Adding to Bash associative arrays inside functions

Question

I'm trying to use associative arrays as a work around for Bash's poor function parameter passing. I can declare a global associative array and read/write to that but I would like to have the variable name passed to the function since many times I want to use the same function with different parameter blocks.

Various Stack Overflow posts have approaches for reading a passed array within a function but not writing to it to allow return values. Pseudo Bash for what I'm trying to do is thus:

TestFunc() {
    local __PARMBLOCK__=${1} # Tried ${!1} as well

    # Do something with incoming array

    __PARMBLOCK__[__rc__]+=1 # Error occured
    __PARMBLOCK__[__error__]+="Error in TestFunc"
}

declare -A FUNCPARM

# Populate FUNCPARM
TestFunc FUNCPARM
if [[ ${FUNCPARM[__rc__]} -ne 0 ]]; then
    echo "ERROR : ${FUNCPARM[__error__]}
fi

Is this kind of thing possible or do I really need to abandon Bash for something like Python?

Did you have any problems with this answer for the question How to pass an associative array as argument to a function in Bash? — Socowi
– Socowi, Commented Jun 9, 2020 at 8:24

Socowi · Accepted Answer · 2020-06-09 08:22:00Z

1

EDIT: Found the duplicate. This is basically the same answer as this one.

You can use a reference variable for that, see help declare:

declare [-aAfFgilnrtux] [-p] [name[=value] ...]
[...]
-n make NAME a reference to the variable named by its value
[...]
When used in a function, declare makes NAMEs local, as with the local command.

f() {
  declare -n paramblock="$1"
  # example for reading (print all keys and entries)
  paste <(printf %s\\n "${!paramblock[@]}") <(printf %s\\n "${paramblock[@]}")
  # example for writing
  paramblock["key 1"]="changed"
  paramblock["new key"]="new output"
}

Example usage:

$ declare -A a=(["key 1"]="input 1" ["key 2"]="input 2")
$ f a
key 2   input 2
key 1   input 1
$ declare -p a
declare -A a=(["key 2"]="input 2" ["key 1"]="changed" ["new key"]="new output" )

This works very well. The only difference to an actual associative array I found so far is, that you cannot print the referenced array using declare -p as that will only show the reference.

answered Jun 9, 2020 at 8:22

Socowi

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2 Comments

Jim Hudd Over a year ago

Perfect! This is exactly what I was looking for. Curiously, in the man page on my Linux system it says "-n" cannot be applied to arrays. I guess that must only refer to non-associative arrays or the manual page is wrong! I don't see the issue of "declare -p" not working being a problem for me.

Socowi Over a year ago

@JimHudd At first "cannot be applied to arrays" stumped me too. However, this means that the reference itself cannot be an array. Still, the referenced variables can be arrays and associative arrays. Example: a=1; b=2; declare -n ref=(a b) fails. a=(1 2); declare -n ref=a works.

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