Code challenge: Bash prompt path shortener

Question

I implemented a prompt path shortener for bash to be included in the PS1 environment variable, which shortens the working directory into something more compact but still descriptive. I'm curious what other ideas may exist.

Here's the challenge:

Create a bash function _dir_chomp which can be included into PS1 like this (line breaks inserted for readability):

PS1='\[\033[01;32m\]\u@\h\[\033[01;34m\] $(
  _dir_chomp "$(pwd)" 20
)\[\033[01;37m\]$(parse_git_branch)\[\033[01;34m\] \$\[\033[00m\] '

with "20" being the parameter for the maximum length as soft limit. These are the examples:

1. /usr/portage/media-plugins/banshee-community-extensions/files becomes /u/p/m/b/files
2. /home/user1/media/video/music/live-sets becomes ~/m/v/m/live-sets (note the ~ character as replacement for $HOME)
3. /home/user2/media does NOT change (20 char limit not exceeded)
4. /home/user1/this-is-a-very-long-path-name-with-more-than-20-chars becomes ~/this-is-a-very-long-path-name-with-more-than-20-chars (last component stays unshortened: soft limit)
5. /home/user1/src becomes ~/src ($HOME always shortened)
6. /home/user1/.kde4/share/config/kresources becomes ~/.k/s/c/kresources (note the prefixing dot is preserved)

Current user is user1.

It's allowed to call external interpreters like awk, perl, ruby, python but not compiled C programs or similar. In other words: external source files are not allowed, code must be inline. Shortest version wins. The length of the bash function body (and called sub functions) counts, means:

_sub_helper() {
  # this counts
}
_dir_chomp() {
  # these characters count (between { and })
  _sub_helper "foobar" # _sub_helper body counts, too
}

Answer 1

Pure Bash:

_dir_chomp () {
    local IFS=/ c=1 n d
    local p=(${1/#$HOME/\~}) r=${p[*]}
    local s=${#r}
    while ((s>$2&&c<${#p[*]}-1))
    do
        d=${p[c]}
        n=1;[[ $d = .* ]]&&n=2
        ((s-=${#d}-n))
        p[c++]=${d:0:n}
    done
    echo "${p[*]}"
}

For purposes of testing, I'm assuming that the question means that the current user is "user1".

Note: Bash 4 has a variable PROMPT_DIRTRIM that shortens the \w escape in PS1 by retaining the number of sub-directories according to its value and replacing the rest with ...

/$ PROMPT_DIRTRIM=2
/$ echo $PS1
\w\$
/$ pwd
/
/$ cd /usr/share/doc/bash
.../doc/bash$

Answer 2

This one is 20 or so characters shorter than my other answer:

_dir_chomp () {
    local p=${1/#$HOME/\~} b s
    s=${#p}
    while [[ $p != ${p//\/} ]]&&(($s>$2))
    do
        p=${p#/}
        [[ $p =~ \.?. ]]
        b=$b/${BASH_REMATCH[0]}
        p=${p#*/}
        ((s=${#b}+${#p}))
    done
    echo ${b/\/~/\~}${b+/}$p
}

Answer 3

This was my own solution when I had the idea for this challenge. The inspiration actually came from Jolexa's Blog.

So here it is, the ruby implementation in readable form:

a = ARGV[1].gsub(%r{^#{ENV['HOME']}}, "~")
b, a = a, a.gsub(%r{/(\.?[^/.])[^/]+(/.*)}, '/\1\2') while
  (a.length > ARGV[2].to_i) && (b != a)
print a

And the actual one-line implementation within the bash function:

_dir_chomp() {
  ruby -e'a="'$1'".gsub(%r{^'$HOME'},"~");b,a=a,a.gsub(%r{/(\.?[^/.])[^/]+(/.*)},"/\\1\\2")while(a.length>'$2')&&(b!=a);print a'
}