Shell Programming and Scripting

#!/bin/bash
clear;
op = 0
until [ $op -ne 2 ]
do
   echo "Select from menu:"
   echo "1. Print Text"
   echo "2. Exit"
   read q

    case $q in
    1) echo "Hello World!!!";;
    2) exit;;
    esac 
done

./file: line 4: [: -ne: unary operator expected
Select from menu:
1. Print Text
2. Exit

until [ "${op}" -ne 2 ]
do
   echo "Select from menu:"
   echo "1. Print Text"
   echo "2. Exit"
   read q

    case $q in
    1) echo "Hello World!!!";;
    2) exit;;
    esac 
done

#!/bin/bash
until [ "${op}" -ne 2 ]
do
   echo "Select from menu:"
   echo "1. Print Text"
   echo "2. Exit"
   read q

    case $q in
    1) echo "Hello World!!!";;
    2) exit;;
    esac 
done

./file: line 2: [: : integer expression expected
Select from menu:
1. Print Text
2. Exit

#!/bin/bash
clear
op=0
until [ $op -eq 2 ]
do
   echo "Select from menu:"
   echo "1. Print Text"
   echo "2. Exit"
   read q

   case $q in
     1) echo "Hello World!!!";;
     2) exit;;
   esac 
done

until:-
This construct tests for a condition at the top of a loop, and keeps looping as long as that condition is
false (opposite of while loop).
until [ condition-is-true ]
do
command(s)...
done
Note that an until loop tests for the terminating condition at the top of the loop, differing from a
similar construct in some programming languages.
As is the case with for loops, placing the do on the same line as the condition test requires a
semicolon.
until [ condition-is-true ] ; do
Example 10-18. until loop
#!/bin/bash
END_CONDITION=end
until [ "$var1" = "$END_CONDITION" ]
# Tests condition here, at top of loop.
do
echo "Input variable #1 "
echo "($END_CONDITION to exit)"
read var1
echo "variable #1 = $var1"
echo
done
exit 0

if you understand the above you have to change your script to the below so that the until loop does not terminate:-

#!/bin/bash
clear;
op = 0
until [ $op -ne 0 ]
do
   echo "Select from menu:"
   echo "1. Print Text"
   echo "2. Exit"
   read q

    case $q in
    1) echo "Hello World!!!";;
    2) exit;;
    esac 
done

in your post you are using "-eq" but in niki22 post he is using "-ne" ..you have invert the condition ...so it will give you true.

BR