Absolute value of a number

Question

I want to take the absolute of a number by the following code in bash:

#!/bin/bash echo "Enter the first file name: " read first  echo "Enter the second file name: " read second  s1=$(stat --format=%s "$first") s2=$(stat -c '%s' "$second")  res= expr $s2 - $s1  if [ "$res" -lt 0 ] then         res=$res \* -1 fi  echo $res 

Now the problem I am facing is in the if statement, no matter what I changes it always goes in the if, I tried to put [[ ]] around the statement but nothing.

Here is the error:

./p6.sh: line 13: [: : integer expression expected 

Answer 1

You might just take ${var#-}.

${var#Pattern} Remove from $var the shortest part of $Pattern that matches the front end of $var. tdlp

---

Example:

s2=5; s1=4 s3=$((s1-s2))  echo $s3 -1  echo ${s3#-} 1 

Answer 2

$ s2=5 s1=4 $ echo $s2 $s1 5 4 $ res= expr $s2 - $s1 1 $ echo $res 

What's actually happening on the fourth line is that res is being set to nothing and exported for the expr command. Thus, when you run [ "$res" -lt 0 ] res is expanding to nothing and you see the error.

You could just use an arithmetic expression:

$ (( res=s2-s1 )) $ echo $res 1 

Arithmetic context guarantees the result will be an integer, so even if all your terms are undefined to begin with, you will get an integer result (namely zero).

$ (( res = whoknows - whocares )); echo $res 0 

Alternatively, you can tell the shell that res is an integer by declaring it as such:

$ declare -i res $ res=s2-s1 

The interesting thing here is that the right hand side of an assignment is treated in arithmetic context, so you don't need the $ for the expansions.