Bash case statement: retrieve list of valid options? - bash

Question

Is there any way to retrieve a list of the options in a case statement? For example if I have this code:
tool=$1
case ${tool} in
brdf)
# Do stuff
;;
drift)
# Do other stuff
;;
*)
echo "ERROR: I don't know this tool. Valid options are: brdf, drift"
exit 1
;;
esac
This is easy to read, but the error message could easily get out of date when adding/removing tools from the list as I need to remember to change the names there too.
The repetition could be avoided using an array something like this:
tool=$1
validtools=(brdf drift)
case ${tool} in
${validtools[0]})
# Do stuff
;;
${validtools[1]})
# Do other stuff
;;
*)
echo "ERROR: I don't know this tool. Valid options are: ${validtools[#]}"
exit 1
;;
esac
But that is pretty horrible to read, and in any case would be even worse to maintain with the hardcoded array indices.
Is there a good way of doing this, perhaps some variable or command that retrieves a list of the available options, or do I just have to remember to update the error message when I add a new option?

Answer 1

The most used way is as your 1st example. See all init scripts in linux.
And it is for reason, because you can use constructions like:
case "$a" in
arg1|arg2|arg3) ... ;;
brg1|brg2) ... ;;
brg2) ... ;;
esac
and would be hard contstuct automatically the right usage message with the all possible variants.
And here is the shopt -s extglob too, what allows you to use extended pattern matching in the case statemens. For examples see this answer: https://stackoverflow.com/a/4555979/632407
But if you want use arrays, try to use associative array, what add a bit of readability. (But it is terrible anyway) :) Like the next:
declare -A arg
initargs() { for a in "$#"; do arg[$a]="$a"; done; }
initargs brd lbrs ubrs
for myarg
do
case "$myarg" in
${arg[brd]}) echo "brd";;
${arg[ubrs]}) echo "ubrs";;
${arg[lbrs]}) echo "lbrs";;
*) echo "Unknown arg =$myarg=. Known are: ${arg[#]}" ;;
esac
done
So the allowed args are: "brd" "lbrs" "ubrs" and the script for the next input
$ bash argtest ubrs badarg brd
produces:
ubrs
Unknown arg =badarg=. Known are: lbrs ubrs brd
brd