Prompt working directory

TL;DR

Shorten ~/very/deep/down/the/rabbit/hole to ⋯/the/rabbit/hole in prompt.

If you’re staring at a shell prompt a few hours every day, you might as well try to modify the prompt to better suit your needs. Some people like a minimalistic prompt with nothing but the working dir and a dollar sign ~ $. Others get an enormous 256-color multi-line beast holding information about the kernel version, battery status, daily horoscope and what not. There’s yet another group which sticks with whatever their system’s default is.

This post will hopefully come in handy for the first and second group.

The Goal

The working directory should be readable no matter how ~/very/deep/down/the/rabbit/hole you’re in in the file system. To maintain readability, the working dir displayed in the prompt should contain no more than three sub directories ⋯/the/rabbit/hole

Bash version 4+ actually has an option to do this PROMPT_DIRTRIM=3, but I find it’s behaviour ludicrous: it behaves (very) differently when the working dir is a sub dir of $HOME or not.

Working dir is a subdir of $HOME:

~ $ PROMPT_DIRTRIM=3
~ $ cd one/
~/one $ cd two/
~/one/two $ cd three/
~/one/two/three $ cd four/
~/one/two/three/four $ cd five/
~/.../three/four/five $

This is useless to me:

• Notice that when in dir four, the path is not truncated at all ~/one/two/three/four $
• The tilde ~ is never truncated ~/.../three/four/five $

Working dir is not a subdir of $HOME:

~ $ PROMPT_DIRTRIM=3
/ $ cd one/
/one $ cd two/
/one/two $ cd three/
/one/two/three $ cd four/
.../two/three/four $ cd five/
.../three/four/five $

This is much better! … or not:

• Most of the time I’m somewhere in $HOME
• I would really like to use a single char ⋯ instead of three dots .... Screen real-estate does not come that cheap. I work with many vim/tmux panes, so cutting some fat here and there does make a difference.

Also, I’ve been wanting to improve my shell scripting skills for some time. This seemed like a nice challenge.

Truncate function

In theory, truncating the $PWD to 3 dirs seemed like a very simple task. In practice, it turned out a (tiny) bit more complicated. In javascript (a language I’m mostly unfamiliar with), this could be achieved with pretty much one line of code:

(Note that this is greatly simplified and doesn’t handle any corner cases.)

PWD.split('/').slice(-3).join('/')

How hard could it be to port this to bash (and zsh)?

One thing I want to stress is that external processes must *not* be created to achieve this task. This function must be as fast as possible and forking off sed, awd and what not, is not a viable option.

The result

After some research on bash/zsh arrays, some head-banging and quite a bit of coffee, I managed to achieve the desired result:

In a subdir of $HOME:

~ $ cd one/
~/one $ cd two/
~/one/two $ cd three/
⋯/one/two/three $ cd four/
⋯/two/three/four $ cd five/

Not in $HOME:

/ $ cd one/
/one $ cd two/
/one/two $ cd three/
⋯/one/two/three $ cd four/
⋯/two/three/four $ cd five/

As a bonus, the separator can be configured, for example it could be = > =

~ $ cd one/
~ > one $ cd two/
~ > one > two $ cd three/
⋯ > one > two > three $ cd four/
⋯ > two > three > four $ cd five/

Plus, with some minor modifications, I got the function working with powerline symbols for my promptline.vim plugin:

{% img /images/promptline_cwd.png %}

The function itself

Hopefully it would be useful to someone else. I certainly learned a lot writing (and re-writing) it. Enjoy!

function truncated_cwd {
  # dir_limit and truncation can be configured
  local dir_limit="3"
  local truncation="⋯"

  local first_char
  local part_count=0
  local formatted_cwd=""
  local dir_sep=" | "

  local cwd="${PWD/#$HOME/~}"

  # get first char of the path, i.e. tilde or slash
  [[ -n ${ZSH_VERSION-} ]] && first_char=$cwd[1,1] || first_char=${cwd::1}

  # remove leading tilde
  cwd="${cwd#\~}"

  while [[ "$cwd" == */* && "$cwd" != "/" ]]; do
    # pop off last part of cwd
    local part="${cwd##*/}"
    cwd="${cwd%/*}"

    formatted_cwd="$dir_sep$part$formatted_cwd"
    part_count=$((part_count+1))

    [[ $part_count -eq $dir_limit ]] && first_char="$truncation" && break
  done

  [[ "$formatted_cwd" != $first_char* ]] && formatted_cwd="$first_char$formatted_cwd"
  printf "%s" "$formatted_cwd"
}

There are a few ways to get the function in the prompt, this is probably the simplest one:

PS1='$(truncated_cwd) \$ '