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Moving Around in a Hurry
Next: 14.14 Automatic Setup When You Enter/Exit a Directory
 

14.13 Which Directory Am I in, Really?

The C shell, and some other shells too, keep their own idea of what your current directory is. The csh will give you the current directory's absolute pathname in $cwd ; bash uses $PWD . But sometimes this can give you the wrong pathname.

Why? Because the cwd variable was added before many versions of UNIX had symlinks ( 18.4 ) (symbolic links). As article 18.7 explains, symlinks can point to directories any place else on the filesystem or even (for some UNIXes) directories on another computer. Poor cwd couldn't cope: it assumed that the current directory was the name of the symlink itself (instead of the directory that the link points to). That led to problems like the one below: cd ing to a "directory" named wpa that's actually a symlink to /work/pwrtools/articles . The value of $cwd , shown in the prompt, is wrong. The /bin/pwd command shows the real current directory ( 14.4 ) (you should type all of /bin/pwd because some shells and users have plain pwd aliased to do echo   $cwd ):

/home/jerry% 

pwd


/home/jerry% 

ls -l wpa


lrwxrwxrwx  1 jerry  23 Sep 8 13:55 wpa -> /work/pwrtools/articles
/home/jerry% 

cd wpa


/home/jerry/wpa% 

/bin/pwd


/work/pwrtools/articles
/home/jerry/wpa%

By now, a lot of C shells have a variable named hardpaths ; the bash variable is nolinks . If you set the shell variable (usually in your shell setup file ( 2.2 ) ), the shell won't be fooled by symlinks. Watch:

/home/jerry/wpa% 

cd


/home/jerry% 

set hardpaths

      
(on bash, 

nolinks=
1
)


/home/jerry% 

cd wpa


/work/pwrtools/articles%

Setting hardpaths or nolinks makes the shell do extra work, so don't bother with it unless you use $cwd .

The dirs ( 14.6 ) command has the same problem. Setting hardpaths or nolinks helps there, too.

If your system has symlinks but your shell doesn't recognize a variable like hardpaths , here are workarounds for the .cshrc file:

alias setprompt 'set prompt="${cwd}% "'
alias cd        'chdir \!* && set cwd=`/bin/pwd` && setprompt'
alias pushd     'pushd \!* && cd .'
alias popd      'popd \!* && cd .'

When you cd , that alias resets the cwd variable to the output of /bin/pwd , then resets the prompt to the new cwd . Using pushd or popd ( 14.6 ) runs the cd alias, too - this changes to the current directory ( . ), which fixes cwd (as well as the dirs command) and resets the prompt.

Whew. Are symlinks worth the work? (I think they are.)

- JP


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