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copied!<p>The following solution can also be used in basic shells such as Bourne without the need for regular expressions. Basically any numeric value evaluation operations using non-numbers will result in an error which will be implicitly considered as false in shell:</p> <pre><code>"$var" -eq "$var" </code></pre> <p>as in:</p> <pre><code>#!/bin/bash var=a if [ "$var" -eq "$var" ] 2>/dev/null; then echo number else echo not a number fi </code></pre> <p>You can can also test for $? the return code of the operation which is more explicit:</p> <pre><code>"$var" -eq "$var" 2>/dev/null if [ $? -ne 0 ]; then echo $var is not number fi </code></pre> <p>Redirection of standard error is there to hide the "integer expression expected" message that bash prints out in case we do not have a number.</p> <p><strong>CAVEATS</strong> (thanks to the comments below):</p> <ul> <li>Numbers with decimal points are <em>not</em> identified as valid "numbers"</li> <li>Using <code>[[ ]]</code> instead of <code>[ ]</code> will always evaluate to <code>true</code></li> <li>Most non-Bash shells will always evaluate this expression as <code>true</code></li> <li>The behavior in Bash is undocumented and may therefore change without warning</li> <li>If the value includes spaces after the number (e.g. "1 a") produces error, like <code>bash: [[: 1 a: syntax error in expression (error token is "a")</code></li> <li>If the value is the same as var-name (e.g. i="i"), produces error, like <code>bash: [[: i: expression recursion level exceeded (error token is "i")</code></li> </ul>

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