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(Limitations of Builtins): Explain why logical directory names are

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generally preferable to physical names.
This commit is contained in:
Paul Eggert 2002-09-02 07:46:31 +00:00
parent 1f064c63b6
commit dcbe26b84a
1 changed files with 48 additions and 13 deletions
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@ -9218,25 +9218,26 @@ You can't use @command{!}, you'll have to rewrite your code.
The use of @samp{break 2}, etcetera, is safe.
@item @command{cd} and @command{pwd}
@item @command{cd}
@c ---------------------------------
@prindex @command{cd}
@prindex @command{pwd}
@acronym{POSIX} 1003.1-2001 requires that @command{cd} and
@command{pwd} must support the @option{-L} and @option{-P} options,
@acronym{POSIX} 1003.1-2001 requires that @command{cd} must support
the @option{-L} (``logical'') and @option{-P} (``physical'') options,
with @option{-L} being the default. However, traditional shells do
not support these options, and their @command{cd} and @command{pwd}
commands have the @option{-P} behavior.
not support these options, and their @command{cd} command has the
@option{-P} behavior.
Portable scripts should assume neither option is supported, and should
assume neither behavior is the default. This can be a bit tricky,
since the @acronym{POSIX} default behavior means that, for example,
@samp{ls ..} and @samp{cd ..} may refer to different directories. It
is safe to use @command{cd @var{dir}} if @var{dir} contains no
@file{..} components. Also, Autoconf-generated scripts check for this
problem when computing variables like @code{ac_top_srcdir}
(@pxref{Configuration Actions}), so it is safe to @command{cd} to
these variables.
@samp{ls ..} and @samp{cd ..} may refer to different directories if
the current logical directory is a symbolic link. It is safe to use
@command{cd @var{dir}} if @var{dir} contains no @file{..} components.
Also, Autoconf-generated scripts check for this problem when computing
variables like @code{ac_top_srcdir} (@pxref{Configuration Actions}),
so it is safe to @command{cd} to these variables.
Also please see the discussion of the @command{pwd} command.
@item @command{case}
@ -9466,7 +9467,41 @@ fi
@item @command{pwd}
@c ----------------
See @command{cd} above.
@prindex @command{pwd}
With modern shells, plain @command{pwd} outputs a ``logical''
directory name, some of whose components may be symbolic links. These
directory names are in contrast to ``physical'' directory names, whose
components are all directories.
@acronym{POSIX} 1003.1-2001 requires that @command{pwd} must support
the @option{-L} (``logical'') and @option{-P} (``physical'') options,
with @option{-L} being the default. However, traditional shells do
not support these options, and their @command{pwd} command has the
@option{-P} behavior.
Portable scripts should assume neither option is supported, and should
assume neither behavior is the default. Also, on many hosts
@samp{/bin/pwd} is equivalent to @samp{pwd -P}, but @acronym{POSIX}
does not require this behavior and portable scripts should not rely on
it.
Typically it's best to use plain @command{pwd}. On modern hosts this
outputs logical directory names, which have the following advantages:
@itemize @bullet
@item
Logical names are what the user specified.
@item
Physical names may not be portable from one installation
host to another due to network filesystem gymnastics.
@item
On modern hosts @samp{pwd -P} may fail due to lack of permissions to
some parent directory, but plain @command{pwd} cannot fail for this
reason.
@end itemize
Also please see the discussion of the @command{cd} command.
@item @command{set}
@c ----------------