Adding debug output scrabbles result from function

Question

I have a function that returns a value.
But I would like to add some echos for easier debugging. But this scrabbles the result.
Example:

function foo {
   echo "In foo"
   VAR="1234"
   echo "Calculated item"
   echo "$VAR"
}

RESULT=$(foo)
echo "RESULT=$RESULT"

I was expecting to get 1234.
Instead the output is:

RESULT=In foo  
Calculated item  
1234

What is the correct way to fix this?

Answer 1

Redirect the debugging output to standard error:

foo () {
   echo 'In foo' >&2
   VAR="1234"
   echo 'Calculated item' >&2
   echo "$VAR"
}

RESULT=$(foo)
printf 'RESULT="%s"\n' "$RESULT"

You could even create a dedicated "debug" function:

debug () {
    printf 'DEBUG: %s\n' "$@" >&2
}

foo () {
   debug 'In foo'
   VAR="1234"
   debug 'Calculated item'
   echo "$VAR"
}

RESULT=$(foo)
printf 'RESULT="%s"\n' "$RESULT"

Output:

$ sh script.sh
DEBUG: In foo
DEBUG: Calculated item
RESULT="1234"