Comments on: Win a book by debugging this shell script

By: Al

Al — Fri, 20 Jun 2008 13:13:01 +0000

The biggest problem I found with for is when dealing with space separated multi-word data input. “For” doesn’t know where the real data begins or ends.

My vote is for extending the subshell with ()’s giving you a chance to deal with vars. Good point. Thank you.

By: Bobby

Bobby — Sat, 14 Jun 2008 19:18:36 +0000

The problem is with the scope of the calls and the variable being set local withing the function. If the variable had been set in the script instead of the funciton the return value would have been 0.

By: Sulakshana

Sulakshana — Wed, 20 Feb 2008 12:49:58 +0000

The
local retVal = 1

and retVal inside the do while loop are treated differently by the shell.

whatever the loop does, the outer retVal has no effect.

So successful or unsuccesful, the function will exit with value =1.

By: Mangesh

Mangesh — Thu, 01 Nov 2007 18:18:08 +0000

This is problem of scope.
Scope of shell is not accessible inside subshell.
If we do not initialize or declare variable “retVal” before while loop (i.e. remove statement retVal=1 ), the script will run fine.

By: Adny

Adny — Tue, 16 Oct 2007 10:28:53 +0000

Exporting retVal won’t work – the subshell will have access to its copy of the parent’s variable, not to the parent’s variable itself

By: admin

admin — Mon, 15 Oct 2007 22:27:27 +0000

PAshaRome,

Yes, thanks for the answer. Do not worry about the english. Not everyone is going to get a copy, I’ll let you know.

Thanks.

By: admin

admin — Mon, 15 Oct 2007 22:24:16 +0000

dosnlinux,

Yes, I could have exported the value. However, it seems inelegant.

By: admin

admin — Mon, 15 Oct 2007 22:22:33 +0000

Jim,

Here’s a test that shows that the while loop is much faster. I am quite surprised…

$ ./whileForLoopTest.sh 55 31 $ ./whileForLoopTest.sh 54 30

Heres the script.

$ cat whileForLoopTest.sh #!/bin/bash run() { for((I=0; I < 500; I++)) do eval "$@" done }


forTest()

{

        for user in $( ps -e -o user )

        do

                :

        done

}
whileTest()

{

        ps -e -o user | \

        while read user

        do

                :

        done

}
start=$( date +%s )

run forTest

expr $( date +%s ) - $start

start=$( date +%s ) run whileTest expr $( date +%s ) - $start

By: PAshaRome

PAshaRome — Mon, 15 Oct 2007 22:05:44 +0000

Sorry for my eng.

There is a scope problem.

retVal inside the loop is not the same of retVal outside the loop.

Tnx. (I would like ti have a copy of book )

By: Jim

Jim — Mon, 15 Oct 2007 21:49:39 +0000

My understanding has always been that a for loop is better for iterating a definable list (which we can get with the grep statement in this example) than the more open ended while loop anyway.

I wonder if there is a way (with bash) to quantify the relative difference. It would seem, at first blush, that the while loop would take more allocated memory and might be slower.

The for loop probably has drawbacks as well.

Anybody have any numbers that they can share?