What is the best way to ensure only one instance of a Bash script is running?

Question

What is the simplest/best way to ensure only one instance of a given script is running - assuming it's Bash on Linux?

At the moment I'm doing:

ps -C script.name.sh > /dev/null 2>&1 || ./script.name.sh

but it has several issues:

1. it puts the check outside of script
2. it doesn't let me run the same script from separate accounts - which I would like sometimes.
3. -C checks only first 14 characters of process name

Of course, I can write my own pidfile handling, but I sense that there should be a simple way to do it.

Answer 1

Advisory locking has been used for ages and it can be used in bash scripts. I prefer simple flock (from util-linux[-ng]) over lockfile (from procmail). And always remember about a trap on exit (sigspec == EXIT or 0, trapping specific signals is superfluous) in those scripts.

In 2009 I released my lockable script boilerplate (originally available at my wiki page, nowadays available as gist). Transforming that into one-instance-per-user is trivial. Using it you can also easily write scripts for other scenarios requiring some locking or synchronization.

Here is the mentioned boilerplate for your convenience.

#!/bin/bash
# SPDX-License-Identifier: MIT

## Copyright (C) 2009 Przemyslaw Pawelczyk <[email protected]>
##
## This script is licensed under the terms of the MIT license.
## https://opensource.org/licenses/MIT
#
# Lockable script boilerplate

### HEADER ###

LOCKFILE="/var/lock/`basename $0`"
LOCKFD=99

# PRIVATE
_lock()             { flock -$1 $LOCKFD; }
_no_more_locking()  { _lock u; _lock xn && rm -f $LOCKFILE; }
_prepare_locking()  { eval "exec $LOCKFD>\"$LOCKFILE\""; trap _no_more_locking EXIT; }

# ON START
_prepare_locking

# PUBLIC
exlock_now()        { _lock xn; }  # obtain an exclusive lock immediately or fail
exlock()            { _lock x; }   # obtain an exclusive lock
shlock()            { _lock s; }   # obtain a shared lock
unlock()            { _lock u; }   # drop a lock

### BEGIN OF SCRIPT ###

# Simplest example is avoiding running multiple instances of script.
exlock_now || exit 1

# Remember! Lock file is removed when one of the scripts exits and it is
#           the only script holding the lock or lock is not acquired at all.