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

Question

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?

See Access variables set inside command substitutions.

pjh
– pjh

2023-02-18 14:22:16 +00:00
Commented Feb 18, 2023 at 14:22 — pjh
– pjh, Commented Feb 18, 2023 at 14:22

jhnc · Accepted Answer · 2023-02-18 15:02:54Z

3

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

edited Feb 18, 2023 at 15:02

answered Feb 18, 2023 at 14:31

jhnc

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2 Comments

Walter A Over a year ago

+1. I was not aware this is possible: f() { declare -n b=$1; b=100; }; a=2; f a; echo "$a" (and you get 100).

jhnc Over a year ago

@WalterA Unfortunately the implementation is not very robust. If one needs to pass complicated data structures around, switching to a "proper" language may be sensible.

j_b · Accepted Answer · 2023-02-18 14:54:40Z

3

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.

edited Feb 18, 2023 at 14:54

answered Feb 18, 2023 at 14:32

j_b

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