BASH Converting String to boolean variable

Question

What's the best way to convert a literal string (e.g. "True" into the appropriate bash boolean variable). For instance java has java.lang.Boolean.valueOf(String)

Right now I'm using this in Bash:

if [ "${answers[2]}" = "TRUE" ] ; then
    clean=true; 
else 
    clean=false; 
fi

is there a way to do it and avoid the IF statement?

edit: to clarify its not by choice that I have String variable containing "TRUE" instead of just using a boolean variable. for full context this is the code

ans=$(yad --title='YadExample' --form --field=Opt1:CHK FALSE  --field=Opt2:CHK FALSE --field=Opt3:CHK TRUE);
#at this point the "yad" program is returning a string seperated by '|', e.g "TRUE|FALSE|TRUE"
IFS="|"
set -- $ans
answers=( $@ )
unset IFS
if [ "${answers[0]}" = "TRUE" ] ; then clean=true; else clean=false; fi
if [ "${answers[1]}" = "TRUE" ] ; then var2=true; else var2=false; fi
if [ "${answers[2]}" = "TRUE" ] ; then var3=true; else var3=false; fi

Solution 1

You could write a function to do the conversion for you:

function boolean() {
  case $1 in
    TRUE) echo true ;;
    FALSE) echo false ;;
    *) echo "Err: Unknown boolean value \"$1\"" 1>&2; exit 1 ;;
   esac
}

answers=(TRUE FALSE TRUE)
clean="$(boolean "${answers[0]}")"
var2="$(boolean "${answers[1]}")"
var3="$(boolean "${answers[2]}")"

echo $clean $var2 $var3

prints

true false true

---

Or, a little fancier:

function setBoolean() {
  local v
  if (( $# != 2 )); then
     echo "Err: setBoolean usage" 1>&2; exit 1 ;
  fi

  case "$2" in
    TRUE) v=true ;;
    FALSE) v=false ;;
    *) echo "Err: Unknown boolean value \"$2\"" 1>&2; exit 1 ;;
   esac

   eval $1=$v
}

answers=(TRUE FALSE TRUE)

setBoolean clean "${answers[0]}"
setBoolean var2 "${answers[1]}"
setBoolean var3 "${answers[2]}"

echo $clean $var2 $var3

Solution 2

You can just use something like:

[ "${answers[2]}" != "TRUE" ] ; clean=$?

You need to reverse the sense of the comparison if you want clean set to 1 on the condition being true.

Your sequence then becomes:

[ "${answers[0]}" != "TRUE" ] ; clean=$?
[ "${answers[1]}" != "TRUE" ] ; var2=$?
[ "${answers[2]}" != "TRUE" ] ; var3=$?

Comments

[bluphoenix]

ah nice. This is pretty much what I was looking for. I wanted to avoid cluttering the code with if statements for something that I might need to do a lot. This function is a good abstraction. Didn't know you could set variables like that from functions, thanks!

[chepner]

I would use declare instead of eval in this case. It's safer (you can't execute arbitrary code from $1), and you're already using bash-specific features.

[andrewdotn]

@chepner When I change eval $1=$v to declare $1=$v, the script stops working :/ I think it’s because it’s being used inside a function, but I’m not completely sure.

[chepner]

Of course. I didn't pay attention to the scope. I guess this is a case where there isn't a good alternative to eval for earlier versions of bash. Version 4.2 of bash does include a -g option for declare so that it can create global functions inside a function.