Odd problem with making a variable the output of a command in a shell script

linux=future · 12-13-2005, 09:04 PM

All I want to do is change the value of a number that was recently typed in. This is a simple script that shows exactly what I want to do:

#!/bin/bash
echo "Enter a number"
read num
num='expr $num - 1'
echo $num

Assuming the user entered 2, the output of the program, according to everything that I have read, should be 1. However, the output is expr $num - 1. I have no idea why it is doing that instead of evaluating. I know the problem is related to surrounding the expression with ''. I tried another script that used the whoami command to check to see if it was being run by root. However, 'whoami' didn't output the user name but instead whoami.

Wynd · 12-13-2005, 09:24 PM

Try using `...` instead of '...'

`...` means "capture the output of the command", whereas '...' means "print it out exactly as it is - no substitutions or anything."

kshkid · 12-13-2005, 09:25 PM

the expression should be evaluated

Code:

#!/bin/bash           
echo "Enter a number" 
read num              
num=`expr $num - 1`   
echo $num

linux=future · 12-13-2005, 09:45 PM

Thaks Wynd! I never noticed that they always shifted to the left a bit...I better get glasses. Evaluates fine now.