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    <p>Advisory locking has been used for ages and it can be used in bash scripts. I prefer simple <code>flock</code> (from <code>util-linux[-ng]</code>) over <code>lockfile</code> (from <code>procmail</code>). And always remember about a trap on exit (sigspec == <code>EXIT</code> or <code>0</code>, trapping specific signals is superfluous) in those scripts.</p> <p>In 2009 I released my lockable script boilerplate (originally available at my wiki page, nowadays available as <a href="https://gist.github.com/571091" rel="noreferrer">gist</a>). 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.</p> <p>Here is the mentioned boilerplate for your convenience.</p> <pre><code>#!/bin/bash # SPDX-License-Identifier: MIT ## Copyright (C) 2009 Przemyslaw Pawelczyk &lt;przemoc@gmail.com&gt; ## ## 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 &amp;&amp; rm -f $LOCKFILE; } _prepare_locking() { eval "exec $LOCKFD&gt;\"$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. </code></pre>
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    1. POWhat is the best way to ensure only one instance of a Bash script is running?
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    1. CO+1 for a better example than the man page
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      1. USFelipe Alvarez
    2. CO+1 for comprehensive boilerplate (and for not having to install procmail)
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    3. COExcellent script; is there a way within this framework to simply check if the lock exists rather than always obtaining a lock when doing so?
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      1. USCarlos P
 

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