How to setup prompt with three nearest folders ../c/Users/test - bash

Question

Here's a sample: ../c/Users/test
I stil have no idea how to achieve this zsh's prompt, if possible also how it will be in bash?
EDIT: the nearest means my pwd will show for i.e: /home/abc/c/Users/test.
If I'm under /home/abc/c or /home/abc or /home, prompt should be /home/abc/c> or /home/abc> or /home>
So only current path that excesses three folders will have .. appended in front and the three nearest folders.

Answer 1

ZSH
In zsh this can be achieved entirely with built-in features. You just have to place
%(4/|../%3d|%d)
in your PROMPT parameter (also known as PS1).
For example:
PROMPT='[%m#%n %(4/|../%3d|%d)]%# '
Would get you something like
[abc#machine ../c/Users/test]%
when the current directory is /home/abc/c/Users/test.
Explanation:
%(x|true-text|false-text): x represents a test, if it evaluates to true it zsh will print what is placed as true-text else it will print the false-text.
4/ is true if the current absolute path has at least 4 elements. For example /home/abc/c/Users/test has 5 elements
So, if the current path has 4 or more elements, the output is ../%3d, where %3d will be replaced with the last 3 elements of the current path. For example ../c/Users/test.
If the current path has less than 4 elements, the output is %d, which will be replaced by the full current path.
BASH
Method 1: simple but not a perfect match
In bash (since version 4) you can achieve very similar results by setting PROMPT_DIRTRIM=3 and placing \w in PS1. This will also only display the last 3 elements of the current path, preceded by either ~/.../ or .../. Which depends on whether the current directory is within the user's home directory.
For example:
PS1='[\u#\h \w]\$ '
PROMPT_DIRTRIM=3
would get you
[abc#machine ~/.../c/Users/test]$
when the current working directory is /home/abc/c/Users/test and
[abc#machine .../share/doc/sometool]$
when the current working directory is /usr/local/share/doc/sometool.
Method 2: complicated but works as asked
For an exact match place the following in your PS1:
$(a=${PWD%/*} a=${a%/*} a=${a%/*}; echo ${PWD/#$a/${a:+..}})
For example
PS1='[\u#\h $(a=${PWD%/*} a=${a%/*} a=${a%/*}; echo ${PWD/#$a/${a:+..}})]\$ '
Important: At least the part that generates the path output needs to be fully quoted, e.g. surrounded by single quotes. Otherwise it would be evaluated at the time of definition and not when the prompt is displayed.
Explanation:
$(command): This is called Command Substitution. It will run command and then be substituted by the resulting output.
The parameter PWD contains the current working directory.
a=${PWD%/*}: The shortest possible match to /* will be removed from the end of $PWD and the resulting value will be assigned to parameter a. That is, the last path element will be removed from $PWD.
a=${PWD%/*} a=${a%/*} a=${a%/*}: this removes the last three path elements from $PWD. If $PWD has three or less elements, then a will be empty at the end. If there are more than three elements, then a contains all element you do not want to be shown, i.e. the ones you want to replace with ...
(Note: While a=${PWD%/*/*/*} also removes the last three path elements, it does not work as intended, if there are less than three elements. In that case the end of $PWD would not match to /*/*/* and nothing would be removed, leaving $a to be identical to $PWD.)
${a:+..}: if a is defined and not null this will be substituted by .., otherwise nothing is substituted. This means if there are path elements to be removed, then ${a:+..} will evaluate to ...
${PWD/#$a/${a:+..}}): if the beginning of $PWD matches $a then it will be replaced by the subtstitution of ${a:+..}. Essentially, if a contains any path elements, than they will be replaced by .., otherwise nothing will be changed.
echo: As this all happens within a Command Substitution, echo is needed in order to output the shortened path.

Answer 2

this seems to do the trick:
PS1='$(pwd|sed -r "sx.+(/[^/]*/[^/]*/[^/]*)\$x..\1x" ):'
eg:
jasen#gonzo:/var/spool/news/comp/lang/c/moderated$ PS1='$(pwd|sed -r "sx.+(/[^/]*/[^/]*/[^/]*)\$x..\1x" ):'
../lang/c/moderated:cd
/home/jasen:
How it works:
PS1 is expanded to produce the prompt.
I use command substitusion $( ... ) to insert output of a command into the prompt.
the command itself is a pipeline
first pwd pints out the current directory
it's output is piped (|) to sed in extended (-r) regular expression mode. sed is given the command.
"sx.+(/[^/]*/[^/]*/[^/]*)\$x..\1x"
this an s substitution command
in this command the symbol that follows the s is the separator here I used x
the phrase [^/]* indicates a seaquence of zero or more non-slashes (like the name of a directory) while the other slashes / represent actual slashes and .+ matches anything at all (but not nothing). and the $ represents end of line.
so starting from dollar it matches lines that end like /name/name/name
the bit after the second X where it says ..\1 is what to replace the match with. in this case .. followed by the bit contained in the bit matched by the parenthesised pattern. ../name/name/name