Is there any mutex/semaphore mechanism in shell scripts?

Question

I\'m looking for mutex/semaphore/concurrency mechanism in shell script. Consider following situation: Unless \"a\" user does not close the shared file, \"b\" user should no

Answer 1

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

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}

)