Continuously watch a socket file, run command when it doesn't exist?

Question

It seems that when my screen is locked for some period of time, my S.gpg-agent.ssh disappears, and so in order to continue using my key I have to re-initialise it.

Obviously, this is a relatively frequent occurrence, so I've written a function for my shell to kill gpg-agent, restart it, and reset the appropriate environment variables.

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This may be a bit of an 'X-Y problem', X being above this line, but I think Y below is more generally useful to know anyway.

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How can I automatically run a command when an extant file no longer exists?

The best I've come up with is:

nohup echo "$file" | entr $command &

at login. But entr runs a command when files change, not just deletion, so it's not clear to me how that will behave with a socket.

Answer 1

According to your comment, cron daemon does not fit.

Watch socket file deletion

Try auditd

# auditctl -w /var/run/<your_socket_file> -p wa 
$ tail -f /var/log/audit/audit.log | grep 'nametype=DELETE'

Howto run a script if event occurred

If you want to run a script on socketile deletion, you can use while loop, e.g.:

tail -Fn0 /var/log/audit/audit.log | grep 'name=<your_socket_file>' | grep 'nametype=DELETE' \
while IFS= read -r line; do
  # your script here
done

thx to Tom Klino and his answer

Answer 2

You don't mention the OS you're using, but if it's linux, you can use inotifywait from the inotify-tools package:

#!/bin/sh

while inotifywait -qq -e delete_self /path/to/S.gpg-agent.ssh; do
    echo "Socket was deleted!"
    # Recreate it.
done