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copied!<p>From the bash4 manpage on Debian:</p> <pre><code>((expression)) The expression is evaluated according to the rules described below under ARITHMETIC EVALUATION. If the value of the expres‐ sion is non-zero, the return status is 0; otherwise the return status is 1. This is exactly equivalent to let "expression". </code></pre> <p>and also ...</p> <pre><code>-e Exit immediately if a pipeline (which may consist of a single simple command), a subshell command enclosed in parentheses, or one of the commands executed as part of a command list enclosed by braces (see SHELL GRAMMAR above) exits with a non-zero status. </code></pre> <p>So what is happening is <code>((var++))</code> increments var from 0 to 1 and returns 0, causing the overall expression to return non-zero, which triggers <code>errexit</code>.</p> <p>Now for the difference between the two different bash versions: this change in <code>((</code> behavior seems to have occurred between 4.0 and 4.1. In 4.0 <code>((</code> apparently did not trigger errexit. See this <a href="http://tiswww.case.edu/php/chet/bash/NEWS">NEWS</a> file for the details. You'll have to scroll down to line 135 or so. The Changelog from the source distribution seems to confirm this.</p> <p>If you just want a variable incremented without using the exit status, there's multiple ways to do it. Maybe some other people could give advice on which is the best, but some possibilities are:</p> <ul> <li><code>var="$((var+1))"</code>, the portable POSIX <code>sh</code> method</li> <li><code>((var++)) || true</code>, forcing the statement to always have a zero exit status (bash only)</li> </ul>

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