Question

Help with shell script indirect variables

Asked by: TwentyFourSeven

Hi,

I'm having difficulty getting my head around indirect variables.

My goal in the code below is to set the "prog_mode" variable based upon the case test outcome.

#!/bin/sh 
echo_bin="/bin/echo"
prog_mode="" 
## YES_NO_INDIRECT 
yes_no_indirect()
{
	# Parameter 1 is the question
	# Parameter 2 is the indirect variable name
while true
	do
	$echo_bin "${1} (y/n)"
	read yesno
	case $yesno in
		Y|y)
			${!2}="TRUE"
			break
			;;
		N|n)
			${!2}="FALSE"
			break
			;;
		*)
			$echo_bin "Invalid selection."
		esac
done
} 
## MAIN
yes_no_indirect "some question here...." "prog_mode"
echo $prog_mode
                                  
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Asked On
2009-11-05 at 04:36:46ID24874103
Topics

Linux Programming

,

Bourne Shell (sh)

,

Bourne-Again Shell (bash)

Participating Experts
2
Points
0
Comments
8

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Answers

 

by: oocytePosted on 2009-11-05 at 05:38:48ID: 25749255

TwentyFourSeven, I have never heard about indirect variables, but I do know indirect variable reference, and that is what ${!var} notation means.  So i'm not sure if the following if what you are looking for.

The ${!var} notation is only available in bash shell.  So the first line have to be #!/bin/bash

So in the following:
$2=prog_mode

Then eval $2="TRUE" is effectively saying prog_mode="TRUE"
This method is available in all flavours of shell.

As for indirect variable reference, see example at bottom of script.

#!/bin/bash
echo_bin="/bin/echo"
prog_mode=""
## YES_NO_INDIRECT
yes_no_indirect()
{
        # Parameter 1 is the question
        # Parameter 2 is the indirect variable name 
        while true; do
                $echo_bin "${1} (y/n)"
                read yesno
                case $yesno in
                        Y|y)
                                eval ${2}="TRUE"
                                break
                                ;;
                        N|n)
                                eval ${2}="FALSE"
                                break
                                ;;
                        *)
                                $echo_bin "Invalid selection."
                                ;;
                esac
        done
} 
## MAIN
yes_no_indirect "some question here...." "prog_mode"
echo $prog_mode 
# Note the missing $ in front of prog_mode.
indirect_ref=prog_mode 
echo ${!indirect_ref}
~
                                              
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by: TwentyFourSevenPosted on 2009-11-05 at 05:48:45ID: 25749352

Hi oocyte,

Yes, I probably did mean indirect reference, oops !  ;-)

Is it possible to do something with eval if I've only got access to the Bourhe sh shell ?

 

by: TwentyFourSevenPosted on 2009-11-05 at 08:01:44ID: 25750846

self solved.

#!/bin/sh 
echo_bin="/bin/echo"
prog_mode="" 
## YES_NO_INDIRECT 
yes_no_indirect()
{
        # Parameter 1 is the question
        # Parameter 2 is the indirect variable name
while true
        do
        $echo_bin "${1} (y/n)"
        read yesno
        case $yesno in
                Y|y)
                        eval ${!2}="TRUE"
                        break
                        ;;
                N|n)
                        eval ${!2}="FALSE"
                        break
                        ;;
                *)
                        $echo_bin "Invalid selection."
                esac
done
} 
## MAIN
yes_no_indirect "some question here...." "prog_mode"
echo $prog_mode

                                              
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by: WanderJahrePosted on 2009-11-09 at 17:22:59ID: 25781705

Hi TwentyFourSeven,

Here's a version that actually works in the Bourne Shell. It also presents a slightly cleaner syntax consistent with your original snippet. Hope this helps.

#!/bin/sh 
echo_bin=/bin/echo
## YES_NO_INDIRECT 
yes_no_indirect()
{
        # Parameter 1 is the question
        # Parameter 2 is the indirect variable name
while :; do
        $echo_bin "$1 (y/n)"
        read yesno
        case $yesno in
                Y|y)
                        result=TRUE
                        break
                        ;;
                N|n)
                        result=FALSE
                        break
                        ;;
                *)
                        $echo_bin "Invalid selection."
                esac
done
eval "$2=$result"
} 
## MAIN
yes_no_indirect "some question here...." prog_mode
$echo_bin $prog_mode

                                              
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by: WanderJahrePosted on 2009-11-09 at 17:40:13ID: 25781771

Dear TwentyFourSeven,

Your solution does not appear to work. Running your code under Bourne shell yeilds the following error:

28: Bad substitution

running it under bash yields

line 15: =TRUE: command not found

The program changes that both oocyte and I offered you, however, do work under both Bourne and Bourne-again shells. The snippet written by oocyte will work in Bash simply by removing the last two lines and the changing the first line to #!/bin/sh.

 

by: TwentyFourSevenPosted on 2009-11-09 at 23:44:38ID: 25783175

Hmmm... let me go check my code, I'm fairly sure it does.

Will be back...

 

by: TwentyFourSevenPosted on 2009-11-09 at 23:46:30ID: 25783190

This code that I wrote myself definitley does.

## YES_NO_INDIRECT 
yes_no_indirect()
{
        # Parameter 1 is the question
        # Parameter 2 is the indirect variable name
while true
        do
        $echo_bin "${1} (y/n)"
        read yesno
        case $yesno in
                Y|y)
                        eval ${2}=1
                        break
                        ;;
                N|n)
                        eval ${2}=2
                        break
                        ;;
                *)
                        $echo_bin "Invalid Command"
                esac
done
}

                                              
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by: WanderJahrePosted on 2009-11-10 at 07:23:55ID: 25786239

Hi TwentyFourSeven

Notice that the code you posted as self solved (ID: 25750846) is different than the code you have just posted (ID: 25783190). In particular lines 12 & 16 (new code) vs lines 15 & 19 (old code). Your corrected code now does indeed work but what you posted as self-solved did not.

20120131-EE-VQP-002

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