Is there any mutex/semaphore mechanism in shell scripts?

See BashFAQ and ProcessManagment for discussions on file locking in Bash.

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There are numerous questions/answers on this topic posted here already on S.O.

I hope this helps.

The BashFAQ noted by shellter has some good examples. The basic idea, which I'm moving here so the page is self-contained, is to use an operation that both tests and sets at the same time: mkdir

mkdir will fail if the directory exists and will make it if it does not. It's an atomic operation and you can use it like so to do a mutex in your shell script (from the above BashFAQ)

# Bourne
lockdir=/tmp/myscript.lock
if mkdir "$lockdir"
then    # directory did not exist, but was created successfully
    echo >&2 "successfully acquired lock: $lockdir"
    # continue script
else    # failed to create the directory, presumably because it already exists
  echo >&2 "cannot acquire lock, giving up on $lockdir"
  exit 0
fi

follow the link for more detail on cleanup and other items.

You can use the flock utility to lock a file / use it as a mutex.

Example:

#!/bin/sh -eu
#advanced bash stuff not needed
: >> lock #create a file if it doesn't exist
{
flock 3 #lock file by filedescriptor

echo $$ working with lock
sleep 2
echo $$ done with lock

} 3<lock

Example usage:

./mx & ./mx & ./mx & #will run one at a time cuz of the lock

(

In reply to massimo's point:

If you don't want to hardcode a filedecriptor number (it should rarely be a problem if you aren't hardcoding 0, 1, or 2, but anyway), then in bash (but not in a POSIX only shell) you can have the system pick a fd for you with:

{
flock $fd
#...
} {fd}<lock

)

You will want to prevent constant polling, and use an interruption like mechanism instead.

For that use a file in memory (run directory), and wait it to be changed by another process:

mutex="/run/user/$(id -u)/mutex"

waitMutex () {
    tail --follow --lines=0 "${mutex}" |
    head -n1 >/dev/null
    echo > "${mutex}"
}