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89 lines
2.9 KiB
Markdown
89 lines
2.9 KiB
Markdown
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# The shift builtin command
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## Synopsis
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shift [n]
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## Description
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The `shift` builtin command is used to \"shift\" the positional
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parameters by the given number `n` or by 1, if no number is given.
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This means, the number and the position of the positional parameters are
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changed. The very first positional parameter is discarded, the second
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becomes the first one, etc.
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Imagine the following set of positional parameters (`$1` to `$4`):
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1 This
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--- ------
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2 is
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3 a
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4 test
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When you use `shift 1`, they will be changed to:
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1 is
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--- ------
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2 a
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3 test
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The [special parameter](/syntax/shellvars#special_parameters) `$#` will
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reflect the final number of positional parameters.
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If the number given is 0, no changes are made to the positional
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parameters.
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### Options
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There are no options.
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### Return status
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Status Reason
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-------- -----------------------------------------------------------------------------------------------------
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0 no error
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1 non-numeric argument
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1 given number (or the default 1) is bigger than the number of actually present positional parameters
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1 given number is negative
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## Examples
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## Portability considerations
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- The `shift` builtin command is specified by POSIX(r).
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- Many shells will throw a fatal error when attempting to `shift` more
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than the number of positional parameters. **POSIX does not require
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that behavior**. Bash (even in POSIX mode) and Zsh return 1 when
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there are no args, and no error output is produced unless the
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[shift_verbose](internals/shell_options#shift_verbose)
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[shopt](commands/builtin/shopt) option is enabled. Ksh93, pdksh,
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posh, mksh, and dash, all throw useless fatal shell
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errors.`$ dash -c 'f() { if shift; then echo "$1"; else echo "no args"; fi; }; f'
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dash: 1: shift: can't shift that many
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` In most shells, you can work around this problem using the
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[command](/commands/builtin/command) builtin to suppress fatal
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errors caused by *special builtins*. \<code\> \$ dash -c \'f() { if
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command shift 2\>/dev/null; then echo \"\$1\"; else echo \"no
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args\"; fi; }; f\'
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no args \</code\> While, POSIX requires this behavior, it isn\'t very
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obvious and some shells don\'t do it correctly. To work around this, you
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can use something like:
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\<code\> \$ mksh -c \'f() { if ! \${1+false} && shift; then echo
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\"\$1\"; else echo \"no args\"; fi; }; f\' no args \</code\> ~~The mksh
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maintainer refuses to change either the `shift` or `command` builtins.~~
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[Fixed](https://github.com/MirBSD/mksh/commit/996e05548ab82f7ef2dea61f109cc7b6d13837fa).
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(Thanks!)
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- Perhaps almost as bad as the above, busybox sh\'s `shift` always
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returns success, even when attempting to shift beyond the final
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argument. \<code\> \$ bb -c \'f() { if shift; then echo \"\$1\";
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else echo \"no args\"; fi; }; f\'
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(no output) \</code\> The above mksh workaround will work in this case
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too.
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## See also
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