How can I use the expanded value of a shell variable in the name of another variable?

Question

I'm starving trying to make the following thing work:

#!/bin/bash
MARCOMIN=1
MARCOMAX=3
ENZOMIN=1
ENZOMAX=3
GIOVANNIMIN=1
GIOVANNIMAX=3
VALUEMARCO=12
VALUEGIOVANNI=4
VALUEENZO=12

for i in MARCO ENZO GIOVANNI; do
        echo $$iMIN is $i\'s MIN
        echo $$iMAX is $i\'s MAX
        echo $VALUE$i is $i\'s VALUE
done

exit 0

It doesn't writes macro as I'd like, though.

me@myhost:~$ bash test.sh
20341iMIN is MARCO's MIN
20341iMAX is MARCO's MAX
MARCO is MARCO's VALUE
20341iMIN is ENZO's MIN
20341iMAX is ENZO's MAX
ENZO is ENZO's VALUE
20341iMIN is GIOVANNI's MIN
20341iMAX is GIOVANNI's MAX
GIOVANNI is GIOVANNI's VALUE

Obviously this is only one of my test. I tried many different ways of writing macros this way but I can't fix it. Found many similar questions here and on SO, but none matched my case.

bash -x test.sh follows:

+ MARCOMIN=1
+ MARCOMAX=3
+ ENZOMIN=1
+ ENZOMAX=3
+ GIOVANNIMIN=1
+ GIOVANNIMAX=3
+ VALUEMARCO=12
+ VALUEGIOVANNI=4
+ VALUEENZO=12
+ for i in MARCO ENZO GIOVANNI
+ echo 21466iMIN is 'MARCO'\''s' MIN
21466iMIN is MARCO's MIN
+ echo 21466iMAX is 'MARCO'\''s' MAX
21466iMAX is MARCO's MAX
+ echo MARCO is 'MARCO'\''s' VALUE
MARCO is MARCO's VALUE
+ for i in MARCO ENZO GIOVANNI
+ echo 21466iMIN is 'ENZO'\''s' MIN
21466iMIN is ENZO's MIN
+ echo 21466iMAX is 'ENZO'\''s' MAX
21466iMAX is ENZO's MAX
+ echo ENZO is 'ENZO'\''s' VALUE
ENZO is ENZO's VALUE
+ for i in MARCO ENZO GIOVANNI
+ echo 21466iMIN is 'GIOVANNI'\''s' MIN
21466iMIN is GIOVANNI's MIN
+ echo 21466iMAX is 'GIOVANNI'\''s' MAX
21466iMAX is GIOVANNI's MAX
+ echo GIOVANNI is 'GIOVANNI'\''s' VALUE
GIOVANNI is GIOVANNI's VALUE
+ exit 0

Answer 1

Bash supports variable indirection:

#!/bin/bash
MARCOMIN=1
MARCOMAX=3
ENZOMIN=1
ENZOMAX=3
GIOVANNIMIN=1
GIOVANNIMAX=3
VALUEMARCO=12
VALUEGIOVANNI=4
VALUEENZO=12

for i in MARCO ENZO GIOVANNI; do
    for j in MIN MAX VALUE; do
        varname="${i}${j}"
        # For VALUE we need reverse order of i and j
        [[ $j = VALUE ]] && varname="${j}${i}"

        printf "%d is %s's %s\n" "${!varname}" "$i" "$j"
    done
done

exit 0

I did a little extra with the inner loop and use of printf in place of echo but the central idea is that given a variable name foo stored in another variable bar you can get the value of $foo by doing ${!bar}.

More: https://stackoverflow.com/questions/8515411/what-is-indirect-expansion-what-does-var-mean

Answer 2

To use a variable expansion within the name of another variable is traditionally accomplished with the eval builtin shell command. This often leads to leaning toothpick syndrome (many backslashes and double escapes), which suggests that there may be a "better" way of accomplishing what you want to do. There are also security issues related to using the eval builtin.

In any case, this is one way to accomplish what you want:

# ...
    eval echo \$${i}MIN is \$i\\\'s MIN
    eval echo \$${i}MAX is \$i\\\'s MAX
    eval echo \$VALUE${i} is \$i\\\'s VALUE
# ...