Alias to change path on the fly

[Jonathan I. Kamens]

  (Note the Followup-To, which probably should have been set to something in
the original posting.  I was all set to flame at the poster for posting a
question to comp.unix.internals that doesn't have anything at all to do with
Unix internals, and then I realized that he posted it to comp.unix.wizards,
which is aliased to comp.unix.internals, at least at my site. :-)

In article <1990Nov8.014515.13882 at cpsc.ucalgary.ca>, paquette at cs-sun-fsa.cpsc.ucalgary.ca (Trevor Paquette) writes:
|> alias host '`echo set path=\($path\)|sed s/itaqc/itahost/|sed s/itasc/itahost/`'
|> alias qc   '`echo set path=\($path\)|sed s/itahost/itaqc/|sed s/itasc/itaqc/  `'
|> alias sc   '`echo set path=\($path\)|sed s/itaqc/itasc/  |sed s/itahost/itasc/`'
|>   But these new aliases give me the following error: 
|> 
|> `echo set path=\($path\)|sed s/itaqc/itahost/|sed s/itasc/itahost/`: Ambiguous

  The C-shell has a problem (discussed several times in comp.unix.shell
recently) with deciding exactly when the output of a backquote substitution
should be expanded into multiple words.  Another example of this is that typing

    kill `cat file-with-list-of-pids-in-it`

won't work because the list of pids will all be treated as one word.  This is
only a problem with the "kill" shell built-in, though, so if you substitute
"/bin/kill" for "kill", it works fine.  The problem you are having is another
manifestation of this.

  The way to get around it is to change your aliases.  For example;

    alias host 'set path= (`echo $path|sed -e s/itaqc/itahost/g -e \\\
      s/itasc/itahost/g`)'

Note that this has the added advantages of using only one invocation of sed
instead of two, and of replacing itasc and itaqc whereever they appear in your
path, not just the first time.

  You could also solve the problem by using eval instead of backquotes,
although you might have to make some minor changes to your aliases in that
case too.  Figuring out how to do this is left as an exercise to the reader
:-).

-- 
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