Substitute a string variable into the name of an array

Question

I am trying to substitute a string variable (an array here), but I am getting an error. Could anyone suggest how to solve this?

COMP="MY"

MY_common_sections_to_fix=( \
      ".rodata" \
      ".data" \
      )

 echo ${${COMP}_common_sections_to_fix[@]}

ERROR:

${${COMP}_common_sections_to_fix[@]}: bad substitution

Which version of bash? There's a convenient builtin available in 4.4. — Charles Duffy
– Charles Duffy, Commented Jun 21, 2017 at 0:43
Since the array you initialized is MY_common_sections_to_fix but you are trying to access L1CC_common_sections_to_fix, that's one level of non-surprise. However, the dynamic naming of variables is also problematic. You could create a variable holding MY_common_sections_to_fix and work out how to exploit ${!varname[@]} or thereabouts — untested, but ${!var} references the variable with the name stored in $var. (Semi-tested: the results do not look promising. Assume that it won't work until further notice!) — Jonathan Leffler
– Jonathan Leffler, Commented Jun 21, 2017 at 0:45
Further notice: it won't work — see shell parameter expansion in the Bash manual. It looks like you can't do indirect expansion of array names. — Jonathan Leffler
– Jonathan Leffler, Commented Jun 21, 2017 at 0:54
BashFAQ #6 goes into which tricks for indirect reference are available on which shells, and which apply to arrays. Note the caveats given attached to the last example in the section as an example of how error-prone doing this in a shell without the pertinent features can be. — Charles Duffy
– Charles Duffy, Commented Jun 21, 2017 at 0:54

Charles Duffy · Accepted Answer · 2017-06-21 00:48:42Z

2

What you're trying to do is indirectly reference an array.

A nameref, if you have a new enough shell (4.3 or later), is the most appropriate tool for the job:

COMP=MY
MY_common_sections_to_fix=( .rodata .data )
declare -n active_sections=${COMP}_common_sections_to_fix
printf '%s\n' "${active_sections[@]}"

This makes active_sections an alias for MY_common_sections_to_fix, and properly emits .rodata and .data as output.

If you don't have bash 4.3 available, hackery with eval is an available (albeit unfortunate) option:

printf -v cmd '%q=( "${%q[@]}" )' active_sections "${COMP}_common_sections_to_fix"
eval "$cmd"

edited Jun 21, 2017 at 0:48

answered Jun 21, 2017 at 0:45

Charles Duffy

299k43 gold badges441 silver badges496 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Charles Duffy Over a year ago

Eh, they used "MY" in the variable name in the question. They use both values, that means either is equally correct -- one or the other needs to be modified to generate a working example.

l'L'l Over a year ago

I thought they were trying to make it L1CC_common_sections_to_fix, but maybe not. The first command works with 3.x if I take out the -n option.

Charles Duffy Over a year ago

@l'L'l, eh? In 3.x, I get declare -- active_sections="MY_common_sections_to_fix" as the result if the -n is removed -- meaning only the name of the target variable, and not its value, is available.

l'L'l Over a year ago

Actually I have no clue what the OP is trying to do, so +1 for trying to make any sense from it.

Collectives™ on Stack Overflow

Substitute a string variable into the name of an array

1 Answer 1

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related