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Locking.md

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This cheat sheet is about file locking techniques.

Filesystem Locking Variants

Solutions that lock files using the filesystem

flock

Note: that you have to create the lock file first!

flock /tmp/myapp.lock <some command>
flock -w 10 /tmp/myapp.lock <some command>

lockfile-*

lockfile-create /tmp/myapp.lock
lockfile-touch  /tmp/myapp.lock
lockfile-remove /tmp/myapp.lock

setlock

setlock -nX lockfile /tmp/myapp.lock

Semaphore Locking Variants

Solutions that more or less safely aquire a lock file

lockfile

lockfile /tmp/myapp.lock
lockfile -15 -r 3 /tmp/myapp.lock     # wait 15s, 3 retries

Pure Shell Locking

Source: https://wiki.bash-hackers.org/howto/mutex

if ( set -o noclobber; echo "locked" > "$lockfile") 2> /dev/null; then
  trap 'rm -f "$lockfile"; exit $?' INT TERM EXIT
  echo "Locking succeeded" >&2
  rm -f "$lockfile"
else
  echo "Lock failed - exit" >&2
  exit 1
fi

Cron job locking

How to ensure cron job runs do not overlap: Use flock -w 0 <lockfile> <cmd>

*/10 * * * * flock -w 0 /tmp/myscript.lock ~/bin/myscript.sh

Another good alternative might be to rely on systemd timer units.

How to find blocking locks

lslocks will give an output listing all active locks:

~ $ lslocks
COMMAND           PID  TYPE  SIZE MODE  M      START        END PATH
thermald          485 POSIX       WRITE 0          0          0 /run...
containerd        573 FLOCK       WRITE 0          0          0 /...
tracker-miner-f  1245 POSIX 16,2M READ  0 1073741826 1073742335 /data/lars/.cache/tracker/meta.
tracker-miner-f  1245 POSIX   32K READ  0        128        128 /data/lars/.cache/tracker/meta.
firefox         21560 POSIX  160K WRITE 0 1073741826 1073742335 /data/lars/.mozilla/firefox/dzr
[...]