How do I write an if statement within a while statement in Bash Shell Script?

Question

I'm currently having simple syntax issues with the following bash shell script. I'm not sure what the syntax is for nesting an if statement into a while statement, or if it's even possible with bash shell scripting (new to all things linux):

#!/bin/bash

myCombo=$((RANDOM%99999+10000));
echo ${myCombo};

myCracker=00000;

while [ (($myCracker<=99999)) ]; do
    if [ $myCracker -eq myCombo ]
    then
        echo "The combination is " ${myCracker} " !"
    else [ $myCracker = $myCracker + 1 ]
    fi
done;

Tom Fenech · Accepted Answer · 2015-02-16 23:45:33Z

2

There were quite a few things wrong with your loop. I've made a number of improvements below:

while (( myCracker <= 99999 )); do
    if (( myCracker == myCombo )); then
        echo "The combination is $myCracker !"
        break
    else
        (( ++myCracker ))
    fi
done

As you're using bash, you can make use of (( arithmetic contexts )), which don't need enclosing in [. Within them, variable names are expanded, so do not require prefixing with $.

Note that your original logic will either loop indefinitely or never echo, which is probably a bug. If myCracker == myCombo is ever true, myCracker won't be incremented so the loop will never terminate. The break statement deals with this.

I left the else branch in deliberately to show the syntax but you could also remove it entirely:

while (( myCracker++ <= 99999 )); do
    if (( myCracker == myCombo )); then
        echo "The combination is $myCracker !"
        break
    fi
done

The break is still useful as it prevents the loop from continuing unnecessarily.

edited Feb 16, 2015 at 23:45

answered Feb 16, 2015 at 23:38

Tom Fenech

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3 Comments

gniourf_gniourf Over a year ago

[I added a break as it seems so appropriate, rollback if you don't like it] This really calls for a for loop :).

kcmallard Over a year ago

Thank you so much for that. This cleared up MANY syntax questions I had about Bash, and I feel as though Bash will come much easier for me with this new knowledge.

Tom Fenech Over a year ago

@gniourf_gniourf thanks, I was just in the process of adding something similar!

JustArchi · Accepted Answer · 2015-02-16 23:44:18Z

1

You can also use extended tests instead of arithmetic contexts if you do simple comparisions. They should be a little faster, and are more common.

while [[ $myCracker -le 99999 ]]; do
    if [[ $myCracker -eq $myCombo ]]; then
        echo "The combination is ${myCracker}!"
        break
    else
        ((myCracker++))
    fi
done

answered Feb 16, 2015 at 23:44

JustArchi

1298 bronze badges

5 Comments

kcmallard Over a year ago

This adds even more knowledge for my skill-bag. Thanks so much for helping out! Looks like I was trying to mix some of this syntax together into a big messy cluster without knowing fully well the syntax rules for certain operations.

JustArchi Over a year ago

I'd suggest sticking with extended tests for everything, and use arithmetic contexts only for arithmetic operations, including incrementing by one. This is because extended tests are more common and handle both numbers (-eq) and strings (=).

Etan Reisner Over a year ago

[[ implicitly introduces an arithmetic context for both the right- and left-hand sides of the numerical operators. This causes problems with variables that evaluate to strings (instead of numbers) since they get evaluated a second time and end up being 0. Compare foo=bar; [[ $foo -eq 0 ]]; echo $? to bar=5; foo=bar; [[ $foo -eq 0 ]]; echo $? to foo=bar; [ $foo -eq 0 ]; echo $?.

JustArchi Over a year ago

No, you're wrong. one="text" two="$one" three="other text" [[ "$one" = "$two" ]]; echo $? # 0 [[ "$one" = "$three" ]]; echo $? # 1

JustArchi Over a year ago

You should use -eq for arithmetic comparisions, and "=" (or "==", which acts the same) for strings.

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