Nick Nick - 1 year ago 55
Linux Question

Run shell script exactly once with cronjob

I have a cron job that calls a script that checks for updates once every hour. If there are updates, it will call
, which in turn calls
. I would like each "new"
to be run exactly once.

I was thinking of writing to a file each time I run
with some unique ID of that file. Before running
, I check if the file contains the ID of
. If so, don't run it.

Edit - more detail:


  • fetch from git repo, pull and merge if any changes

  • Run


I want
to run just once, because next hour, the cron will run again, pull from git, and run
again. Is there some way for each unique
to run just once?

After writing that out, maybe the best way is just to only run
if there was a change from the git repo?

This seems like overkill and hard to maintain. Is there a simpler way to write a bash script that only runs one time?

Answer Source

At the start of the script do this:

if [ -f stampfile ]; then
touch stampfile

This checks for a file called stampfile (specify a path to anywhere convenient where this may be stored). If it's there, just exit. If it's not there create it with touch.

Then let the script do its thing.

A slight variation: This can also be used to avoid having two instances of a script running at the same time. The script would then rm -f stampfile at the end of its run.

In this case, if the script is killed, the stampfile will be "stale" (stamp present but script not alive). To detect a stale stampfile, put the PID of the script into it instead of touching it.

To check if there's another instance running, and managing the stampfile:

if [ -f stampfile ]; then
    if kill -0 $(<stampfile) 2>/dev/null; then
        rm -f stampfile    # stale
echo $$ >stampfile

# rest of script

rm -f stampfile
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