Bash array which contains environment variables to be used as function arguments

Question

I am trying to store some function parameters in a bash array, but some of them are defined from the content of environment variables and they do not actually "expand" to their actual values (I hope to be clear enough):

Example:

fileprefix='20200222_*'
test=1
args=( '-F' )
[[ "${test}" == 1 ]] && args+=( '-la "${fileprefix}"' )

But when executing it it raises an error:

$ ls "${args[@]}"

ls: invalid option -- ' '
Try 'ls --help' for more information.

and:

$ echo "${args[@]}"

-F -la "${fileprefix}"

but:

$ echo "${fileprefix}"
20200222_*

I would expect:

$ ls "${args[@]}"

-rw-r--r-- 1 user user 0 Jul  4 19:37 20200222_a.txt
-rw-r--r-- 1 user user 0 Jul  4 19:38 20200222_b.txt
-rw-r--r-- 1 user user 0 Jul  4 19:39 20200222_c.txt
-rw-r--r-- 1 user user 0 Jul  4 19:40 20200222_d.txt

Hence my question; how would you "expand" the content of the ${fileprefix} variable when executing the ls command with the array of parameters?

I didn't found something interesting here: https://tldp.org/LDP/Bash-Beginners-Guide/html/sect_10_02.html

Answer 1

... args+=( '-la "${fileprefix}"' )

Here, '-la "${fileprefix}"' is quoted, so it's stored as one element of the array, with the space, quotes, dollar sign, braces and everything. The ${fileprefix} isn't expanded, because the string is in single quotes. ls eventually gets that as one complete argument, and then complains about the space not being a valid option character.

Since you want ${fileprefix} to be expanded and used as a filename pattern / glob, you need to leave it unquoted. (It will also be word-split, which will matter if it ever contains whitespace (or you change IFS to something other than the default).) Also, you need to make -la a distinct element.

So:

args+=( -la ${fileprefix} )

(Note that echo "${args[@]}" is not a good way to look at an array: echo will print all arguments it gets joined by spaces, so it's hard to tell one array element with a space from two without, e.g. foo bar vs. foo and bar. printf ":%s\n" "${array[@]}" would be mildly better, at least until your data starts to contain newlines too, or the array can be empty.)