Sometimes we need a single instance of a script to run at a time. Meaning, the script itself should detects whether any instances of himself are still running and act accordingly.

When multiple instances of one script running, it’s easy to cause problems. I’ve ever seen that about 350 instances of a status checking script running there without doing anything, but eat lots of system resource.


It's simple to implement it in Bash Shell:

# This is to examine whether the lockfile existed
[ -f "${0}.lock" ] && exit -1
# Create the lock file
lockfile ${0}.lock
sleep 40
# Release the lock file manually
rm -f ${0}.lock

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# This is to examine whether the lockfile existed
[ -f "${0}.lock" ] && exit -1
# Create the lock file
lockfile ${0}.lock
sleep 40
# Release the lock file manually
rm -f ${0}.lock

lockfile /tmp/lock; sleep 10; rm -f /tmp/lock

lockfile /tmp/lock && echo "Finaly, I'am running..."; rm -f /tmp/lock

lockfile -r0 /tmp/lock && echo "Finaly, I'am running..."; rm -f /tmp/lock

(Hi, Mod, thanks for your note, I'd pay attention next time)

Thanks, Ripat, nice examples, so the code can be eased like this

# Create the lock file
lockfile -r0 ${0}.lock && {
     # Your code goes here!
}
# Release the lock file manually
rm -f ${0}.lock

lock_on()
{
    local f=$1
    local freefd=6  ## do not use fd 5

    ## make sure the file be there
    mkdir -p "$( dirname $f )"
    touch "$f"

    ## find a free fd
    while (( freefd <= 9 )); do
        [[ -L /dev/fd/$freefd ]] || break
        (( freefd++ ))
    done

    (( freefd == 10 )) && return 1

    ## open the lock file
    eval "exec $freefd< \"$f\""
}

is_locked()
{
    local f=$1

    fuser "$f" &> /dev/null
}

lock="/tmp/.$( basename $0 ).lock"
is_locked "$lock" && exit 1
lock_on "$lock"
## do something here