Boolean in Shell Scripting

Question

I have a problem with boolean for while loop. As such, I switch to for loop instead. But still, I cannot change the value of a boolean after the condition is met.

doFirst= true
    for (( j=1; j<=7; j++))
    do
            letter="A"
            seatChoses=$letter$j

            flagRand=$(echo $flightSeatBooked | awk -v flseatRand=$flightSeatBooked -v orseatRand=$seatChoses '{print match(flseatRand, orseatRand)}')
            if  $doFirst ; then
            **$doFirst= false** // Here is the error!
                if [ $flagRand -eq 0 ]; then
                    echo "System generated a slot, "$seatChoses" for you. [Y or N]"
                fi
            fi

    done

Answer 1

There is no such thing as a boolean value in a shell script (that is, something you can store in a variable, and treat as a boolean). true and false are commands; true exits with value 0, and false exits with a nonzero value. An if statement in bash taks a command; if that command returns 0, then the then clause is executed, otherwise the else clause is.

doFirst= true

This line doesn't do what you expect at all. In a shell script, you cannot have any spaces after the equals sign. The space means you're done with the assignment, and now writing a command. This is equivalent to:

doFirst="" true

Furthermore, if you have an assignment before a command (like this), that doesn't actually perform the assignment in the shell. That sets that environment variable in the environment for that command alone; the assignment has no effect on anything outside of that command.

if  $doFirst ; then

This expands the $doFirst variable, and tries to interpret the result as a command. Oddly, if $doFirst is undefined (which it is, as I explain above), this takes the then branch. At that point, you make your first mistake again, trying to set a variable to be false, and again, nothing happens; $doFirst is left undefined. You make the further mistake of trying to assign $doFirst; you use $ to get the value of a variable, when setting, you use the bare name.

My recommendation would be to not try to use booleans in Bash; just use strings instead, and check the value of the string. Note that I remove the space, so now I'm setting it to that exact string; and there is no command, so this sets the variable within the shell, not in the environment for a single command:

doFirst=true
# ...
   if [ $doFirst = true ]; then
       doFirst=false
   # ...

Answer 2

Are you actually putting a space between the = and the "true"/"false" or is that a formatting error? That's one of your problems.

Another, as mentioned by Anders Lindahl in the comment section, is that when you set a variable in shell scripting, you cannot use the $ in the front. You must say

doFirst=false

Again, note that there are no spaces around the equals sign.