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copied!<p><em>"When referencing a variable, it is generally advisable to enclose its name in double quotes"</em> -- <a href="http://tldp.org/LDP/abs/html/quotingvar.html" rel="nofollow">http://tldp.org/LDP/abs/html/quotingvar.html</a></p> <p><code>if [ $step -ge 1 ] && [ $step -le 52 ]</code> can be replaced as </p> <p><code>if [ "$step" -ge 1 -a "$step" -le 52 ]</code></p> <p><code>if [[ "$step" -eq 0 ]]</code> can be replaced as <code>if [ "$step" -eq 0 ]</code></p> <p>Also, suppose you have the following script:</p> <pre><code>#!/bin/bash if [ $x -eq 0 ] then echo "hello" fi </code></pre> <p>You get this error when you run the script -- <code>example.sh: line 2: [: -eq: unary operator expected</code></p> <p>But using <code>if [ "$x" -eq 0 ]</code></p> <p>You get a different error when you run the script -- <code>example.sh: line 2: [: : integer expression expected</code></p> <p>Thus, it is always best to put variables inside quotes...</p> <p><code>if [[ .... ]]</code> syntax is particularly useful when you have <code>regex</code> in the condition statement -- <a href="http://honglus.blogspot.com/2010/03/regular-expression-in-condition.html" rel="nofollow">http://honglus.blogspot.com/2010/03/regular-expression-in-condition.html</a></p> <p>EDIT: When we deal with strings --</p> <pre><code>#!/bin/bash if [ $x = "name" ] then echo "hello" fi </code></pre> <p>You get this error when you run the script -- <code>example.sh: line 2: [: =: unary operator expected</code></p> <p>But, if you use <code>if [ "$x" = "name" ]</code> it runs fine (i.e. no errors ) and <code>if</code> statement is evaluated as <code>false</code>, as value of <code>x</code> is <code>null</code> which does not match <code>name</code>. </p>

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