How to get green/red terminals under OpenBSD?

Question

For linux machines I can use:

# vi ~/.bashrc # red/green terminal colors regarding exit code

export PROMPT_COMMAND='PS1="`
if [[ \$? = "0" ]];
then echo "\\[\\033[0;32m\\]";
else echo "\\[\\033[0;31m\\]";
fi`[\u@\h \w]\[\e[m\] "'
export PS1

to get green terminal when exit code is 0, and get red prompt when exit code is not 0.

How can I do this under OpenBSD? (the default ksh)

(I was trying to do it, but with no luck - using ssh to connect to the OpenBSD machine from my notebook - ubuntu/gnome-terminal. )

Answer 1

Each shell has its own way of expanding the prompt. OpenBSD's ksh is pdksh. See its prompt settings in the manual, under the PS1 parameter.

The way ksh works is that it performs parameter substitution, command substitution and arithmetic substitution (all the $ expansions) on the value of PS1 before printing the prompt. So you set PS1 once and for all, but the value is regenerated each time. (You can do that in bash too, if you put shopt -s promptvars in your .bashrc.)

OpenBSD's ksh supports backslash escapes similar to bash. With other versions of ksh, you'd need to translate the bash prompt escape sequences into $ substitutions as well.

PS1='$(if (($? == 0)); then print -n "\\033[32m"; else print -n "\\033[31m"; fi)'
PS1="$PS1"'[\u@\h \w]'$(print '\033')'[0m '

The simple translation above has a few defects:

• It forks a process each time, to analyze $?. There is a way to avoid this, but it's hackish in ksh88.
• It causes redrawing bugs, because of the zero-width control sequences. This can be avoided with the gross hack documented in the manual.

Here's some more complex code that uses arithmetic to avoid spawning a subshell and takes care of delimiting non-printable sequences.

escape=$(print '\033')
ctrla=$(print '\001')
PS1=$(print '\001\015')
PS1=$PS1$ctrla$escape'[$(($? ? 31 : 32))m'$ctrla
PS1=$PS1'[\u@\h \w]'
PS1=$PS1$ctrla$escape'[0m'$ctrla' '

Answer 2

The problem is only bash has PROMPT_COMMAND. Try this instead:

PS1='\[$(if (($?)); then tput setaf 1; else tput setaf 2; fi)\]'\
'[\u@\h \w]\['"$(tput sgr0)"'\]'

^{¹ Caveat: I haven't tested this on ksh, but it avoids PROMPT_COMMAND and works in bash.}

^{² tput uses your system's terminfo databases. This is generally more portable and maintainable than hard-coding escape sequences, provided terminfo is installed correctly.}