Variable scope in KSH v5.2.14 99/07/13.2

UltramaticOrange · 08-19-2008, 05:15 PM

So, I've a situation like the following and I'm looking for a work-around to get expected output.

The real-world version of this script does necessitate the use of "while read..." instead of for loops.

sample code:

Code:

#!/bin/ksh

myvar="hello there, world "

echo -e "1\n2\n3\n4" | while read "f1"
do
        echo -e "1\n2\n3\n4" | while read "f2"
        do
                echo -e "1\n2\n3\n4" | while read "f3"
                do
                        myvar="$myvar $f1"
                        echo $myvar
                done
                echo $myvar
                exit 0
                echo "I do not print"
        done
        echo "I still print"
done

Actual output:

Code:

hello there, world 1
hello there, world 1 1
hello there, world 1 1 1
hello there, world 1 1 1 1
hello there, world
I still print
hello there, world 2
hello there, world 2 2
hello there, world 2 2 2
hello there, world 2 2 2 2
hello there, world
I still print
hello there, world 3
hello there, world 3 3
hello there, world 3 3 3
hello there, world 3 3 3 3
hello there, world
I still print
hello there, world 4
hello there, world 4 4
hello there, world 4 4 4
hello there, world 4 4 4 4
hello there, world
I still print

EXPECTED output (or "The output under KSH 'Version M 1993-12-28 s+'"):

Code:

hello there, world 1
hello there, world 1 1
hello there, world 1 1 1
hello there, world 1 1 1 1
hello there, world 1 1 1 1

Mr. C. · 08-19-2008, 05:51 PM

Perhaps this explains:

Quote:

Note: Some shells (but not this one) execute control structure commands
in a subshell when one or more of their file descriptors are redi-
rected, so any environment changes inside them may fail. To be porta-
ble, the exec statement should be used instead to redirect file
descriptors before the control structure.

UltramaticOrange · 08-19-2008, 07:04 PM

Ahhha. Souds likely. When I'm back at work I'll try leading with a dot+space and see if I can't convince it to run the loop within the current ksh instance.

What was your source? In the man page someplace?

Mr. C. · 08-19-2008, 07:53 PM

My ksh man page.

That while and other redirected control structures (sometimes) run in a sub-shell is an old sh implementation artifact from years gone by. Later shell implementations duplicated this brain-dead behavior to preserve compatibility.

The tip is indicating that you should use exec to setup redirections. I don't recall if/how you get around a pipe in ksh.

UltramaticOrange · 08-20-2008, 10:41 AM

Now that I understand the problem I was able to do a lot of research (see "resources" below) I was able to work out a solution that works for me that doesn't involve creating a temporary file. I'm surprised that this isn't one of the solutions offered up in everything that I've found.

Something to keep in mind is that the only information that I care about is the output from the inner most loop.

For anyone who comes along after me and tries this it is important that you only echo the data you care about (which is $myvar in the example)

Code:

#!/bin/ksh

myvar="hello there, world "

echo -e "1\n2\n3\n4" | while read "f1"
do
        echo -e "1\n2\n3\n4" | while read "f2"
        do
                echo -e "1\n2\n3\n4" | while read "f3"
                do
                        myvar="$myvar $f1"
                        echo "$myvar"
                done | awk -F"string not in data" '{print $1}' | tail -n 1
                exit 0
        done | awk -F"string not in data" '{print $1}' | tail -n 1
done | awk -F"string not in data" '{print $1}' | tail -n 1

To capture the output of these nested loops, you can add a back-tick (`) before and after the outermost loop:

Code:

#!/bin/ksh

myvar="hello there, world "

myvar=`echo -e "1\n2\n3\n4" | while read "f1"
do
        echo -e "1\n2\n3\n4" | while read "f2"
        do
                echo -e "1\n2\n3\n4" | while read "f3"
                do
                        myvar="$myvar $f1"
                        echo "$myvar"
                done | awk -F"string not in data" '{print $1}' | tail -n 1
                exit 0
                #echo "I do not print"
        done | awk -F"string not in data" '{print $1}' | tail -n 1
        #echo "I still print"
done | awk -F"string not in data" '{print $1}' | tail -n 1`

echo "$myvar"

output:

Code:

hello there, world  4 4 4 4

resources:
http://www.faqs.org/faqs/unix-faq/fa...section-8.html
http://bashcurescancer.com/win-a-boo...ll-script.html
http://wooledge.org:8000/BashFAQ#hea...b6db59946ee451