Monday, 4 April 2011

Bash, PS1, PROMPT_COMMAND and other fun

I've just spent 30 mins trying to understand the flow between the various files that set a bash prompt on fedora to do the following: (assumption is here that you're in a colour xterm)

I want:
* my username to change colour depending if I still have a valid kerberos token
* the hostname of the machine I'm on to be in RED if I'm root
* displayed path to be as simple as possible
* $ if I'm a minion, # if root as mormal
* command line editing to work sensibly, no wierdisms on wrapping long lines

so - I used to have on my machine something like the following:
if [ "$PS1" != "" ] ; then
klist -s
if [ $? -eq 0 ] ; then
PS1='\[\033[32m\]\u@\h\[\033[0m\]:\w\$ '
else
PS1='\[\033[36m\]\u\[\033[32m\]@\h\[\033[0m\]:\w\$ '
fi
fi

and something similar for root.

But on my laptop (F14) I wanted it system wide, so went down the approach of customising /etc/bashrc where you find calls to
PROMPT_COMMAND=/etc/sysconfig/bash-prompt-xterm

1) /etc/sysconfig/bash-prompt-* aren't included, so you're on your own
2) It must point to an executable script that is run every time before displaying the prompt
3) PS1 is still displayed *AND* if you use tab completion / ctrl-l, its *only* PS1 thats displayed on your screen, not the output from PROMPT_COMMAND

so: DON'T do the following:

cat /etc/sysconfig/bash-prompt-xterm
#!/usr/bin/env bash
# set green username if we have a valid kerberos token, else cyan
klist -s
if [ $? -eq 0 ] ; then
K='[32m'
else
K='[36m'
fi
# set hostname in red if we're root, green otherwise
if [ ${USER} = 'root' ] ; then
U='[31m'
else
U='[32m'
fi

printf "^[%s%s^[[37m@^[%s%s^[[0m:%s " ${K} ${USER} ${U} ${HOSTNAME%%.*} "${PWD/#$HOME/~}"
#echo -ne "\033]0;${USER}@${HOSTNAME%%.*}:${PWD/#$HOME/~}\007"
(where ^[ is 'ctrl-v, esc' in vim)
because you end up with stuff like

aelwell@pcitgtelwell:~[aelwell@pcitgtelwell ~]$

until you press ctrl-l and end up with just
[aelwell@pcitgtelwell ~]$ (ie, $PS1)


but *DO* make the call to see if you have a valid token and set the xterm titlebar in /etc/sysconfig/bash-prompt-xterm, but if you're altering PS1, then do so in the traditional places of /etc/bashrc and (as suggested in that file) a custom modification shell script in /etc/profile.d/ directory.

Ho Hum. Hope this clears up for anyone else trying to work out what the sysconfig/bash-prompt-* files do.

oh, and does anyone know a lighter call to see if a token is still valid than 'klist -s'?

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