Is there any mutex/semaphore mechanism in shell scripts?

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.