Shell – Pass multiple commands to flock

command lineflocklockshell

flock -x -w 5 ~/counter.txt 'COUNTER=$(cat ~/counter.txt); echo $((COUNTER + 1)) > ~/counter.txt'

How would I pass multiple commands to flock as in the example above?

As far as I understand, flock takes different flags (-x for exclusive, -w for timeout), then the file to lock, and then the command to run. I'm not sure how I would pass two commands into this function (set variable with locked file's contents, and then increment this file).

My goal here is to create a somewhat atomic increment for a file by locking it each time a script tries to access the counter.txt file.

Best Answer

Invoke a shell explicitly.

flock -x -w 5 ~/counter.txt sh -c 'COUNTER=$(cat counter.txt); echo $((COUNTER + 1)) > ~/counter.txt'

Note that any variable that you change is local to that shell instance. For example, the COUNTER variable will not be updated in the calling script: you'll have to read it back from the file (but it may have changed in the meantime), or as the output of the command:

new_counter=$(flock -x -w 5 ~/counter.txt sh -c 'COUNTER=$(cat counter.txt); echo $((COUNTER + 1)) | tee ~/counter.txt')
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