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copied!<p>Let's describe them here.</p> <p>First of all, there are basically 3 different test methods</p> <ul> <li><code>[ EXPRESSION ]</code>, which is exactly the same as <code>test EXPRESSION</code></li> <li><code>[[ EXPRESSION ]]</code></li> <li><code>(( EXPRESSION ))</code>, which is exactly the same as <code>let "EXPRESSION"</code></li> </ul> <p>Let's go into the details:</p> <h1><code>test</code></h1> <p>This is the grandfather of test commands. Even if your shell does not support it, there's still a <code>/usr/bin/test</code> command on virtually every unix system. So calling <code>test</code> will either run the built-in or the binary as a fallback. Enter <code>$ type test</code> to see which version is used. Likewise for <code>[</code>.</p> <p>In most basic cases, this should be sufficient to do your testing.</p> <pre><code>if [ "$a" = test -o "$a" = Test ]; if test "$a" = test -o "$a" = Test; </code></pre> <p>If you need more power, then there's...</p> <h1><code>[[]]</code></h1> <p>This is a bash special. Not every shell needs to support this, and there's no binary fallback. It provides a more powerful comparison engine, notably <a href="http://www.gnu.org/software/bash/manual/bashref.html#Pattern-Matching" rel="nofollow">pattern matching</a> and regular expression matching.</p> <pre><code>if [[ "$a" == [Tt]es? ]]; # pattern if [[ "$a" =~ ^[Tt]es.$ ]]; # RE </code></pre> <h1><code>(())</code></h1> <p>This is a bash special used for arithmetic expressions, and is true if the result of the calculation is non-zero. Not every shell needs to support this, and there's no binary fallback.</p> <pre><code>if (( x * (1 + x++) )); if let "x * (1 + x++)"; </code></pre> <p>Note that you can omit the <code>$</code> sign when referencing variables within <code>(( ... ))</code> or <code>let</code>.</p>

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