 14-Apr-2012 12:21

Great Article and good writing. I came looking for one answer (which I found) and found that I could not stop reading (normally a sign of a good article).

Having been around as the last of the Telex's died out, having designed UART circuits, worked on Xenix and Unix Systems, and naturally Linux systems, your article plugged some reasonable holes I had on the subject...

Thanks again...

Anonymous

Tue 17-Apr-2012 16:35

This is a very good article and I've read and enjoyed it multiple times (no, really!).

One thing I think it does lack is that when it's talking about signals, it should mention about Linux's new(ish) signalfd.

Anonymous

Sat 5-May-2012 14:56

Great and insightful article!!!

From this article I gain general knowledge about architecture

of Linux devices(LowLevelDriver<->LineDiscipline<->HighLevelDriver).

It's really a core of Linux I/O.

Thank you very much.

___________________________

Sichkar Dmytro from Ukraine

dmbios@mail.ru

Anonymous

Tue 15-May-2012 23:52

Thank you so much. Great article!

Anonymous

Thu 24-May-2012 15:22

Good stuff, thanks!

Anonymous

Mon 28-May-2012 02:48

Thanks.

whizziwig

David Blackman

Mon 4-Jun-2012 17:16

Can you explain why sometimes my terminal gets diseased, and the line I'm typing overwrites itself, or the cursor is in the wrong place, or similar symptoms? is there a way to fix it?

Anonymous

Mon 4-Jun-2012 19:11

Love the article, but I did have a comment -- namely that I wouldn't describe the DEC VT100 as solid state because of its CRT. Again, though, the article is great! Thanks!

Anonymous

Tue 5-Jun-2012 07:20

Absolutely terrific!

Anonymous

Tue 5-Jun-2012 19:07

That brings back old memories. The TTY shown is from the 1940's were 5 bit rather than the later 8 bit (really 7 bit plus parity) machines seen in the late 60's and 70's. Next retro core memory come back.

RC Roeder

Anonymous

Wed 6-Jun-2012 03:02

Fantastic! awesome article you should think about to write a book or something, It clarified me some things, even having 5+ years of Unix experience.

Al Mejida.

Anonymous

Tue 12-Jun-2012 18:59

Thanks a lot for writing this!

Anonymous

Fri 6-Jul-2012 15:46

"yes" program , produced in 2009 !

Thank you David McKenzie for your contribution to open source community !

I am just wondering what in your background that enabled the FSF to accept such worthless contribution ?

Member of what masonic lodge or what church or son of a war hero or billonarties you have to be so they accept that piece of crap ?

For reference:

yes command - otputs a line on tty until killed !

coded and added to Linux in 2009.

Anonymous

Fri 6-Jul-2012 15:48

"yes" program , produced in 2009 !

Thank you David McKenzie for your contribution to open source community !

I am just wondering what in your background that enabled the FSF to accept such worthless contribution ?

Member of what masonic lodge or what church or son of a war hero or billonarties you have to be so they accept that piece of crap ?

For reference:

yes command - otputs a line on tty until killed !

coded and added to Linux in 2009.

interesing point although might be harsh on a boy who modified a hello world program and managed to add it to Linux.

Anonymous

Fri 6-Jul-2012 15:50

"yes" program , produced in 2009 !

I disagree an entire source code of the yes in TARball may be a good example of how to add a new command to Linux, sort of like a new command template.

www.LinuxCAD.com

lft

Linus Åkesson

Fri 6-Jul-2012 16:07

"yes" program , produced in 2009 !

...

Member of what masonic lodge or what church or son of a war hero or billonarties you have to be so they accept that piece of crap ?

Your angry ignorance is amusing. Do you also consider echo or /dev/zero crap? Unix commands are simple by design, so that they can be combined easily. For instance, off the top of my head, here's a way to list the first 100 powers of two: (echo 1; yes 'p2*')|dc|head -n 100

Anonymous

Fri 6-Jul-2012 21:17

int main( in argc, char** arg)

{

if ( argc < 2 )

{

printf( "\nNo program, prints line to tty until killed !... please put now my name in Linux.\n" );

exit(1);

}

while(1)

{

printf( argv[1] );

};

}

Anonymous

Fri 13-Jul-2012 17:37

Thank you!!

Anonymous

Sat 14-Jul-2012 03:33

Shell is allowing me to type in only 256 characters. e.g.

bash-3.00$ cat

SunStudio12u1-SunOS-SPARC-

Minal Patil

Can Web Master please delete this post so that the page would not be made so wide in some browser to be unreadable?

Thanks.

lft

Linus Åkesson

Sat 14-Jul-2012 11:39

Can Web Master please delete this post so that the page would not be made so wide in some browser to be unreadable?

Thanks.

I've added some "max-width" attributes that should fix the problem. What browser do you use?

Anonymous

Sat 14-Jul-2012 22:18

lft wrote:

Can Web Master please delete this post

I've added some "max-width" attributes that should fix the problem. What browser do you use?

Thanks for being so responsive.

I did realize, later, that it was just elinks that laid out the page to be annoyingly wide.

$ elinks http://www.linusakesson.net/programming/tty/index.php

http://www.linusakesson.net/programming/tty/index.php

(Use "]" or mouse to click near the right border of the terminal to scroll right.)

I also found, later, that removing the posting containing a unusually long single-word helps, but a little scrolling left and right is still needed in elinks.

Since this is restricted to an uncommon browser, don't worry about it.

(I started using elinks on one computer due to the recent development on Linux desktop that resulted in poor support for older video cards where Firefox would freeze the entire desktop when rendering some common web content. elinks worked great for text content.)

By the way, thanks for the page. It was a great read.

lft

Linus Åkesson

Sun 15-Jul-2012 20:29

lft wrote:

Can Web Master please delete this post

I've added some "max-width" attributes that should fix the problem. What browser do you use?

Thanks for being so responsive.

I did realize, later, that it was just elinks that laid out the page to be annoyingly wide.

I use elinks quite a lot myself, actually, for a fast, nonsense-reduced browsing experience. The page is not wider than the screen on my setup (elinks version 0.12pre5 with CSS enabled).

Anonymous

Wed 19-Sep-2012 21:01

It's not to keep application simple that the line discipline is in the kernel. The unix philosophy is quite the contrary, to keep the KERNEL simple, and deport to user space the handing of complexities that they don't want inside the kernel. (See for example interrupted system calls and signal handling, but that's another topic).

The reason why the line discipline is inside the kernel, is to avoid context switches at the reception of each character (which in the early times of small core memories, would imply swap-outs and swap-ins!). So the line discipline keeps in a kernel buffer a line of input, and since it's simple enough to test for a specific byte and decrement a counter to implement the backspace "editing" (and a few other simple editing functions), it's done there.

The alternative, is to use the raw mode, where the characters are forwarded to the application as soon as they're received, which is needed for more sophisticated editors, like the (at the time) famously known Eight Megabytes And Constantly Swapping editor (emacs). And indeed, since emacs had to use this raw mode, which implies a context switch at the reception of each character typed, it was constantly swapping when the computers hadn't enough memory to keep emacs and all the other programs in core.

Anonymous

Thu 20-Sep-2012 02:54

Thanks a lot! Was looking for this kind of information all over the place, never found anything good. If you got more please keep it coming.

Anonymous

Thu 27-Sep-2012 22:03

The most concise/intuitive article I've read so far on the subject. Diagrams were especially helpful. If you manage to write a book on Linux is such manner, no doubt it's going to be a bestseller...

Thanks,

Val.

Anonymous

Fri 5-Oct-2012 17:38

ZZZZZZZZZZZZZZZZZZZZZZZZZZ to long, jesus christ write up a summery for us lazy people

too many Zs. write a shorter insult with proper spelling for us non-moron people

Anonymous

Sun 11-Nov-2012 20:09

Thanks for the page. You help me a lot.

Anonymous

Thu 29-Nov-2012 10:27

Great job!

I have just a suggestion.

This article can be used as a first step by people (like me...) who don't have a deep knowledge on the topic,

so why not to add a "References" section to help going into more depth?

Cheers!

Anonymous

Sun 30-Dec-2012 09:26

Thank you very much for this great article!

Anonymous

Fri 4-Jan-2013 15:38

Excellent!! Thank you very much for this article. Watching and trying to understand technological issues via their historical context and being showed how they evolved make it really a pleasure to follow your explanations and very easy to get them.

Anonymous

Wed 6-Feb-2013 01:27

Very useful article, thanks.

A couple of clarifications that would be great:

1. It seems that there is a 1:1 correspondence between a session and the associated tty (bearing in mind that the associated tty may be "none") - is that true? In other words, can one session include processes with different ttys, and can processes in two different sessions be associated with the same tty? (and if so... what does that mean?!)

2. What are the exact rules for automatic raising of SIGHUP? It seems that this is raised by the TTY driver, right? Does the POSIX specification specify when this should happen and who should receive it, and does Linux follow that? I have a confusing situation involving ssh -t raising SIGHUP on exit, while logging out of an interactive ssh login apparently does not.

Thanks

Anonymous

Mon 11-Mar-2013 17:54

Wow! Thanks Linus for the article.

This is how I learned about things in the past, from people who knew their craft well. Compare this with how we do it today - don't think, google first, rummage through incoherent posts, forums, mailing lists and if you are patient enough might be able to put pieces together to get just the clues/pointers to the information you want! So much for the 'age of information'.

Keep up the good work!

Thanks.

Anonymous

Tue 2-Apr-2013 11:01

Real very informative! Kudos to your effort and thanks for hard work!

ulzha

Sun 7-Apr-2013 14:59

Fantastic, tack så mycket!

I'm not writing an OS, but writing an improved screen or GUI terminal I do consider.

In particular I was interested in whether there was an API to tell foreground job's output and background job's output apart (e.g. associating a PID with each chunk output) to highlight them understandably or something. Now I figure that I just might be able to implement that by wisely trapping SIGTTOUs perhaps...

Anonymous

Sat 18-May-2013 11:57

So much for the 'age of information'.

Well, it is the age of information. Noone said it was the age of wisdom.

Anonymous

Sat 18-May-2013 12:08

"yes" program , produced in 2009 !

Thank you David McKenzie for your contribution to open source community !

I am just wondering what in your background that enabled the FSF to accept such worthless contribution ?

Member of what masonic lodge or what church or son of a war hero or billonarties you have to be so they accept that piece of crap ?

For reference:

yes command - otputs a line on tty until killed !

coded and added to Linux in 2009.

Actually, "yes", which is part of GNU coreutils, has been around since, like forever, being an implementation of the same-named Unix command.

David MacKenzie is the author of many of coreutils' commands, including chgrp, chmod, chown, date, dirname, expand, fold, ginstall, groups, head, mkdir, mkfifo, mknod, nice, printenv, printf, rmdir, stty, su, tty, uname, unexpand, and obviously yes; and is co-author of many others. But he is probably best known for autotools, which is one of the most central pieces of free software, as any distribution maintainer could tell you.

Now, what have YOU done for GNU or Linux or free software?

Anonymous

Sat 24-Aug-2013 21:48

So I was searching for information about the TTY system, and where did I, by chance, end up? Thanks for the great article. :-)

/radiantx

Anonymous

Fri 6-Sep-2013 12:10

really great post ! Thanks a lot for the general understanding of the how, instead of the usual "to do this, do that" ! And I'll stop here because having three exclamation mark in one sentence is a clear sign of mental disorder, but thanks again.

Anonymous

Sun 15-Sep-2013 20:58

Excellent post on TTY!! Thank you -- Aananth C N

Anonymous

Thu 10-Oct-2013 05:42

thank you. very interesting.

Anonymous

Wed 11-Dec-2013 14:19

I'll join the crowd here to say thank you for the time you spent on this article, I've appreciated it a lot.

Anonymous

Fri 13-Dec-2013 15:13

Thank you. I don't think this article will EVER be outdated :P

Anonymous

Fri 27-Dec-2013 15:55

Have you tried ls /dev/tty.* to get a listing of connected devices? On my Mac OS X this cues me into which tty device to use. I'm running a DEC Writer III. :)

Anonymous

Tue 11-Feb-2014 19:38

Thanks for this enlightening article! It is a bit wordy, but I completely expected that.

I am currently working on writing a toy OS

You work for Microsoft?

Anonymous

Wed 12-Feb-2014 02:14

First teletype was made in Germany around the 30s or 40s. One of the many types of teletypes was the Hellschreiber. There were also other types but all worked the same: you punch letters and they are remote printed to one or more receiving stations. After the war in the operations of confiscating the german patents, the US developed the teletype further. That's missing from your article.

Anonymous

Fri 14-Feb-2014 08:32

The Hellschreiber device is not a teletype, it's more of a facsimile machine. You pressed a letter, and a _bitmap_ of the letter was transmitted (twice to account for mechanical asynchrony). On the receiving side, marks and spaces were literally penned onto the paper, creating two copies of the text, one atop the other. Due to said asynchrony, it was often skewed, but because two copies were printed, nonetheless legible.

This is a very, very different mode of operation from the teletypes described above.

Anonymous

Fri 14-Feb-2014 08:45

I don't know of the history of teletypes in Germany, but the Teletype Model 15 was first produced in 1930 and was in wide use before and after World War II. Both in military and civilian contexts.

The Model 15 wasn't the first model produced by the Teletype corporation, but it was probably the most widely used pre-war model. It might have been the most widely used model, period.

Like I said, I don't really know anything about German teletypes, but looking at the Wikipedia page for Hellschreiber, that device is quite different than the Teletype corporation's devices. It looks like the Hellschrieber sends pixels and might actually be more similar to FAX machines than Teletype devices.

Anonymous

Mon 17-Feb-2014 07:42

man! you made me see the light!

Anonymous

Thu 27-Feb-2014 07:55

Hi--how are things in Sweden?

Every once in a while I get up the ambition to complain about the width of text on a web page, and you're the lucky winner today--sorry ;-)

This could be a good article--from the looks of it, it probably is--but why is it (and so many other web pages today) so wide?

Checking one line at random, it is 130 characters wide:

echo "Meanwhile, however, the computers — still quite large and primitive, but able to multitask — were becoming powerful enough to" | wc

1 20 130

Oh, and I'm ignoring the stuff in the left hand panel / column--I simply horizontally scroll so that panel is not visible.

.....

You mean you aren't browsing this page with a teletype?

Anonymous

Thu 10-Apr-2014 12:05

Writing to a TTY which is stopped due to flow control, ...... will block your process

I can't understand? The process will not blocked when TTY is stopped by flow control(ctrl+S), the foreground process will continue running. The only difference is I can't see the display until I type ctrl+Q again.

Thanks for you article

nyu

Anonymous

Thu 10-Jul-2014 11:03

Writing to a TTY which is stopped due to flow control, ...... will block your process

You're right Nyu... and this running process may be blocked when the TTY kernel buffer is full of non-displayed characters, if it outputs too much on stdout/stderr.

I guess the author has taken a shortcut when writing this, as the flow control stop is often used to block a too verbose process and to be able to read few lines before let it go on again.

Yves

Anonymous

Mon 14-Jul-2014 11:16

great post!!

Anonymous

Thu 24-Jul-2014 09:39

Hi!

I'm struggling with RS485 communication:

Is it possible to configure a tty to automatically raise the RTS line before sending and lower the RTS line after sending?

Thank you for your input,

Helmut

Anonymous

Mon 28-Jul-2014 11:13

the VT100 was not a colour terminal as the text suggests, though.

Anonymous

Mon 28-Jul-2014 22:07

Unix purist <- I WAS HERE +1 great article

Anonymous

Tue 5-Aug-2014 09:29

Great article - I will recommend it to my network.

Kind regards,

Christian from Germany

Anonymous

Tue 2-Sep-2014 16:01

the VT100 was not a colour terminal as the text suggests, though.

seems so, just checked the manuals online. http://www.vt100.net/docs/

http://www.vt100.net/docs/

do you perhaps know which terminal introduced color attributes for escape sequences?

Anonymous

Thu 4-Sep-2014 23:13

The Plan 9 operating system completely does away with TTY, signals, and ioctl. Everything in Plan 9 is either a file or represented as a file.

Anonymous

Sat 13-Sep-2014 12:33

The basic idea is that every pipeline is a job, because every process in a pipeline should be manipulated (stopped, resumed, killed) simultaneously.

Not just pipelines, every command is a job in shell's parlance. Job is a userspace thing, only maintained by shell, not kernel.

http://www.gnu.org/software/bash/manual/bashref.html#Shell-Commands

Niz

Anonymous

Wed 24-Sep-2014 12:34

By far the most amazing article about TTY I have ever read, thanks!

Anonymous

Sun 12-Oct-2014 17:04

Great article. Thank you! After 15 years of using unix and linux i finally got to the description of ttys :)

Anonymous

Fri 14-Nov-2014 01:31

Fantastic article. Thanks for doing all of the legwork. :)

-pf

Anonymous

Sun 16-Nov-2014 02:37

Thank you so for the article!

And now, I know the meaning of "tty" and "pty"!

Maya2003

Anonymous

Wed 3-Dec-2014 15:42

Great article, thanks a lot!

Anonymous

Wed 14-Jan-2015 20:09

This is the best article I have ever read about tty.

Thanks a lot!

Anonymous

Mon 9-Feb-2015 02:38

Thank you !!! :)

Anonymous

Sun 15-Feb-2015 22:47

Yup, interesting intro to the TTY area, I think, and fun trying out the terminal tweaks..but any ideas of where I can find a good intro to using TTY in Linux, without the writer assuming I know all about networking/the Linux kernel in advance??? Please and thank-you.

Anonymous

Sun 15-Feb-2015 22:47

zdennis

Zach Dennis

Sun 8-Mar-2015 23:07

The O'Reilly book titled "Termcap & TermInfo" should get you started: http://www.amazon.com/termcap-terminfo-OReilly-Nutshell-Linda/dp/0937175226

http://www.amazon.com/termcap-terminfo-OReilly-Nutshell-Linda/dp/0937175226

Anonymous

Sat 11-Apr-2015 22:14

best tty explanation ever written, hands down.

thank you so very much.

Anonymous

Tue 14-Apr-2015 23:21

I'm not especially new to bash programming or serial ports in general, but have never delved into stty and echo.

So I have a simple script (and I'm not sure I have the stty settings right), that should send "tx c" to the serial port. However, the best I can tell using minicom, if I perform the following from the command line:

echo tx c > /dev/ttyO1

the serial device receives:

tx.{

echo aaa > /dev/ttyO1 echos aay

echo bbbbb > /dev/ttyO1 echos bbbbz

echo abcdefghijklmnop > /dev/ttyO1 echos abeefgkijkmmnoz

WHAT am I missing?

Anonymous

Tue 21-Apr-2015 16:45

This is best ever article on TTY. What an awesome job. Please keep writing such articles.

Chakri

Anonymous

Fri 8-May-2015 12:00

The best article about tty ever! no kidding.

Anonymous

Sat 25-Jul-2015 18:08

very useful! thanks~

Anonymous

Thu 30-Jul-2015 04:41

I am new to Linux but this article is so clear even i can taste 60% of them. I will read it again a couple days later because this is the best article i ever read about linux.

Anonymous

Sat 15-Aug-2015 19:45

amazingly clear on a very fundamental piece of technology that I never really knew about. Just the build up of the subject is a great way to approach any kind of technical writing. I'll definitely keep this post in mind when I try to tackle writing about a complicated technical subject.

Anonymous

Sun 16-Aug-2015 03:57

"yes" program , produced in 2009 !

Thank you David McKenzie for your contribution to open source community !

I am just wondering what in your background that enabled the FSF to accept such worthless contribution ?

Member of what masonic lodge or what church or son of a war hero or billonarties you have to be so they accept that piece of crap ?

For reference:

yes command - otputs a line on tty until killed !

coded and added to Linux in 2009.

HISTORY

The yes command appeared in Version 7 AT&T UNIX.

The code for this in GNU is ridiculously long, but in other systems, the code is shorter than the license or the above comment

include <stdio.h>

int

main(int argc, char *argv[])

{

if (argc > 1)

for (;;)

puts(argv[1]);

else

for (;;)

puts("y");

}

i336_

David Lindsay

Tue 18-Aug-2015 16:55

This is really cool. I've just discovered this; I might have more to add after I've properly read through it. While I remember, though, I want to mention one of the better resources I've found for terminal escape sequences: an old online manpage for dtterm(5), at

http://www2.phys.canterbury.ac.nz/dept/docs/manuals/unix/DEC%5C_4.0e%5C_Docs/HTML/MAN/MAN5/0036%5C_%5C_%5C_%5C_.HTM

Also, the following Github Gist is updated extremely frequently (6 days ago, as I type this); it tracks terminal support for 24-bit color, a fairly new capability that's slowly gaining traction. You should be able to just expect it to be available 3-5 years from now; terminals in hyper-current distros like Arch likely have the support already.

https://gist.github.com/XVilka/8346728

Anonymous

Tue 22-Sep-2015 04:41

Thanks for a great explanation of infrequently discussed, arcane, but essential part of the Unix/Linux "reptile brain". Required knowledge for anyone who hopes to use Unix/Linux beyond a novice level.

Anonymous

Mon 30-Nov-2015 09:35

Amazing. Very very needed. Thank you very much.

Anonymous

Fri 4-Dec-2015 08:14

Very nice summary for those born too late :-)

It might be worth mentioning that UARTs/serial ports have control lines to block transmission when the received data can't be processed as fast as it's received.

This way the UART itself would stop transmitting and the whole process was handled in hardware.

The sending process would have to wait (be blocked) for the UART to transmit it's data anyway, as the hardware would mostly be slower than the software anyway.

However when modems came in to play, the terminals UART and the hosts UART were no longer directly wired to each other and modems would only transmit data and no control lines between them. This situation required another method of flow control that would have to be transmitted in-band with the data. So software flow control via device control characters was invented.

Anonymous

Thu 7-Jan-2016 12:45

Anonymous

Tue 12-Jan-2016 16:07

Really good document,thanks for your great efforts

Anonymous

Fri 5-Feb-2016 14:34

I am learning linux... and it is best article to understand what is going under the hood:) thanks for your time

Anonymous

Sat 20-Feb-2016 01:55

Good job demystifying TTY for me. Keep it up.

Cheers

Anonymous

Thu 10-Mar-2016 01:06

This and all OK.. But.. I have one doubt for long time... In case of terminal emulator we can navigate/scroll using mouse.. How do we scroll/navigate inside TTY (I am a Linux beginner.. So sorry if it's a stupid question)

lft

Linus Åkesson

Tue 29-Mar-2016 22:35

Hi! That is not handled by the TTY layer, it is done by the terminal emulator itself. Whenever something scrolls off-screen, it is appended to a scrollback buffer, which you can then view using GUI controls or keyboard shortcuts.

Anonymous

Wed 30-Mar-2016 22:02

Yes! More than I was looking for, but I'm not complaining. Brilliant article, and now I know enough to stay the fuck away until I must.

Anonymous

Sat 9-Apr-2016 01:04

really cool thanx to share your knowledge

Anonymous

Sun 10-Apr-2016 03:34

Hello Linus! I have a question about what happens when you kill a process in raw mode. If you launch, say, vim in a terminal and then run killall -9 vim from a separate terminal, you would expect the original terminal to be left in raw mode after vim exits (as if you had run stty raw or something). However, surprisingly, it is left in canonical mode. I have tried this with multiple shells, terminal emulators, and OS's, and only urxvt behaves differently from this. My question is, what mechanism resets the terminal in this case?

-- kc

lcu

Luis Colorado

Thu 14-Apr-2016 14:19

-- kc

Historically, the UNIX tty driver used to reset terminal settings to known values (300bps, some parity, cooked mode, etc.) in close() callback (which was called by the kernel only on last close to the device only) but this was found nonsense, because normal initial session programs like old /etc/getty (yes, it was stored there) were to initialize it fully. This had some drawbacks, because this last close could happen when not needed and the reasons to conserve settings between close where significant.

This requirement has been dropped from linux kernel and now linux allows you to fix terminal settings from a previous shell command without losing those settings because of a last close issue.

Anonymous

Fri 22-Apr-2016 04:21

Thank you sir. You have been helpful.

Anonymous

Mon 16-May-2016 21:57

Thank you. I don't think this article will EVER be outdated :P

The images of it already are on high resolution monitors.

Anonymous

Thu 23-Jun-2016 20:00

Best article regarding this mess!

Anonymous

Fri 15-Jul-2016 21:14

This is just awesome! I have old phone co UNIX books that don't go into this depth! Thx for this info

Anonymous

Sat 16-Jul-2016 19:19

Nice one. Thanks.

Anonymous

Sat 23-Jul-2016 14:39

I'm using python on Debian os.

I've interfaced bluetooth to uart.

The data coming from bluetooth is received and can be monitored on ttyS0.

Can someone tell me how to copy that data.

I want to paste it in a text file

Anonymous

Mon 29-Aug-2016 22:34

"yes" program , produced in 2009 !

.... cut ....

For reference:

yes command - otputs a line on tty until killed !

coded and added to Linux in 2009.

Interesting! I remember I made a joke with yes program to my friend in 1996 by using Slackware Linux I.e. Linux kernel so new.

Anonymous

Mon 29-Aug-2016 23:03

I'm not especially new to bash programming or serial ports in general, but have never delved into stty and echo.

echo tx c > /dev/ttyO1

the serial device receives:

tx.{

echo aaa > /dev/ttyO1 echos aay

echo bbbbb > /dev/ttyO1 echos bbbbz

echo abcdefghijklmnop > /dev/ttyO1 echos abeefgkijkmmnoz

WHAT am I missing?

Not sure about your system but in my old linux systems with serial port, device was ttyS01 but not tty01.

Anonymous

Mon 29-Aug-2016 23:11

Shift+page up/down

This will do the job both in fyi terminal emulators and the ones on TTYs (ctrl+alt+F1, etc.).

Anonymous

Mon 29-Aug-2016 23:23

I'm not especially new to bash programming or serial ports in general, but have never delved into stty and echo.

echo tx c > /dev/ttyO1

the serial device receives:

tx.{

echo aaa > /dev/ttyO1 echos aay

echo bbbbb > /dev/ttyO1 echos bbbbz

echo abcdefghijklmnop > /dev/ttyO1 echos abeefgkijkmmnoz

WHAT am I missing?

Not sure about your system but in my old linux systems with serial port, device was ttyS01 but not tty01.

It seems ttyO1 works for you.

Maybe outrun a CR and LF at the end of the echoed words will help.

Indeed CR may not be needed.

Try this:

echo bbbbb\n\r > /dev/ttyO1

Hope it works *fingers crossed* :)

Anonymous

Mon 29-Aug-2016 23:29

I have been with Linux and UNIX systems since 1994. They were good old days and I am learning these amazing knowledge just now from this article.

Fantastic job Linus. Thank you so much!

Bedri Özgür Güler

Anonymous

Thu 6-Oct-2016 19:39

the text should be arranged to wrap to the width of the (reader's) window

Whilst I agree with you entirely, this article wraps just as you request.

Of course, it may have been altered in the many years since you wrote your comment :-) but credit where credit is due!

Anonymous

Sun 20-Nov-2016 21:14

Wow, very impressive style of writing, thank you very much.

Great article.

Would be awesome something like that about handling keycodes(infocmp & friends).

Anonymous

Sun 11-Dec-2016 23:47

I have a small question.

Can you tell me when the /dev/ttyXXX or /dev/pts/X are added in the /dev directory ?

Initially, I thought that each program had its own /dev/ttyXXX device. When is the kernel adding a new /dev/ttyXXX device ?

Let say that I add the following lines in /etc/rc.local

Start these program during the init.

/home/user/prog1 &

/home/user/prog2 &

/home/user/prog3 &

Will it create one or three new /dev/ttyXXX device ?

(or will it reuse another one that is already present?)

Thanks for this article.

Anonymous

Sun 12-Feb-2017 05:50

ZZZZZZZZZZZZZZZZZZZZZZZZZZ to long, jesus christ write up a summery for us lazy people

That was the "summery"; you are an insult to lazy people everywhere.

But you have redeemed yourself with a great toast: To long, jesus christ! >clink!<

Anonymous

Sat 18-Feb-2017 19:46

Thank you Linus. Really helped me understand this stuff.

Anonymous

Mon 13-Mar-2017 07:49

This was extremely helpful, thanks!

Anonymous

Wed 15-Mar-2017 12:28

In The Hitchhiker's Guide to the Galaxy, Douglas Adams mentions an extremely dull planet, inhabited by a bunch of depressed humans and a certain breed of animals with sharp teeth which communicate with the humans by biting them very hard in the thighs. This is strikingly similar to UNIX, in which the kernel communicates with processes by sending paralyzing or deadly signals to them. Processes may intercept some of the signals, and try to adapt to the situation, but most of them don't.

This is the only thing I could find about it, do you have any reference for that?

Anonymous

Sun 7-May-2017 22:25

amazing article!

thank you very much

Anonymous

Tue 13-Jun-2017 21:18

Just wanted to leave another pile of thanks for this great article.

Regards,

jvlns

Anonymous

Mon 3-Jul-2017 22:01

Linus,

I read this a few years ago and have just re-read today. This is one of the best and clearest explanations of anything, never mind something as dry and arcane as tty.

You have a great talent for this, many thanks.

Anonymous

Mon 17-Jul-2017 20:16

Thanks for that nice write-up. Actually, I just came here because I don't want linux to mess with my payload over the serialport, is there a quick way to tell the tty to simply forward everything and _not_ mess with it?

I've tried stty "raw" option, it's still messing around though.

Anonymous

Mon 24-Jul-2017 05:03

I have been always wondering how tty works for years and could not get a clear idea until today I stumbled on this write up, an admirable article! Thank you!

Anonymous

Tue 26-Sep-2017 00:17

"To facilitate moving the terminal emulation into userland,(...) inside pseudo terminals, à la screen(1) or ssh(1)..."

What means "à la screen" ? do you speak french too ;) ?

Really interesting and original web site, i wonder where you find the time to do so many things (time consuming things)...

Thanks for your writing

Anonymous

Mon 11-Dec-2017 19:21

@OP, Btw do you have any "to the metal" UNIX/linux books to recommend that has information regarding such stuff?

Anonymous

Wed 20-Dec-2017 22:15

Amazing article!

Anonymous

Thu 11-Jan-2018 03:39

You are a Rocket Scientist!

Anonymous

Tue 6-Feb-2018 17:48

10 years old, still the best article on TTYs. Still getting comments. Not a record ( I presume ) but noteworthy.

Thank you very much Linus. I understand my particular problem - my expect script has to send a \n character at login - much better now.

To long !, Jesus Christ.

Anonymous

Thu 22-Feb-2018 02:08

Returning to Unix/Linux after 30 year gap. If only all documentation was as clear as this. Excellent. Thanks.

Anonymous

Thu 22-Feb-2018 05:58

An awesome article, it helps a lot! Thanks from china.

Anonymous

Sun 22-Apr-2018 00:39

Thank you very much for this amazing article!

Indeed, some things had been demystified :)

Anonymous

Fri 18-May-2018 05:00

Beautifully written and comprehensive enough for UNiX beginners like me. Thank you!

Anonymous

Wed 18-Jul-2018 20:44

Thanks for the introduction of TTY!

Anonymous

Sat 28-Jul-2018 00:46

I thought that it would be helpful to leave a comment about something that confused me.

Why, when -tostop was set on the terminal, did the editor get sent a SIGTTOU but the backgrounded (sleep 5; echo ...) did not?

I used strace on the editor process and found that it was calling ioctl() right before receiving a SIGTTOU:

--- SIGCONT {si_signo=SIGCONT, si_code=SI_USER, ...} ---

ioctl(5, SNDCTL_TMR_STOP or TCSETSW, {...}) = ...

--- SIGTTOU {si_signo=SIGTTOU, si_code=SI_KERNEL} ---

--- stopped by SIGTTOU ---

In the Linux kernel source, I found that the code that handles an ioctl request for TCSETSW will unconditionally check if the process is in the foreground process group of the tty, and send SIGTTOU if it isn't.

But, at least in the write handler for N_TTY, the same check is *only made if the TOSTOP bit is set*.

So, -tostop will allow a backgrounded process to write to the terminal but it will not allow a backgrounded process to call ioctl(..., TCSETSW, ...) on it.

Anonymous

Wed 12-Sep-2018 17:03

which terminal introduced color attributes for escape sequences?

DEC's VT241?

Anonymous

Wed 12-Sep-2018 17:07

However when modems came in to play, the terminal's UART and the host's UART were no longer directly wired to each other and modems would only transmit data and no control lines between them. This situation required another method of flow control that would have to be transmitted in-band with the data. So software flow control via device control characters was invented.

This flow-control pre-dates modems. The ^S/^Q characters _are_ the ASR-33 teletype's codes for turning off and on (respectively) the paper tape reader. Systems would issue them to prevent their processing programs from being overrun by automated input from tape. Later they were generalized into line disciplines, etc.

Anonymous

Thu 8-Nov-2018 08:42

Excellent comprehensive article on tty, thx Linus!

Anonymous

Fri 9-Nov-2018 22:35

Great and informative read indeed! I feel a bit more confident tinkering my terminal now. Also learnt quite a lot on signal and process concepts. Thanks :)

Anonymous

Sat 29-Dec-2018 20:10

Thanks for an informative article. I really learned something!

-Tom

Anonymous

Wed 2-Jan-2019 17:13

Brilliant article! Thanks a lot!

Anonymous

Tue 29-Jan-2019 19:59

Thank you for the wonderful article!

Anonymous

Wed 27-Feb-2019 09:17

I am loving you hard.

Catatonic

Anonymous

Mon 11-Mar-2019 14:12

superb, fantastic -- opened a new world for me.

Thank you

Anonymous

Fri 24-May-2019 14:34

Thanks! Great article!

Anonymous

Wed 5-Jun-2019 06:01

ZZZZZZZZZZZZZZZZZZZZZZZZZZ to long, jesus christ write up a summery for us lazy people

Haha, this article IS the summary

Anonymous

Fri 7-Jun-2019 00:01

How do a program should propperly handle a "/dev/pts"? I mean, I am able to open that device for read and write, the program is effectively writing and reading stuff, but when I want to close the program gracefully and close the file descriptor used to open the device, the program crashes with no clue on what has happened.

Thanks for the excellent work!

Anonymous

Sat 8-Jun-2019 23:57

Having read your wonderful article, am feeling obliged to give my thanks. BIG thanks! :)

Anonymous

Thu 20-Jun-2019 21:03

Amazing stuff! Thank you!

Anonymous

Thu 1-Aug-2019 18:33

Absolutely amazing post.

Anonymous

Mon 5-Aug-2019 06:09

This is a fantastic guide. As you say, it's hard to find straightforward information about how the tty system works.

Anonymous

Thu 29-Aug-2019 23:58

Thanks for the great post!!

Anonymous

Tue 24-Sep-2019 03:54

lucid explanation. much makes sense on terminals in stevens/kerrisk linux api guide

thanks!

Waverley

Jim Fergusson

Sat 5-Oct-2019 13:35

a really excellent article on a subject that mystified me - (and I'm an old guy who remembers punched tape 'telex' as the main communication channel between our company and it's Connecticut USA parent) - thanks so much for this Mr Akesson!

Anonymous

Mon 11-Nov-2019 01:19

For me, background processes are able to write to the terminal. For example,

[ ~]$ python -c "import time; time.sleep(10); print('hello');"

[2]+ Stopped python3 -c "import time; time.sleep(10); print('hello');"

[ ~]$ bg

[2]+ python3 -c "import time; time.sleep(10); print('hello');" &

[ ~]$ hello

Anonymous

Fri 27-Dec-2019 21:05

Trying to use GDB GUI application, with two terminals, and you find out this information bringing some light when you want control on the execution screen. New decade and still useful info.

Anonymous

Sat 29-Feb-2020 19:59

Total shit. Doesn't explain how the tty works, nor how the shell connected to tty/pty.

Ruslan63

Ruslan Poddubny

Thu 12-Mar-2020 13:02

If it's possible: How is related /dev/ack with /dev/tty?

Anonymous

Fri 7-Aug-2020 13:10

wow

Anonymous

Tue 18-Aug-2020 07:18

O.M.G.!!!

Anonymous

Mon 31-Aug-2020 23:52

Great article, thank you for putting this together!

Anonymous

Mon 7-Sep-2020 18:33

I didn't get exactly how pty came into picture and what is the usage of that.

Looking forward to get more info on that.

Anonymous

Wed 30-Sep-2020 06:55

Really informative! good article!

Anonymous

Tue 13-Oct-2020 22:11

is this troll ..vik

Anonymous

Tue 3-Nov-2020 23:23

Thank you! Still informative in 2020! After so many years, there's still not article explains TTY better than this.

Anonymous

Sun 17-Jan-2021 14:03

Thanks for your article！

Anonymous

Sun 28-Feb-2021 22:16

Dear Mr. Linus Åkesson,

Thank you for the extremely informative article. I'm an amateur programmer and Linux newbie and trying to wrap my head around TTY as I'm exploring the capabilities of the new "iSH" iPad app which runs Alpine Linux inside an x86 emulator on iPadOS on an iPad. The Alpine Linux wikis ay that Alpine Linux can be set up to do graphics and a GUI, but I've not been able to so via the iSH app. When I try to do so, my X or Xorg or xterm programs usually either tell me that they "Can't open display"or that it "Cannot find a free VT: Not a tty". When I use the Linux tty command, it shows the active tty as /dev/pts/0 which I understand would be expected since I am in a Bash shell inside Linux typing the command. I wrote the app developer who indicated that the App isn't really set up to graphics. Setting the env $DISPLAY to the host:0.0 also does not help.

My questions for you that I'm confused about is how could the App not be able to do graphics, access the display, or find a free tty if it is able to display the text characters on the terminal? Why can't X11 use the same procedure that the Bash Linux shell uses to display text characters on my iPad to display graphics? I did notice that the same difficulty comes up when using Alpine Linux Docker containers--they seem to only natively display text characters in a terminal window. I read that one can set up such Alpine Linux Docker containers to display graphics/GUI information but to do so one has to set up the Docker container to specifically transmit such graphic information to the local computer's tty display. I'm guessing that the difficulty Alpine Linux Docker containers might have in natively displaying graphic/GUI information (as opposed to text character shell information) might be related to the same limitation I might be having in trying to get the Alpine Linux kernel in the iSH app to display graphic/GUI information on my iPad but don't understand mechanistically why.

If you could explain the difference between Linux being able to display character text information on a tty display versus being able to display graphic/GUi information on the same tty display (perhaps with some supplemental diagrams to your original article), I (and suspect others) would greatly appreciate it.

Thanks,

MIke

Anonymous

Thu 11-Mar-2021 16:15

Dear Mr. Linus Åkesson,

(..)

MIke

Hey MIke,

Not Linus Akesson but I might clear up the confusing here, I admit it is a topic that can lead to much confusion. I took a quick peek into "iSH" but keep in mind iOS is not my forte, nor do I fully know how "iSH" is implemented. In that regard I do not know what is possible and what is not but I give my educated guess at the end.

To clear up any confusion about a "tty" and "bash": both have absolutely nothing to do with anything graphical, be it rendering text on a terminal or doing anything GUI related.

"bash" is simply a shell like there are many other shells ("sh", "zsh", "ksh"). To really oversimplify things: "bash" reads input bytes (your commands you type for example) and spawns processes. These processes produce output. Bash does not intercept this output nor does it render text. For example, if you type "ls" and press enter, it reads the command, creates a new process ("ls") and that process produces output.

A "tty" neither has anything to do with rendering text. You can read and write to a "tty". The article explains the "tty"'s role: It is a passive object with some data fields. Just like a normal file you can read and write to the "tty". A keyboard can for example write to a "tty", "bash" can read that data from the "tty". The "ls" process can write to the "tty" and a >virtual terminal< (e.g. xterm) can read that data from the "tty". The "tty" also contains data about processes for example who the foreground process is in order to direct signals to the right process and process groups.

Then who is responsible for the actual rendering? The (virtual) terminal (not the "tty") plays a central role here. If we only focus on a virtual terminals we have basically two cases mentioned in the article.

1. The virtual terminal process resides in kernel space (called the console).It reads from the "tty", say the output from the "ls" process and renders the text. It interacts with the VGA driver to do so.

2. The virtual terminal process resides in user space: An example of virtual terminal is "xterm", "gnome-terminal" is another, "st", the one I use, but here are many more. The virtual terminal has the same tasks as the kernels virtual console but instead of interacting with the VGA driver it interacts with a display server (Xorg for example). The display server in turns interacts with the videocard drivers to render graphical output.

In short a "tty" has so little to do with rendering anything that a "tty" does not even need to be connected to a virtual terminal. A ssh server for example reads from a socket, writes to a "tty" and returns the output back over the connection to a ssh client. The ssh client writes the results to a "tty" and your virtual terminal process reads it and presents it to the screen.

As far as the 'iSH' app is concerned: it seems that it merely emulates a virtual terminal. To do anything GUI related in a linux enviroment you should have a display server running (e.g. Xorg) that interacts with the linux video display drivers. Opening "xterm" in "iSH" is like starting a new virtual terminal from a virtual terminal you are already in ("iSH"). So in order to start a graphical user interface you have to install a display server and all display drivers which I doubt "iSH" can do (hence the response from the developer 'it is not really set up to do anything graphical').

I hope this helps,

Bennie

Anonymous

Fri 12-Mar-2021 03:28

Wow, Bennie, thanks for that thorough response, and it does help. I thought the TTY might be the answer to most of my questions, but I now understand that it is much more complex than that.

Have a good week,

Mike

Anonymous

Sat 13-Mar-2021 16:34

A well formulated question deserves a proper answer I think. Besides, you did your research by even contacting the app-developer. Happy to help :).

To go a bit off topic regarding the article (since noone is really posting here anyway) and to say a bit about running linux on an ipad I think you have three options. What you want depends on your goals (but running iOS apps inside linux will not be possible).

1. Either/or linux/iOS: jailbreak

If you want to run a full fletched linux OS on the ipad an option is to jailbreak the ipad and try to install linux. This is hard because Apple does not want you to and a failed installation might render the ipad useless. Also you will not be able to run any iOS apps anymore obviously.

2. Interact with the ipad linux-like: 'iSH'

To interact with the ipad like running xterm is using the 'iSH' app, the reason I think it was created . I'm not totally sure what the app is capable of but I guess you can do pretty much anything with your system as on a normal linux distribution, as long as it does not require GUI applications (except perhaps starting iOS apps you already have installed).

3. Running linux on top of iOS: Virtualisation Software

The last option I can think of is to run linux in a virtual machine (I checked and VirtualBox seems to have a port for ipads). Here you run linux on top of iOS in an app such as VirtualBox. The downside is when in emulating software such as VirtualBox things run more slowly, because it has to emulate the environment. Also, it will only grant access to linux application and its own filesystem; interacting directly with files and apps under iOS is not what it is intended to do. The upside is that you can keep using you ipad in the normal way and switch to linux whenever you want (with a graphical user interface if you like).

Hope you are having a great weekend :),

Bennie

Anonymous

Sat 13-Mar-2021 17:04

I forgot to mention some resources if you are interested:

To get a kind of overview how graphics work under linux have a look at the graphics server (note that Xorg is not the only player in town anymore. Wayland is a newly developed display server):

https://en.wikipedia.org/wiki/X.Org%5C_Server

If you want to go down the rabbit hole:

Bovets 'Understanding the linux kernel' is an indepth look into how the kernel works.
A more specific look at the role and code of a tty driver is Coberts 'Linux Device Drivers' (chapter 18 is devoted to tty drivers)

Another tip is looking at the source code of the mentioned 'st' terminal (https://st.suckless.org/). It is thé terminal with the least bloat and easiest to understand code in my opinion (take for example x.c where xlib is included, the interface to the X server to make 'draw' requests).

https://st.suckless.org/

Anonymous

Fri 2-Apr-2021 00:31

I wonder when I'm in bash and erasing text with backspace what stops me from erasing the bash prompt at the beginning of the line?

Anonymous

Tue 6-Apr-2021 18:43

Thank you very much, Bennie, for all the additional information! ’ll check out those additional resources you recommended.

Have a good week.

Sincerely,

Mike

Anonymous

Sun 18-Apr-2021 12:11

Anytime Mike :)

I wonder when I'm in bash and erasing text with backspace what stops me from erasing the bash prompt at the beginning of the line?

Eventhough it is a bit more complex (because bash works with a builtin readline library that handles backspaces) it basically boils down to this: the prompt is a simple string that gets printed before reading in your input. If for example you'd run in bash:

while true:

printf "my prompt $ "

read

done

you'd get the same situation. I guess you would not expect the 'read' command to erase the printf output string. In other words: the prompt is not part of the input buffer but a simple string that gets printed before each read call.

Bennie

Anonymous

Tue 20-Jul-2021 20:18

Why aren't UART driver in the graph for typical desktop system? From the article, it seems only the hardware of Terminal and UART are gone. UART setting still exists in later parts of the article.

ben

Ben in Seattle

Mon 2-Aug-2021 03:49

I wonder when I'm in bash and erasing text with backspace what stops me from erasing the bash prompt at the beginning of the line?

That's an excellent question!

Modern terminals only show what they receive, not what they send. When you hit a key, the terminal sends it to your UNIX box which then interprets it and (usually) sends it back. The illusion that what you type immediately shows up on the terminal is due to your shell choosing to echo back the keys you've hit. The backspace character (^H) is echoed back just like any other key by your shell until there is nothing left to erase except the prompt. At that point the shell simply chooses to not echo the backspace, thus keeping the prompt intact. Some shells even choose to send a different character, ^G, which rings the bell on the terminal to let you know you've hit the end.

ben

Ben in Seattle

Mon 2-Aug-2021 04:18

Great article.

For another great source on TTY devices you can go to:

http://publib.boulder.ibm.com/infocenter/aix/v6r1/index.jsp?topic=/com.ibm.aix.genprogc/doc/genprogc/ttysys.htm

Enjoy,

Ori

That site has been revamped and IBM only offers the manuals in PDF format. You can now (2021) find the TTY Subsystem for AIX documentation here:

http://public.dhe.ibm.com/systems/power/docs/aix/61/genprogc%5C_pdf.pdf#TTYSYS

Anonymous

Tue 17-Aug-2021 19:49

So as a convenience, the operating system provides an editing buffer and some rudimentary editing commands (backspace, erase word, clear line, reprint)

Can anybody supply examples of how these listed commands may be applied to an incoming command? When can the machine decide to deprive the command of all word (i.e. erase it from the command)?

Anonymous

Thu 23-Dec-2021 18:58

Marvelous article. Big thanks for that. I use the pipes for decades but never understood it down to the ground of the kernel, not better until now... good idea to start at the beginning with the teletypers ;)

Anonymous

Tue 4-Jan-2022 03:11

So as a convenience, the operating system provides an editing buffer and some rudimentary editing commands (backspace, erase word, clear line, reprint)

Can anybody supply examples of how these listed commands may be applied to an incoming command? When can the machine decide to deprive the command of all word (i.e. erase it from the command)?

I don't think the buffer allows one to erase a word from a command that has been sent.

An example of how the buffer works is when you're typing a command in a console and, before hitting enter, decide to delete some characters to fix a typo.

Anonymous

Tue 1-Feb-2022 17:50

hello

Anonymous

Thu 21-Apr-2022 19:15

Great article!

Anonymous

Mon 25-Apr-2022 07:55

Fantastic stuff!!

Thank you for writing it.

Anonymous

Wed 27-Apr-2022 15:57

This is by far the best tty article to be found on the web. Thanks!

Anonymous

Sun 29-May-2022 19:26

Thanks for a great article. We need a lft-o-pedia for purely technical stuf :)

Anonymous

Thu 9-Jun-2022 13:36

Anytime Mike :)

I wonder when I'm in bash and erasing text with backspace what stops me from erasing the bash prompt at the beginning of the line?

while true:

printf "my prompt $ "

read

done

Bennie

Anonymous

Fri 1-Jul-2022 22:07

Thanks a lot – those explanations are really helpful and as clear as they can possibly be. The topic itself is quite special: As soon as you use a terminal and a shell, you will encounter all these things – if you want it or not. You can simply let them happen and rely on the fact that everything just works … somehow. But with the help of this article, you can actually understand what’s going on behind the scenes. Great!

Anonymous

Fri 8-Jul-2022 08:01

Anonymous

Tue 2-Aug-2022 07:54

I'd just like to point out another awesome article about "What happens when you press a key in your terminal?" by Julia Evans (b0rk)

https://jvns.ca/blog/2022/07/20/pseudoterminals/

kind regards

-mrizvic

Anonymous

Mon 10-Oct-2022 15:28

Thank you for your explanation!

linrl3