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    <p>To make locking reliable you need an atomic operation. Many of the above proposals are not atomic. The proposed lockfile(1) utility looks promising as the man-page mentioned, that its "NFS-resistant". If your OS does not support lockfile(1) and your solution has to work on NFS, you have not many options....</p> <p>NFSv2 has two atomic operations:</p> <ul> <li>symlink</li> <li>rename</li> </ul> <p>With NFSv3 the create call is also atomic.</p> <p>Directory operations are NOT atomic under NFSv2 and NFSv3 (please refer to the book 'NFS Illustrated' by Brent Callaghan, ISBN 0-201-32570-5; Brent is a NFS-veteran at Sun).</p> <p>Knowing this, you can implement spin-locks for files and directories (in shell, not PHP):</p> <p>lock current dir:</p> <pre><code>while ! ln -s . lock; do :; done </code></pre> <p>lock a file:</p> <pre><code>while ! ln -s ${f} ${f}.lock; do :; done </code></pre> <p>unlock current dir (assumption, the running process really acquired the lock):</p> <pre><code>mv lock deleteme &amp;&amp; rm deleteme </code></pre> <p>unlock a file (assumption, the running process really acquired the lock):</p> <pre><code>mv ${f}.lock ${f}.deleteme &amp;&amp; rm ${f}.deleteme </code></pre> <p>Remove is also not atomic, therefore first the rename (which is atomic) and then the remove.</p> <p>For the symlink and rename calls, both filenames have to reside on the same filesystem. My proposal: use only simple filenames (no paths) and put file and lock into the same directory.</p>
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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. COWhich pages of NFS Illustrated support the statement that mkdir is not atomic over NFS?
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      1. USmaxschlepzig
    2. COThnks for this technique. A shell mutex implementation is available in my new shell lib : https://github.com/Offirmo/offirmo-shell-lib, see "mutex". It uses `lockfile` if available, or fallback to this `symlink` method if not.
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    3. CONice. Unfortunately this method does not provide a way to automatically delete stale locks.
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      1. USRichard Hansen
 

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