Range variables in for loop custom definable

Question

I have a question with range variables in for loops. In for loops I know you can use {..} to define a range. But I want it to be customer definable. So the script asks for a range value, and I want it to be flexible. like this:

#!/bin/bash
while true; 
    do
    echo "Some explenation..."
    read -p "Possible values are: ALL or RANGE (to define a specific range):" answer
    case $answer in
            #ALL
            [aA][lL][lL] )
            echo "ALL is selected, 1 to 250"
            RANGE="{1..250}"
            break
            ;;
            #RANGE
            [rR][aA][nN][gG][eE] )
            echo "Range is selected, please define a custom range. Notation should be like: <start>..<stop>"
            echo "or: <single> <single>"
            echo "or a combination of both. Like: 1..23 28 29 101..145 180 212"
            read RANGE
            break
            ;;
            #OTHERS
            * )
            echo "Typo error!! Not correct, type again ALL, PRI, SEC or 
RANGE"
            ;;
    esac
    done
echo ""
echo "Range for this execution is set to:"
echo "$RANGE"
for NEXI in $RANGE
    do
    sleep 0.2
    echo "Nexus number: $NEXI"
    done
echo ...

But the Output when I enter {1..10} 52 54 {120..128} is:

Nexus number: {1..10}
Nexus number: 52
Nexus number: 54
Nexus number: {120..128}

while I expected it to be like:

Nexus number: 1
Nexus number: 2
Nexus number: 3
Nexus number: 4
Nexus number: 5
Nexus number: 6
Nexus number: 7
Nexus number: 8
Nexus number: 9
Nexus number: 10
Nexus number: 52
Nexus number: 54
Nexus number: 120
Nexus number: 121
... etc.

fully written out.

What can I change to achieve this?

Thanks.

Answer 1

The problem is that brace expansion happens before variable expansion. As explained in man bash:

The order of expansions is: brace expansion; tilde expansion, parameter and variable expansion, arithmetic expansion, and command substitution (done in a left-to-right fashion); word splitting; and pathname expansion.

This means that when you have something like $var which contains {1..3}, and do echo "$var", the shell looks for possible brace expansions before expanding the variable to its value. Since the braces are in the variable, that means that at that point the shell sees no braces so no brace expansion is performed.

One workaround would be to eval the variable if it is a range. Add these lines before your for loop:

if [[ $RANGE =~ ^[0-9]+\.\.[0-9]+$ ]]; then
        RANGE=$(eval echo {$RANGE})
fi

Now, on a more general note, it is usually a good idea to avoid using CAPITAL variable names in shell scripts. Environment variables are usually capitalized and that can lead to mixups if you happen to use the same name as a defined env variable. So use lower case variable names to be on the safe side.

Also, this is a really annoying program to debug and equally annoying to use. Don't prompt your user for input at run time! Instead, change your script and make it take arguments. Typing values is hard, annoying, error prone and can't be automated.

Answer 2

Set your shell to ksh and this code should work.

#!/usr/bin/ksh