echoing a variable with a variable in its name?

Speedy2k · 12-29-2011, 10:07 PM

I am building a script right now with variables with variable in their name created in a while loop. i would like to be able to output an invremental variable with echo like this: echo $RC$COUNT but i don't find the correct way to do it?

Code:

#!/bin/bash
#NOMBRE DE RELAIS
NR=8
#NOMBRE D'ENTREES NUMERIQUES
NIN=8
#NOMBRE D'ENTREES ANALOGIQUES
NIA=2
#NE PAS EDITER EN BAS DE CECI
#LES VALEURS DES RELAIS VONT ETRES STOCKER DANS LES VARIABLES: Rx
#LES VALEURS DES COMPTEURS DES RELAIS VONT ETRE STOCKER DANS LES VARIABLES: RCx
#LES VALEURS DES ENTREES NUMERIQUES VONT ETRES STOCKER DANS LES VARIABLES: INx
#LES VALEURS DES COMPTEURS DES ENTREES NUMERIQUES VONT ETRES STOCKER DANS LES VARIABLES: INCx
#LES VALEURS DES ENTREES ANALOGIQUES VONT ETERS STOCKER DANS LES VARIABLES: IAx
####################################################
#On va donner la valeur "" au 8 RELAIS et la valeur "0" au 8 compteurs
COUNT=1
while [ $COUNT -le $NR ]
do
	eval R${COUNT}=""
	eval RC${COUNT}="0"
	COUNT=$(( $COUNT + 1 ))
        echo $RC$COUNT
done

Telengard · 12-30-2011, 12:59 AM

I'm not sure what you want to achieve, but here are my two best guesses.

arrays

Code:

$ fruit[0]="apple"; fruit[1]="orange"
$ i=0; while [ $i -le 1 ]; do echo ${fruit[$i]}; i=$(( $i + 1 )); done
apple
orange
$

indirect expansion

Code:

$ animal="horse"; color="brown"
$ for i in animal color; do echo ${!i}; done
horse
brown
$

HTH

Speedy2k · 12-30-2011, 07:45 AM

I don't really understand how to achieve that with an array or even if i am explaining myself correctly, but what i achieve to do is echoing a incremental variable into a loop.
So here it will loop until count equal 8. So in the first loop i want to echo $RC1 in this case will be zero on all 8, but on the next loop it will echo the value of $RC2 etc. in this case will always be 0.

David the H. · 12-30-2011, 12:13 PM

Everything you describe is just screaming for arrays.

Arrays are one of the most useful, and least used, features of the modern shell. So take some time to learn how to use them. Seriously. They're very simple really, just a linked list of variables, all with the same name, but with individual index numbers to differentiate the individual entries.

Basically, instead of RC1, RC2, etc. you use RC[1],RC[2], etc. (actually the count is zero-based, so you'd generally be starting with [0]).

http://mywiki.wooledge.org/BashGuide/Arrays

Your loop above could be written like this, for example (also using a c-style for loop for counting instead):

Code:

for (( i=1;i<=8;i++ )); do			#Loops through the numbers 1-8
	R[i]=""					#sets the R array element with the value of $i to empty
	RC[i]="0"				#sets the RC array element with the value of $i to 0
	echo "The value of RC[$i] is ${RC[i]}"	#echo back the current array element
done

echo "${RC[@]}"		#to list all entries at once.

I highly suggest you read the whole BashGuide, from the link I gave. It gives good, clear explanations of all the fundamental concepts you should understand for bash scripting.

As Telengard also pointed out, there are other techniques for indirect variable use, but they are generally unnecessary when you have arrays available.

http://mywiki.wooledge.org/BashFAQ/006

rknichols · 12-31-2011, 11:47 AM

I agree that what you are doing cries out for arrays, but to directly answer your question, you need to use the 'eval' built-in command to re-parse the line:

Code:

XX21=something
BASE=XX
COUNT=21
eval echo \$$BASE$COUNT

Use of 'eval' can get very messy very quickly because any character special to the shell is getting parsed twice, so you sometimes end up needing a lot of backslash characters (and perhaps some minor cursing and use of "set -x" to see what the heck is happening) to get the final effect right.

David the H. · 12-31-2011, 01:01 PM

eval also has potentially serious security implications, and for the same reason--it executes the line twice. Unless you know exactly what the input values will be, and exactly what happens when the line is eval'd, you run the danger of bad things happening (either accidentally or by design).

http://mywiki.wooledge.org/BashFAQ/048

My position is, basically, do not use eval. Of course I can't add ever because there are times when it genuinely is the only solution. But 99% of the time as soon as you find yourself typing "e..v..a...", it's time to stop what you're doing and reevaluate the whole process instead. There are almost certainly other, better ways to do what you want.