Shell Programming and Scripting

Hi all
I have a small doubt. Can we do an if compare using if (( )). In what conditions will it fail.
Regards,
Ranjith

(( )) can be used if you are doing numeric comparisons. It will not work well with the String comparison for which you have to use [[ ]].

An Example

Code:

/export/home/test/mons>if (( 10 == 10 ))
> then
> echo "Equal"
> else
> echo "Not equal"
> fi
Equal

Code:

/export/home/test/mons>if (( "hai" == "hai" ))
> then
> echo "Equal"
> else
> echo "Not equal"
> fi
ksh:  hai == hai : bad number

Code:

/export/home/test/mons>if [[ "hai" == "hai" ]] 
> then
> echo "Equal"
> else
> echo "Not equal"
> fi
Equal

If the expression within (( )) comes out to be a 0, then the if fails.

Code:

[/tmp]$ cat test.sh
#! /bin/ksh

L=0
if ((L=L+1)) ; then
echo $L
else
echo "Failed"
fi; 
if ((L=L-1)) ; then
echo $L
else
echo "Failed"
fi; 
[/tmp]$ ./test.sh
1
Failed
[/tmp]$

I have a small script flow like this
#! /bin/ksh
testscpt.sh
pgmcc=$?
if (( $pgmcc -ne 0 && $pgmcc -ne 99 ))
then
echo "Correct condition check"
else
echo "Failed"
fi

FIrst, I am getting syntax error with the if condition. Second, the testscpt.sh always returns 0. What will happen to this check if the testscpt.sh returns value other than 0 or 99.

Should I use '!=' instead of '-ne' when I use (( )). Is it like C coding.
Regards,
Ranjith

First, how can you get two exit values from a single running of the script ?
if (( $pgmcc -ne 0 && $pgmcc -ne 99 )) should be
if (( $pgmcc -ne 0 || $pgmcc -ne 99 ))

I think you should change ((expr)) to [[expr]] to satisfy your requirements.

THe testscpt.sh returns multiple return codes based on the error conditions. But it returns only one code at any particular time. We had a problem with the script a few days back and on analysing found the issue due to this check. THe script shown above is a small representation of the the actual code.
Normally, the testscpt.sh always returns 0, but a few days back, it failed due to DB issues and returned 1. The if condition should have caught that, but it failed and it led to lot of processing issues. We are currently planning to make changes to this part of the code.
Even, we are not sure why the '&&' has been included. Is it due to the way (( )) evaluates its expression.
Thanks for your inputs. We were lost/perplexed from the morning.