How to do locking a file from a shell script?

Question

I would like to synchronize two Bash scripts via a file lock. How to do this? Zsh has zsystem flock call, but is there a more "shellish" method that's also available to Bash?

I searched "bash lock file" and found this: - How do I synchronize (lock/unlock) access to a file in bash from multiple scripts? Try man flock, might be all you need. — yarl, Sep 10 '16 at 08:32

score 11 · Answer 1 · answered Sep 10 '16 at 08:52

11

Apart from locking the file itself, you could create a file used as a lock (a lockfile). There are a bunch of utilities for this, e.g. procmail has lockfile, and in Debian, the liblockfile-bin package has dotlockfile. Procmail should be common enough to be available on most systems.

But basically it comes down to something like:

tempfile=$(mktemp ./lock.XXXX)
lockfile=./lockfile
if ln $tempfile $lockfile ; then
    echo got it
    # do something
    rm $lockfile # after you're done
else 
    echo did not succeed
fi
rm $tempfile

ln will not clobber the target if it already exists, unlike something like echo > file, instead you get an error you can check.

NOTE: if you want to do locks over NFS, use some tool made for that purpose. The semantics of concurrent access to files over NFS are... interesting to say the least.

answered Sep 10 '16 at 08:52

ilkkachu

138,973

This indeed allows atomic lock, but cannot cleanup. If script is interrupted, the lock stays in place and there's no way for an automatic cleanup. One solution would be to write the caller PID in the file (which cannot work long-term as PIDs are eventually reused, but at least makes a stale lock removal possible). – Stéphane Gourichon Sep 09 '19 at 12:27
@StéphaneGourichon, yes, though in Bash you can trap 'rm $lockfile' EXIT, which should remove the lock file on exit unless the shell is somehow prevented from running the exit trap. If that's still an issue, then perhaps the flock` utility mentioned in the linked question should be used. – ilkkachu Sep 09 '19 at 12:45
@Michał Krzysztof Feiler, (I don't know if the ping works) ln -T is a GNU feature, and even with it, the locking won't work if the lock file is a directory instead. Though with -T, ln would give an error, but in any case the lockfile needs to have a known name, and it must be possible to create it. Anyway, when you write edits, there's no need to mark them with "EDIT", just edit the text to include the new information in a natural way. – ilkkachu Oct 09 '19 at 13:33

How to do locking a file from a shell script?

1 Answers1