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October 6th, 2008
fix_path, programming, shell  [html]
Working several hours a day on the command line now, I've gotten interested in what I can do with the command prompt itself. For a long time, my command prompt looked like:
user@host /path/to/cwd $
This was quite serviceable, though when deep into a directory hierarchy (especially one laid out by a windows user) /path/to/cwd might be more like a three line monster.

Fixing this, though, isn't too hard. I've made several environment variables already that correspond to common places I want to go to. Because cd $FOO_SRC is way nicer than cd ~/code/current/foo/foo-1.2.4-rcA/src/. So why not make the command prompt know about this? So we could do:

user@host ~ $ cd $FOO_SRC
user@host $FOO_SRC $
So I write a little python program, fix_prompt.py:
           import sys
           import os

           # if it happens to match but it really short, ignore it as
           # its probably not what we want

           # these variables contain paths that would make little
           # sense to abbreviate to (except HOME, where we just want
           # to deal with it nicer)
           IGNORE_VARS=["PWD", "HOME", "OLDPWD"]

           # in order to still show home as ~ and /home/user as ~user
           # we need to have some special logic.  If you wanted a dir
           # /foobar to show up as the dir %, then you could add an
           # entry for that here.
           SPECIAL_REPLACEMENTS=[["/home/" + os.environ["USER"], "~"],
                                 ["/home/", "~"]]

           # we want to sort a pair of lists by the lengths of their
           # first elements, so we need a comparator function
           def longest_a_first_comp(x,y):
               return len(y[0])-len(x[0])

           in_path = sys.argv[1] if sys.argv[1:] else ""

           # get all the environment variables, reversed in mapping so
           # we can do reverse variable expansion. Or is it
           # variable condension?  variable condensation?
           env = [[v,k] for k,v in os.environ.items()]

           # sort them by length.  This is so if we have:
           #    $FOO_SRC="/home/user/src/foo"
           #    $SRCS="/home/user/src"
           # we will take the longer one.  I think this is always what
           # a user wants.

           for r in env:
               # the substitution only happens if it would shorten the path
               if in_path.startswith(r[0]) \
                      and r[1] not in IGNORE_VARS\
                      and len(r[0]) > IGNORE_LENGTH\
                      and len(r[1]) < len(r[0]):
                   # prefix with a dollar sign so it looks like an
                   # environment variable.  The dollar sign needs to
                   # be escaped, and it's escape needs to be escaped.
                   print "\\$" + in_path.replace(r[0], r[1], 1)

           for r in SPECIAL_REPLACEMENTS:
               if in_path.startswith(r[0]):
                   print in_path.replace(r[0], r[1], 1)

           # if we didn't do any condensations, yield the original path
           print in_path
Then I tell bash to call it, in ~/.bashrc:
           PS1="\u@\h \w $ "
           promptFunc() {
               PS1="${PS1}$(python ~/bin/fix_path.py $(pwd)) $"

           if [ $(whoami) == "myusername" ]
               # so if someone does an "su" at my terminal its really obvious
               unset PROMPT_COMMAND
Someday I may be on a slow machine that can't run python snappily like I want. Then I'll rewrite in C. Shouldn't be too bad.

One thing that surprises me about this is that I've not been able to find anyone else doing it online. Maybe I'm not calling it the right thing. "Path shortening"? "Path condensing"? The closest I found was some people putting in elipsies in the middle of things. It's sort of a reverse expansion of an environment variable.

Now if only "shopt -s cdable_vars" would let me do tab completion on them.

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