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83 lines
2.0 KiB
Plaintext
83 lines
2.0 KiB
Plaintext
http://www.opengroup.org/onlinepubs/9699919799/
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Open Group Base Specifications Issue 7
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http://www.opengroup.org/onlinepubs/9699919799/utilities/V3_chap01.html
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Shell & Utilities
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It says that any of the standard utilities may be implemented
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as a regular shell built-in. It gives a list of utilities which
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are usually implemented that way (and some of them can only
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be implemented as built-ins, like "alias"):
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alias
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bg
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cd
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command
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false
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fc
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fg
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getopts
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jobs
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kill
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newgrp
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pwd
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read
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true
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umask
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unalias
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wait
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http://www.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html
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Shell Command Language
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It says that shell must implement special built-ins. Special built-ins
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differ from regular ones by the fact that variable assignments
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done on special builtin are *PRESERVED*. That is,
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VAR=VAL special_builtin; echo $VAR
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should print VAL.
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(Another distinction is that an error in special built-in should
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abort the shell, but this is not such a critical difference,
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and moreover, at least bash's "set" does not follow this rule,
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which is even codified in autoconf configure logic now...)
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List of special builtins:
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. file
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: [argument...]
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break [n]
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continue [n]
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eval [argument...]
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exec [command [argument...]]
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exit [n]
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export name[=word]...
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export -p
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readonly name[=word]...
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readonly -p
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return [n]
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set [-abCefhmnuvx] [-o option] [argument...]
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set [+abCefhmnuvx] [+o option] [argument...]
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set -- [argument...]
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set -o
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set +o
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shift [n]
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times
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trap n [condition...]
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trap [action condition...]
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unset [-fv] name...
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In practice, no one uses this obscure feature - none of these builtins
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gives any special reasons to play such dirty tricks.
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However. This section also says that *function invocation* should act
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similar to special built-in. That is, variable assignments
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done on function invocation should be preserved after function invocation.
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This is significant: it is not unthinkable to want to run a function
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with some variables set to special values. But because of the above,
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it does not work: variable will "leak" out of the function.
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