How to modify a global variable within a function and return a boolean in bash? [duplicate] - bash

Question

This question already has answers here:
How to return an array in bash without using globals?
(21 answers)
Closed 2 days ago.
I'm working with this:
GNU bash, version 4.4.20(1)-release (x86_64-pc-linux-gnu)
I have a script like below:
#!/bin/bash
map2=()
result=""
f() {
tmpA=(12 34 7844);
map2=("${tmpA[#]}");
echo true;
return;
}
result=$(f)
echo result=$result : array=${map2[#]}
Which returns:
result=true : array=
if I replace result=$(f) simply by f it returns:
result= : array=12 34 7844
I could not find the way to modify the global array but also get the return value.
Any idea on how to achieve this?

Answer 1

Any environment changes made inside $( ... ) are lost when it exits.
However, bash allows a way of changing arguments passed "by reference" by using the declare -n command:
#!/bin/bash
map2=()
result=""
f() {
declare -n resultRef=$1
declare -n map2Ref=$2
local tmpA=(12 34 7844)
map2Ref=("${tmpA[#]}")
map2Ref[4]=999
resultRef=true
echo during: resultRef=$resultRef : arrayRef=${map2Ref[*]} : tmpA=${tmpA[*]}
}
f result map2
echo after: resultRef=$resultRef : arrayRef=${map2Ref[*]} : tmpA=${tmpA[*]}
echo result=$result : array=${map2[*]}
Variables declared in this way behave like local - they are discarded when the function returns.
during: resultRef=true : arrayRef=12 34 7844 999 : tmpA=12 34 7844
after: resultRef= : arrayRef= : tmpA=
result=true : array=12 34 7844 999

Answer 2

Using command substitution (i.e., the $(f) construct), you are creating a subshell. Subshells inherit variables from their parent shells, but a subshell cannot modify its parent shell environment.
If you just call your function and check its exit code $? then you should be able to generate your desired output:
#!/bin/bash
map2=()
result="false"
my_func() {
tmpA=(12 34 7844)
map2=("${tmpA[#]}")
}
if my_func ; then
result="true"
fi
printf "result=%s : array=%s\n" "$result" "${map2[#]}"
Output:
result=true : array=12 34 7844
Note that you can use bash -x yourscript to enable xtrace for more debugging output and you can paste your script into https://www.shellcheck.net/ for help as well.