UNIX for Dummies Questions & Answers

I figured this out, but I wanted to pose the question.

I was writing a while loop that required counting the amount of times the job had run, for example.

An example below is what I had:

Code:

variable=0

while :
do
    variable = `expr $variable + 1`
done

That didn't do what I needed it to (allow multiple attempts of input). However the following did:

Code:

variable=0

while :
do
    (( variable = variable + 1 ))
done

I was just curious why the double parenthesis handled that while the expr statement didn't. I'm still learning

Remove the blank space in between and re-try:-

Code:

variable=`expr $variable + 1`

Or

Code:

variable=$( expr $variable + 1 )

Ahh ok cool - thank you. That did it. The guy I'm learning from at work does it in double parenthesis. Is there any reason to do it one way or the other?

Double parentheses are used for arithmetic operations and they enable you to omit the dollar signs on integer and array variables and include spaces around operators for readability.

I appreciate it. Makes sense to me