Cannot exit bash script correctly

Question

I have a made a script on bash.

#!/bin/bash

zen(){
mark=$(zenity --scale \
    --text 'FREQUENCY' \
    --value=$la \
    --min-value=0\
    --max-value=5000 \
    --step=1)
}
la=500

echo "Script for shim. Regulary frequency"
zen
while [ true ] 
do

case $? in

    0) echo $mark
       la=$mark
       #zenity --notification --window-icon="info" --text="Thank you!" --timeout=1
       zen
    ;;
    1) 
       # exit 1
       # sl -e || break
       # break
       # return 1       
    ;;
esac 
done
echo "thanks for using!"

It works okay, excluding exit point. # stands before options I have tried, and every one of them does not allow to exit this script correctly and instead of "thanks for using!" or just nothing I am getting in terminal:

This option is not available. Please see --help for all possible usages.

This option is not available. Please see --help for all possible usages.

This option is not available. Please see --help for all possible usages.

This option is not available. Please see --help for all possible usages.

This option is not available. Please see --help for all possible usages.

This option is not available. Please see --help for all possible usages.

This option is not available. Please see --help for all possible usages.

This option is not available. Please see --help for all possible usages.

This option is not available. Please see --help for all possible usages.

This option is not available. Please see --help for all possible usages.

This option is not available. Please see --help for all possible usages.

This option is not available. Please see --help for all possible usages.

^XThis option is not available. Please see --help for all possible usages.

This option is not available. Please see --help for all possible usages.

This option is not available. Please see --help for all possible usages.

This option is not available. Please see --help for all possible usages.

.........................................

It looks like it is problem with zenity, when I am trying to exit the script. I looked this error up and only sound thought was to upgrade zenity, which I have done but it has given me nothing new...

So how can I solve it and break this script correctly..?

My OS is Ubuntu Server 16.04

EDIT

By my script I would like to achieve repeating question from zenity till the moment when user clicks 'cancel'

Answer 1

$? is the exit status of the last command that was run. In your case, that's of the [ command (which you use to test whether the true string is non-empty as a condition of your while loop).

You almost never need to use $? explicitly. Just do

la=500
while
  mark=$(zenity --scale \
      --text 'FREQUENCY' \
      --value="$la" \
      --min-value=0 \
      --max-value=5000 \
      --step=1)
do
  echo "$mark"
  la=$mark
done

Or simply:

mark=500
while
  mark=$(zenity --scale \
      --text 'FREQUENCY' \
      --value="$mark" \
      --min-value=0 \
      --max-value=5000 \
      --step=1)
do
  echo "$mark"
done

Answer 2

There is a missing space before the backslash in your zenity call which may cause the error:

zen(){
mark=$(zenity --scale \
    --text FREQUENCY \
    --value=$la \
    --min-value=0 \
    --max-value=5000 \
    --step=1)
}

la=500

echo "Script for shim. Regulary frequency"
zen
zen_ec=$?
while true
do

    case $zen_ec in

        0) echo $mark
           la=$mark
           #zenity --notification --window-icon="info" --text="Thank you!" --timeout=1
           zen
        ;;

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