💾 Archived View for thrig.me › blog › 2023 › 11 › 11 › a-small-at-gotcha.gmi captured on 2024-08-18 at 18:09:21. Gemini links have been rewritten to link to archived content
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The gotcha may not be obvious,
"The working directory, the environment (except for the variables BASH_VERSINFO, DISPLAY, EUID, GROUPS, PPID, SHELLOPTS, SSH_AGENT_PID, SSH_AUTH_SOCK, TERM, TERMCAP, UID, and _), and the umask are retained from the time of invocation."
http://man.openbsd.org/man1/at.1
but may become apparent if the working directory the job was scheduled in is no longer available. Thus, a safe default when you do not care about the working directory would be to chdir to a "safe" directory before entering the at job.
$ ( cd / && echo echo echo | at +1m )
...
A test script would run something like the following: remove (or deny permission to) the working directory the test code runs under.
#!/bin/sh
set -e
d=`mktemp -d`
cd "$d"
f=`mktemp`
printf '(pwd;ls;cd /tmp;pwd;ls) | tee "%s"' "$f" |
at +1m
rmdir "$d" # whoops!
#chmod 000 "$d" # whoops?
sleep 61
printf '%s\n' "$f"
$ sh atdirtest.sh
commands will be executed using /bin/ksh
job 1699666500.c at Sat Nov 11 01:35:00 2023
/tmp/tmp.4ncZGxKiQ1
$ cat /tmp/tmp.4ncZGxKiQ1
$
The job did not run at all; assuming local mail is setup (it may not be, or may be broken, or could be accumulating to some long neglected /var/mail file that now has a few million entries in it)† there will be a mail message with an error. Maybe.
Your "at" job on gear.thrig.me
"/var/cron/atjobs/1699666500.c"
produced the following output:
sh: <stdin>[39]: cd: /tmp/tmp.YEpGSEczxB - No such file or directory
Execution directory inaccessible
What use is at? I use it for scheduling short-term reminders
#!/bin/sh
# tellme tea +2m
# tellme show 11:59
x11=${DISPLAY?"DISPLAY is not set"}
msg=${1?"message at-timespec"}
shift
cd # go home to avoid PWD being removed
printf 'DISPLAY="%s" xembiggen "%s"\n' "$x11" "$msg" |
at "$@" 2>/dev/null || echo >&2 "at failed"
that others might use a phone or something like remind for.
https://dianne.skoll.ca/projects/remind/
The root directory (/) is typical for daemons. Directories like $HOME could go away or be unavailable, especially if they are on remote-mounted filesystems, and the remote end or the network goes down. Other directories may make more sense depending on what the subsequent code expects and how the system is setup.
† One time the database admins finally fixed the mail transport agent, and my pager got hit with a backlog of 900+ messages, which set the pager to ringing for a while, until the pager company deactivated it, which made being on-call slightly more difficult than usual, and I had to call up the pager company and tell them to reactivate the pager, after cleaning out the mail spool. Why people who didn't know how to run a mail server were trying to run a mail server is probably a good question.
Yes, this was back when pagers were a thing. No, I haven't ever owned a smartphone.
tags #at #unix