mirror of
https://github.com/rawiriblundell/wiki.bash-hackers.org
synced 2024-12-25 22:20:39 +01:00
66 lines
2.9 KiB
Markdown
66 lines
2.9 KiB
Markdown
====== The shift builtin command ======
|
|
|
|
===== Synopsis =====
|
|
<code>shift [n]</code>
|
|
|
|
===== Description =====
|
|
|
|
The ''shift'' builtin command is used to "shift" the positional parameters by the given number ''n'' or by 1, if no number is given.
|
|
|
|
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.
|
|
|
|
Imagine the following set of positional parameters (''$1'' to ''$4''):
|
|
|
|
^1 |This |
|
|
^2 |is |
|
|
^3 |a |
|
|
^4 |test |
|
|
|
|
When you use ''shift 1'', they will be changed to:
|
|
|
|
^1 |is |
|
|
^2 |a |
|
|
^3 |test |
|
|
|
|
The [[syntax:shellvars#special_parameters|special parameter]] ''$#'' will reflect the final number of positional parameters.
|
|
|
|
If the number given is 0, no changes are made to the positional parameters.
|
|
|
|
|
|
==== Options ====
|
|
|
|
There are no options.
|
|
|
|
==== Return status ====
|
|
|
|
^Status ^Reason ^
|
|
|0 |no error |
|
|
|1 |non-numeric argument |
|
|
|1 |given number (or the default 1) is bigger than the number of actually present positional parameters |
|
|
|1 |given number is negative |
|
|
|
|
|
|
===== Examples =====
|
|
|
|
===== Portability considerations =====
|
|
|
|
* The ''shift'' builtin command is specified by POSIX(r).
|
|
* Many shells will throw a fatal error when attempting to ''shift'' more than the number of positional parameters. **POSIX does not require that behavior**. Bash (even in POSIX mode) and Zsh return 1 when there are no args, and no error output is produced unless the [[internals/shell_options#shift_verbose | shift_verbose]] [[commands/builtin/shopt | shopt]] option is enabled. Ksh93, pdksh, posh, mksh, and dash, all throw useless fatal shell errors.<code>
|
|
$ dash -c 'f() { if shift; then echo "$1"; else echo "no args"; fi; }; f'
|
|
dash: 1: shift: can't shift that many
|
|
</code> In most shells, you can work around this problem using the [[commands:builtin:command | command]] builtin to suppress fatal errors caused by //special builtins//. <code> $ dash -c 'f() { if command shift 2>/dev/null; then echo "$1"; else echo "no args"; fi; }; f'
|
|
no args
|
|
</code> 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:
|
|
|
|
<code> $ mksh -c 'f() { if ! ${1+false} && shift; then echo "$1"; else echo "no args"; fi; }; f'
|
|
no args
|
|
</code>
|
|
<del>The mksh maintainer refuses to change either the ''shift'' or ''command'' builtins.</del> [[https://github.com/MirBSD/mksh/commit/996e05548ab82f7ef2dea61f109cc7b6d13837fa | Fixed]]. (Thanks!)
|
|
* Perhaps almost as bad as the above, busybox sh's ''shift'' always returns success, even when attempting to shift beyond the final argument. <code> $ bb -c 'f() { if shift; then echo "$1"; else echo "no args"; fi; }; f'
|
|
(no output)
|
|
</code> The above mksh workaround will work in this case too.
|
|
|
|
===== See also =====
|
|
|