Change Korn prompt to show shorter path for working directory

Question

I have changed my .kshrc to show my prompt as follows: 511 /export/home/students/cninja $ where 511 is the command number and ../students/cninja is the current working directory.

Is it possible to show a shortened version of the working directory, the prompt then showing something like: 511 ~/cninja $ or 511 ../students/cninja $ or what have you?

This is what my line looks like in .kshrc:

PS1=' ! $PWD $'; export PS1

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Update: I haven't been able to determine whether I have a ksh88 or ksh93 shell. Most of the echo *what have you* commands return a blank stare and a prompt from the shell, and echo ${.sh.version} returns /bin/ksh: ${.sh.version}: bad substitution.

Answer 1

There exist a few implementations of ksh:

• The original (or AT&T) one by David Korn with two major variants (ksh88 and ksh93) each one having different versions with a different feature set
• pdksh (the public-domain Korn shell) with a few derivatives.
• zsh. Zsh has a ksh emulation mode which it activates when called as ksh.

You may want to use zsh as your ksh, or even switch altogether to zsh which is a much more powerful tool as an interactive shell.

If you call zsh as ksh. You can have the prompt expansion you want by doing (add it to your $ENV file):

setopt PROMPT_PERCENT # restore zsh prompt expansion disabled by the ksh mode
PS1='%! %~$ '

Otherwise, if you do need to stick with the AT&T or PD ones, you could aproximate it with:

PS1='! $(pwd | sed "s,^$HOME,~,")$ '

Which would work for ~ but not for ~other-user.

Example:

$ echo ${.sh.version}
Version JM 93u+ 2012-02-29
$ PS1='! $(pwd | sed "s,^$HOME,~,")$ '
85 ~$ cd /tmp
86 /tmp$ cd
87 ~$

From a vague recollection from decades ago, it could be that ksh88 doesn't expand command substitutions inside $PS1. In that case, you could still use some trick like:

one=1
tilde='~'
PS1='! ${tilde[0${one#${PWD##"$HOME"*}1}]}${PWD#"$HOME"}$ '

(which I've now verified works on all of ksh93u+ on Debian, ksh88i on Solaris, MirBSD ksh 40.9.20120630 (pdksh derivative) on Debian, and zsh 4.3.17 (when called as ksh) on Debian, so I'd be surprised if there was any ksh where it didn't work)

The idea being that if $PWD starts with $HOME, then ${PWD##"$HOME"*} is empty, so ${one#1} is empty, so we get ${tilde[0]} that is $tilde. But if it doesn't then ${PWD##"$HOME"*} is $PWD, then ${one#${PWD}1} is $one, so we get ${tilde[1]} which is empty as the $tilde array as only one element.

Answer 2

I thought the following would work, but it turned out to be a bash specific solution. Leaving this answer for reference purpose.

export PS1=' ! $( basename $PWD )'

Maybe it requires backtics instead of the $( ) construct.