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66 lines
2.8 KiB
Plaintext
66 lines
2.8 KiB
Plaintext
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====== The shift builtin command ======
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===== Synopsis =====
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<code>shift [n]</code>
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===== Description =====
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The ''shift'' builtin command is used to "shift" the positional 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 changed.
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The very first positional parameter is discarded, the second becomes the first one, etc.
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Imagine the following set of positional parameters (''$1'' to ''$4''):
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^1 |This |
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^2 |is |
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^3 |a |
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^4 |test |
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When you use ''shift 1'', they will be changed to:
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^1 |is |
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^2 |a |
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^3 |test |
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The [[syntax:shellvars#special_parameters|special parameter]] ''$#'' will reflect the final number of positional parameters.
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If the number given is 0, no changes are made to the positional 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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|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 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>
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$ 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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</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'
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no args
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</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:
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<code> $ mksh -c 'f() { if ! ${1+false} && shift; then echo "$1"; else echo "no args"; fi; }; f'
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no args
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</code>
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<del>The mksh maintainer refuses to change either the ''shift'' or ''command'' builtins.</del> [[https://github.com/MirBSD/mksh/commit/996e05548ab82f7ef2dea61f109cc7b6d13837fa | Fixed]]. (Thanks!)
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* 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'
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(no output)
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</code> The above mksh workaround will work in this case too.
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===== See also =====
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