Which of the quotation marks in this shell script are unnecessary?

foo &
foopid="$!"
while kill -0 "$foopid"
do
    message="`foostatus`"
    if test "$message" = "Terminated"
    then
        exit 0
    else
        echo foo status: $message
    fi
done
exit 0

A few notes:

• "$!" is the process ID number of the last "background" task in sh (i.e. the most recent command invoked with a '&' after it).
• "kill -0" is being used to test whether the process still exists; signal 0 does nothing, but kill -0 will fail if the process no longer exists. So the 'while' loop is iterating for as long as the process exists.
• The hypothetical "foostatus" command yields human-readable status output such as "foo is waiting for printer #3".

a) "$!"

Answer: Unnecessary. The process ID is a simple number, no funny characters.

b) "$foopid"

Answer: Again unnecessary. We know that this has the value of a previous access to "$!", so it's going to be just a number.

c) "`foostatus`"

Answer: Necessary. The status may contain spaces, wildcards, anything; and we want it all to go into the variable 'message' without further interpretation.

d) "$message"

Answer: Necessary. Without the quotes, the message will be re-parsed, and if it contains spaces (which the example suggests it usually will), it will occupy multiple arguments to test, and test will give a syntax error.

e) "Terminated"

Answer: Unnecessary. All of the characters in the string are letters. The quotation marks don't do anything because there are no special characters to suppress the interpretation of.