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</code></pre></div> <h2 id=description>Description<a class=headerlink href=#description title="Permanent link">¶</a></h2> <p>The <code>shift</code> builtin command is used to "shift" the positional parameters by the given number <code>n</code> or by 1, if no number is given.</p> <p>This means, the number and the position of the positional parameters are changed. The very first positional parameter is discarded, the second becomes the first one, etc.</p> <p>Imagine the following set of positional parameters (<code>$1</code> to <code>$4</code>):</p> <table> <thead> <tr> <th>Positional Parameter</th> <th>Value</th> </tr> </thead> <tbody> <tr> <td>1</td> <td>This</td> </tr> <tr> <td>2</td> <td>is</td> </tr> <tr> <td>3</td> <td>a</td> </tr> <tr> <td>4</td> <td>test</td> </tr> </tbody> </table> <p>When you use <code>shift 1</code>, they will be changed to:</p> <table> <thead> <tr> <th>Positional Parameter</th> <th>Value</th> </tr> </thead> <tbody> <tr> <td>1</td> <td>is</td> </tr> <tr> <td>2</td> <td>a</td> </tr> <tr> <td>3</td> <td>test</td> </tr> </tbody> </table> <p>The <a href=../../../syntax/shellvars/#special_parameters>special parameter</a> <code>$#</code> will reflect the final number of positional parameters.</p> <p>If the number given is 0, no changes are made to the positional parameters.</p> <h3 id=options>Options<a class=headerlink href=#options title="Permanent link">¶</a></h3> <p>There are no options.</p> <h3 id=return-status>Return status<a class=headerlink href=#return-status title="Permanent link">¶</a></h3> <table> <thead> <tr> <th>Status</th> <th>Reason</th> </tr> </thead> <tbody> <tr> <td>0</td> <td>no error</td> </tr> <tr> <td>1</td> <td>non-numeric argument</td> </tr> <tr> <td>1</td> <td>given number (or the default 1) is bigger than the number of actually present positional parameters</td> </tr> <tr> <td>1</td> <td>given number is negative</td> </tr> </tbody> </table> <h2 id=examples>Examples<a class=headerlink href=#examples title="Permanent link">¶</a></h2> <h2 id=portability-considerations>Portability considerations<a class=headerlink href=#portability-considerations title="Permanent link">¶</a></h2> <ul> <li>The <code>shift</code> builtin command is specified by POSIX®.</li> <li> <p>Many shells will throw a fatal error when attempting to <code>shift</code> more than the number of positional parameters. <strong>POSIX does not require that behavior</strong>. Bash (even in POSIX mode) and Zsh return 1 when there are no args, and no error output is produced unless the <a href=../../../internals/shell_options/#shift_verbose>shift_verbose</a> <a href=../shopt/ >shopt</a> option is enabled. Ksh93, pdksh, posh, mksh, and dash, all throw useless fatal shell errors. <div class=highlight><pre><span></span><code><a id=__codelineno-0-1 name=__codelineno-0-1 href=#__codelineno-0-1></a>$ dash -c 'f() { if shift; then echo "$1"; else echo "no args"; fi; }; f'
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<a id=__codelineno-0-2 name=__codelineno-0-2 href=#__codelineno-0-2></a> dash: 1: shift: can't shift that many
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</code></pre></div> In most shells, you can work around this problem using the <code>command</code> builtin to suppress fatal errors caused by <em>special builtins</em>. <div class=highlight><pre><span></span><code><a id=__codelineno-1-1 name=__codelineno-1-1 href=#__codelineno-1-1></a>$ dash -c 'f() { if command shift 2>/dev/null; then echo "$1"; else echo "no args"; fi; }; f'
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<a id=__codelineno-1-2 name=__codelineno-1-2 href=#__codelineno-1-2></a>no args
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</code></pre></div> While, POSIX requires this behavior, it isn't very obvious and some shells don't do it correctly. To work around this, you can use something like: <div class=highlight><pre><span></span><code><a id=__codelineno-2-1 name=__codelineno-2-1 href=#__codelineno-2-1></a>$ mksh -c 'f() { if ! ${1+false} && shift; then echo "$1"; else echo "no args"; fi; }; f'
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<a id=__codelineno-2-2 name=__codelineno-2-2 href=#__codelineno-2-2></a>no args
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</code></pre></div> <del>The mksh maintainer refuses to change either the <code>shift</code> or <code>command</code> builtins.</del> <a href=https://github.com/MirBSD/mksh/commit/996e05548ab82f7ef2dea61f109cc7b6d13837fa>Fixed</a>. (Thanks!)</p> </li> <li> <p>Perhaps almost as bad as the above, busybox sh's <code>shift</code> always returns success, even when attempting to shift beyond the final argument. <div class=highlight><pre><span></span><code><a id=__codelineno-3-1 name=__codelineno-3-1 href=#__codelineno-3-1></a>$ bb -c 'f() { if shift; then echo "$1"; else echo "no args"; fi; }; f'
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<a id=__codelineno-3-2 name=__codelineno-3-2 href=#__codelineno-3-2></a>(no output)
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</code></pre></div> The above mksh workaround will work in this case too.</p> </li> </ul> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date">November 7, 2024</span> </span> <span class=md-source-file__fact> <span class=md-icon title=Created> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M14.47 15.08 11 13V7h1.5v5.25l3.08 1.83c-.41.28-.79.62-1.11 1m-1.39 4.84c-.36.05-.71.08-1.08.08-4.42 0-8-3.58-8-8s3.58-8 8-8 8 3.58 8 8c0 .37-.03.72-.08 1.08.69.1 1.33.32 1.92.64.1-.56.16-1.13.16-1.72 0-5.5-4.5-10-10-10S2 6.5 2 12s4.47 10 10 10c.59 0 1.16-.06 1.72-.16-.32-.59-.54-1.23-.64-1.92M18 15v3h-3v2h3v3h2v-3h3v-2h-3v-3z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date">November 7, 2024</span> </span> </aside> <h2 id=__comments>Comments</h2> <script src=https://giscus.app/client.js data-repo=flokoe/bash-hackers-wiki data-repo-id=R_kgDOJ3Nr6Q data-category="Giscus Page Comments" data-category-id=DIC_kwDOJ3Nr6c4CXq9t data-mapping=pathname data-strict=1 data-reactions-enabled=1 data-emit-metadata=0 data-input-position=top data-theme=preferred_color_scheme data-lang=en data-loading=lazy crossorigin=anonymous async>
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