Re: Run script after package install, update?

[Greg Wooledge <greg@wooledge.org>]

On Sun, Jun 15, 2025 at 11:07:41 -0400, Boyan Penkov wrote:
> So ultimately, my problem is addressed; however, the larger question
> is still open: is there in fact a straightforward way for a user, not
> the package maintainer, to tell the package management system: "If and
> only if your operation touched package x, also do this one thing
> locally."?  I can think of about a million use cases for this: "if you
> touched maildir-utils, run mu  index." "if you touched offlineimap,
> run offlineimap" "if you touched etckeeper, re-commit the /etc files"
> and so on ...

Since the apt documentation is such complete rubbish, the only ways
to actually figure out how anything *works* are source diving and
experimenting.  I do not feel like trying to source-dive through apt
and its libraries, so I tried an experiment.

According to the apt.conf(5) man page:

       Pre-Invoke, Post-Invoke
           This is a list of shell commands to run before/after invoking
           dpkg(1). Like options this must be specified in list notation. The
           commands are invoked in order using /bin/sh; should any fail APT
           will abort.

That's clear as mud.  What is "list notation"?  Again, I had to experiment.
I tried looking for examples, learned that the
/usr/share/doc/apt/examples/configure-index.gz file documented at the end
of the man page does not exist, discovered that the file
/usr/share/doc/apt/examples/configure-index exists instead, read that, and
found nothing helpful in deciphering what an apt.conf.d/* file should
look like.

I looked at other files in /etc/apt/apt.conf.d/ for inspiration, and
tried searching for the word "list" in the man page, but ultimately it
came down to experimenting until I got it right.

Here's what I did:

1) I created the file /etc/apt/apt.conf.d/99localexperiment with the
following contents:

====================================================================
DPkg::Post-Invoke { /usr/local/sbin/dpkg-experiment; };
====================================================================

2) I created the script /usr/local/sbin/dpkg-experiment with the following
contents:

====================================================================
#!/bin/sh
exec > /var/tmp/dpkg-experiment
printf '%s args' "$#"
if test "$#" != 0; then
    printf :
    printf ' <%s>' "$@"
fi
echo; echo
ps -fp "$$,$PPID"
echo
env
====================================================================

and gave it 755 permissions.

3) I installed a package by running "sudo apt-get install sl".

Here's the /var/tmp/dpkg-experiment file that was created as a result:

====================================================================
0 args

UID          PID    PPID  C STIME TTY          TIME CMD
root      487061  487060  0 11:19 pts/27   00:00:00 sh -c /usr/local/sbin/dpkg-experiment
root      487062  487061  0 11:19 pts/27   00:00:00 /bin/sh /usr/local/sbin/dpkg-experiment

SUDO_GID=1000
DPKG_FRONTEND_LOCKED=true
MAIL=/var/mail/root
LC_TIME=C
USER=root
HOME=/root
COLORTERM=rxvt-xpm
SUDO_UID=1000
LOGNAME=root
TERM=rxvt-unicode-256color
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
DISPLAY=:0
LANG=en_US.utf8
XAUTHORITY=/home/greg/.Xauthority
LS_COLORS=ln=31:ex=35:cd=44;37:bd=44;37:pi=32
SUDO_COMMAND=/usr/bin/apt-get install sl
SHELL=/bin/bash
SUDO_USER=greg
PWD=/tmp
====================================================================

Conclusion: there is nothing at all in the environment or arguments
passed to the Post-Invoke script(s) that indicate what has been done.
Any such scripts created by the local sysadmin will have to take their
own investigative steps to try to figure out what happened, and what
they should do about it.

Unless of course there's some *other* hook that we don't know about
because the documentation is so poor.