嵌套的for循环和变量命名

Question

I am trying to use a for loop to name my variables in the following manner:

for (( a=1; a<=10; a++ )); do
    for (( b=1; b<=100; b++ )); do
        line[$a]position[$b]="This is LINE $a at POSITION $b."
        eval echo "${line[a]position[b]}"
    done
done

Is this possible and my code is just wrong?

Answer 1

No, this is not possible the way you try it. First of all, [] are not allowed in names (see definitions in the manual), so you'd have to use a name like line${a}position$b .

One ( mostly frowned upon ) option is to use eval :

#!/bin/bash

for (( a=1; a<=2; a++ )); do
    for (( b=1; b<=2; b++ )); do
        varname=line${a}position$b
        eval $varname=\"This is LINE \$a at POSITION \$b.\"
        echo "${!varname}"
    done
done

This first creates the string to be used as the variable name, varname . It becomes, for each loop, line1position1 , line1position2 and so on.

Then, on this line, we assign your string to the variable with that name:

eval $varname=\"This is LINE \$a at POSITION \$b.\"

The quoting is such that the string expands to, for example for the first loop,

line1position1="This is LINE $a at POSITION $b."

and the value of line1position1 becomes This is LINE 1 at POSITION 1. .

To access the value of this variable, we have to use indirect expansion : the ! in ${!varname} indicates that varname is not the variable name we want, but expands to it.

This all being said, Bash 4.3 introduced namerefs , which allow to solve this much more elegantly:

#!/bin/bash

for (( a=1; a<=2; a++ )); do
    for (( b=1; b<=2; b++ )); do
        declare -n refvar=line${a}position$b
        refvar="This is LINE $a at POSITION $b."
        echo "$refvar"
    done
done

No eval required here.

The output for both solutions is this:

This is LINE 1 at POSITION 1.
This is LINE 1 at POSITION 2.
This is LINE 2 at POSITION 1.
This is LINE 2 at POSITION 2.