How to serialize command execution on linux?

Question

I want to make sure that only one script can be run by any regular user of a system at a time. There can be multiple users logged in and each of them should only be able to run a command after any running commands of other users have finished.

A long time ago on UNIX I was using the batch command with a proper "one task only" queue definition to serialize scripts execution. It solved lot of lock management problems (it needed only a simple timeout to set in the scripts).

Now on Linux the batch command performs differently, one task is run at each minute, tasks run in parallel until 1.5 load average is reached.

I made my own lock management shell library to serialize execution but I wonder if there is a standard command for doing that.

Answer 1

flock is really excellent for this. You can use flock in a wrapper around your shell script, use it on the command line, or incorporate it into your script itself.

The best thing about flock is that while it waits, it doesn't wait in a busy loop.

It also always cleans up the lock when your process exits / flock exits.

Methods based on atomic file/directory creation can get locked out if the process exits without cleaning up (or if there is a kernel panic, or power failure, ...).

With flock, the Linux kernel does the cleanup.

From the manual,

(
flock -s 200  
# ... commands executed under lock ...
) 200>/var/lock/mylockfile

In this form you can wrap a specific block of code in your shell script.

Or you can run it like this,

/usr/bin/flock /tmp/lockfile command

If you don't want to block/wait indefinitely, you can specify a timeout:

-w  --timeout <secs>     wait for a limited amount of time

Or just use a non blocking argument:

-n  --nonblock           fail rather than wait

Answer 2

Maybe what you want here is a hard limit of number of processes your user can run? Make sure you include their shell's process and any other necessary things that will need to be run during their session.

This can be set in /etc/security/limits.conf Add a line like:

testuser hard nproc 2

Note that this will throw an error about not being able to fork if there are too many things trying to run at once. Change 2 to suit your needs and 'testuser' to the user you want to limit.

Answer 3

Use mkdir which is an atomic process to lock other out. Only one at the time kan make a directory:

#!/bin/bash
if ! mkdir /tmp/TheLock 2> /dev/null ; then
    echo Error, user \'$(stat -c %U /tmp/TheLock)\' has the task
else
    echo Yes, I got it\!
    sleep 1 # do stuff
    rmdir /tmp/TheLock
fi

If you run the script first, it will read out:

Yes, I got it!

If somebody try to run it at the same time, the output would be:

Error, user 'Emmanuel' has the task