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    <p>Another option is to use shell's <code>noclobber</code> option by running <code>set -C</code>. Then <code>&gt;</code> will fail if the file already exists.</p> <p>In brief:</p> <pre><code>set -C lockfile="/tmp/locktest.lock" if echo "$$" &gt; "$lockfile"; then echo "Successfully acquired lock" # do work rm "$lockfile" # XXX or via trap - see below else echo "Cannot acquire lock - already locked by $(cat "$lockfile")" fi </code></pre> <p>This causes the shell to call:</p> <pre><code>open(pathname, O_CREAT|O_EXCL) </code></pre> <p>which atomically creates the file or fails if the file already exists.</p> <hr> <p>According to a comment on <a href="http://mywiki.wooledge.org/BashFAQ/045" rel="noreferrer">BashFAQ 045</a>, this may fail in <code>ksh88</code>, but it works in all my shells:</p> <pre><code>$ strace -e trace=creat,open -f /bin/bash /home/mikel/bin/testopen 2&gt;&amp;1 | grep -F testopen.lock open("/tmp/testopen.lock", O_WRONLY|O_CREAT|O_EXCL|O_LARGEFILE, 0666) = 3 $ strace -e trace=creat,open -f /bin/zsh /home/mikel/bin/testopen 2&gt;&amp;1 | grep -F testopen.lock open("/tmp/testopen.lock", O_WRONLY|O_CREAT|O_EXCL|O_NOCTTY|O_LARGEFILE, 0666) = 3 $ strace -e trace=creat,open -f /bin/pdksh /home/mikel/bin/testopen 2&gt;&amp;1 | grep -F testopen.lock open("/tmp/testopen.lock", O_WRONLY|O_CREAT|O_EXCL|O_TRUNC|O_LARGEFILE, 0666) = 3 $ strace -e trace=creat,open -f /bin/dash /home/mikel/bin/testopen 2&gt;&amp;1 | grep -F testopen.lock open("/tmp/testopen.lock", O_WRONLY|O_CREAT|O_EXCL|O_LARGEFILE, 0666) = 3 </code></pre> <p>Interesting that <code>pdksh</code> adds the <code>O_TRUNC</code> flag, but obviously it's redundant:<br> either you're creating an empty file, or you're not doing anything.</p> <hr> <p>How you do the <code>rm</code> depends on how you want unclean exits to be handled.</p> <p><strong>Delete on clean exit</strong></p> <p>New runs fail until the issue that caused the unclean exit to be resolved and the lockfile is manually removed.</p> <pre><code># acquire lock # do work (code here may call exit, etc.) rm "$lockfile" </code></pre> <p><strong>Delete on any exit</strong></p> <p>New runs succeed provided the script is not already running.</p> <pre><code>trap 'rm "$lockfile"' EXIT </code></pre>
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    1. POQuick-and-dirty way to ensure only one instance of a shell script is running at a time
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    1. COVery novel approach... this appears to be one way to accomplish atomicity using a lock file rather than a lock directory.
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      1. USMatt Caldwell
 

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