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In tcsh, I have the default:

prompt [%m:%c3] %n%# 

which gives prompts like:

[woehler:hacking/c/hello] ajcarr% 

and

[woehler:~] ajcarr% 

In other words, the current directory and up to the next two above it in the path.

In ksh93 or bash, the substitution of $HOME by ~ is easy, as is extracting the name of just the current directory, but I have yet to find a way of replicating the %c3 behaviour of tcsh. At present in ksh93 I have:

[ajcarr@Woehler] hello $ 

and

[ajcarr@Woehler] ~ $ 

Does anyone have any suggestions about how to do this?

For bash, you could achieve similar results by setting the PROMPT_DIRTRIM variable:

$ PS1='[u@h] w$ '
[schaller@r2d2] ~$ pwd
/home/schaller
[schaller@r2d2] ~$ PROMPT_DIRTRIM=3
[schaller@r2d2] ~$ cd /home/schaller/tmp/513924/another/directory/here
[schaller@r2d2] ~/.../another/directory/here$

In ksh93:

PS1='$PWD#$PWD%?/*/*/*?/ $ '
share/doc/libnl-3-dev $ _

PS1='[$HOSTNAME%%.*:$PWD#$PWD%?/*/*/*?/] $USER% '
[host:share/doc/libnl-3-dev] user% _

If you want it to also replace $HOME with ~, something nastier is needed:

PS1='$(d=$PWD/#$HOME/"~";printf %s "$d#$d%?/*/*/*?/") $ '
~/w/maemo $ cd sb2-pathmaps
w/maemo/sb2-pathmaps $ _

PS1='$(d=$PWD/#$HOME/"~";printf %s "[$HOSTNAME%%.*:$d#$d%?/*/*/*?/]") $USER% '
[host:w/maemo/sb2-pathmaps] user% _

All this should also work in bash, though bash has its own prompt escapes (eg. h for $HOSTNAME%%.*) and path shortening mechanism (with PROMPT_DIRTRIM).

Also, the nastier variant will be really nasty, because bash, unlike ksh93, will fork() a separate process for each $(...; printf ...) command substitution.

zsh has prompt escapes quite similar but not identical to tcsh:

zsh$ PS1='[%m:%3c] %n%# '
[host:share/doc/libnl-3-dev] user% _