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Constructing a Useful Bash Prompt

DATE: 2017-08-14

AUTHOR: John L. Godlee

This is what my bash prompt looks like at the moment:

Current bash prompt

Here is what the various parts refer to:

Annotated bash prompt

And here is a link to a .bashrc,

What is a bash prompt?

The bash prompt is a piece of text placed at start of a command line interface using the bash shell. The primary function of the prompt is to let the user know that the computer is ready for the next command, the secondary function is to provide the user with some information about the status of the current session.

The default bash prompt looks like this:

Default bash prompt

so fire up your terminal of choice and see that your bash prompt looks similar. The default prompt shows the currently logged in username, the hostname (i.e. the name of th computer), the current directory path relative to ~ and finally a $, which marks the end of the prompt and the start of the area that you can type commands.

Customising the bash prompt

The .bashrc, among other things related to how bash functions, is where you can define a custom bash prompt. .bashrc is normally found in the root (~) directory. Check if you have a ~/.bashrc using:

cat ~/.bashrc

If bash says that there is no such file, create one using:

touch ~/.bashrc

Finally, start editing .bashrc using your favourite text editor, e.g. vim:

vim ~/.bashrc

Super basic

To start with, let's replace the default bash prompt with something really simple like this:

Dollar sign minimal prompt

To achieve this, type the following into your .bashrc:

PS1='\

Then exit the text editor and enter the following into the terminal, which forces bash to reload .bashrc:

source ~/.bashrc

PS1 refers to only one of five bash prompts, with PS1 being the default prompt that is shown most of the time; PS2,PS3,PS4 and PROMPT_COMMAND are used under special conditions. This article[1] has a good description of what they're used for.

1: http://www.thegeekstuff.com/2008/09/bash-shell-take-control-of-ps1-ps2-ps3-ps4-and-prompt_command/

Incorporating variables

The simplest way to add a variable (i.e. something which changes depending on environment conditions, like the current working directory) is to use one that bash understands by default, a full list of which can be found here[2].

2: http://www.gnu.org/software/bash/manual/bashref.html#Controlling-the-Prompt

Start by adding the time to our simple $ prompt:

PS1='\T \

Bash prompt with time

Note the space between \T and \$. Adding spaces between variables can help everything to look neater.

Read through the list of available variables and add a few to your own prompt.

Colours

To change the colours parts of the bash prompt, wrap the variable in ANSI escape sequences, like so:

PS1='\[\e[31m\]\w\[\e[m\] \T \

bash prompt with colours

This makes the directory path appear in red text. 31m is the section of that sequence that actually defines the colour red.

ANSI escape sequences can also be used to change the background colour of the text, add underlines, make the text bold, or high contrast. I often refer to both this wikipedia page on ANSI colour codes[3] and this blog post on jafrog.com[4] when choosing colour codes.

3: http://en.wikipedia.org/wiki/ANSI_escape_code#colors

4: http://www.jafrog.com/2013/11/23/colors-in-terminal

A more elaborate example which makes the current directory light-cyan coloured, with bold font:

PS1='\[\e[96;1m\]\w\[\e[m\] \T \

Bash prompt with complex colours

The ;1 is the part which makes the text bold. Note how [\e[m\] always needs to be placed at the end of the coloured part of the bash prompt, to return the colours to normal.

Try colouring the time (\T) so it is underlined and magenta.

Unicode characters

Another way escape sequences can be used is to add unicode characters to the prompt, whether this is a cool lightning bolt, a tick or a cross, or any of the other 136,000 characters available in Unicode 10.0.

Wikipedia has an excellent resource for browsing unicode characters

Browse through the wikipedia page and pick your chosen character, then note down its code, e.g. u2602 for the umbrella symol. The syntax for adding this to your bash prompt is as follows:

PS1=


\u2602'

Note that inserting unicode can also by accomplished just by copying and pasting the unicode character itself into the .bashrc.

Bash prompt with unicode

Note how the $ isn't escaped this time, as we don't actually want to see the $ in our prompt

Formatting

Brackets, spaces, hyphens, colons can all be used to great effect in your bash prompt to help separate the different parts. As an example, try setting this in your .bashrc and source ~/.bashrc to see the results:

PS1='[\T] {\u:\h} -- \w'

Bash prompt with brackets for formatting

Building PS1 incrementally

After a while, if your bash prompt becomes long and complicated, your PS1 code may start to look busy and hard to read, but it's easy to restructure the PS1 code to increase readability. In the example below, a long and complicated prompt is rewritten so each part is on a separate line and spaces are given their own lines to make them easier to see. Additionally, this allows each part to have its own comment, further increasing readability:

# Messy one liner
PS1='┏[\T] \u@\h \[\e[31m\]\w\[\e[m\]\n┗$ '

# Tidy on multiple lines
PS1=


\u250F'		# Elbow
PS1+='[\T]'	# Time
PS1+=' '	# Space 
PS1+='\u@\h'	# User@hostname
PS1+=' ' 	# Space
PS1+='\[\e[31m\]\w\[\e[m\]'	# current dir
PS1+='\n'	# New line
PS1+=


\u2517'	# Elbow
PS1+='\


	# $
PS1+=' '	# Space

Multi-line bash prompt

Conditional statements and functions

From here onwards I am writing about stuff I don't fully understand and have just gathered from other websites and tutorials.

If you want to expand the number of variables you can use in your bash prompt you can take create functions in your .bashrc, which you then source in your PS1 code. The example below (which I adapted from here[5]) shows how to present a unicode tick if the previous command was successful, and a unicode cross if it was unsuccessful, add the following to your .bashrc:

5: https://www.reddit.com/r/linux/comments/2uf5uu/this_is_my_bash_prompt_which_is_your_favorite/co7ygus/?utm_content=permalink&utm_medium=front&utm_source=reddit&utm_name=linux

SUCCESS='[✓]'
FAIL='[✗]'

error_test () {
    if [ $? == 0 ]; then
        echo -e $SUCCESS
    else
        echo -e $FAIL
    fi
}

## Bash prompt
PS1="\$(error_test)"

Bash prompt with conditional status

Note the __stat. As far as I know the __ aren't necessary, the function could just be called stat, but it seems to be a common convention in bash prompt functions.

Sourcing external scripts

I use the git-prompt.sh shell script[6], which is part of the contributed materials in the git github repo, to get the current git branch displayed in my bash prompt. Download and save the script in the link as ~/.git-prompt.sh. Then add the following to your .bashrc:

6: http://www.github.com/git/git/tree/master/contrib/completion/git-prompt.sh

source ~/.git-prompt.sh

PS1='$(__git_ps1 "[%s]")'

Bash prompt with git repository

git-prompt.sh also has some variables which can be set from within .bashrc. Details can be found by reading the preamble of the script in a text editor.

Now that you've read through the tutorial, try to construct your own bash prompt, adding the bits that you find useful. Refer back to the linked .bashrc[7] for some inspiration.

7: https://github.com/johngodlee/bash_prompt_tut/tree/master/files