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

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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.

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