Map unknown associative array element

Question

How can I map an unknown value alongside other known values in bash using an associative array, so that:

#!/bin/bash

array=("foo" "foo" "bar" "something else" "also something else" "bar")

declare -A map
map["foo"]="1"
map["bar"]="2"
map[unknown]="?"

for KEY in "${array[@]}"; do
   echo ${map[$KEY]}
done

outputs:

I do not know in which index my element is located, nor the length of the array to be mapped. I have tried doing map[${array[@]}]="?" before setting the other values, but this does not work

If KEY is set to foo and map["foo"]="" then what should echo ${map[$KEY]} output - a) an empty string or b) the character ?? — Ed Morton
– Ed Morton, Commented Nov 17 at 0:26

F. Hauri - Give Up GitHub · Accepted Answer · 2025-11-14 16:44:32Z

Default value

See Parameter Expansion in bash man page:

man -Pless\ +/parameter:-word bash

  ${parameter:-word}
       Use  Default  Values.  If parameter is unset or null, the expan‐
       sion of word is substituted.  Otherwise, the value of  parameter
       is substituted.

So

#!/bin/bash

array=("foo" "foo" "bar" "something else" "also something else" "bar")

declare -A map
map["foo"]="1"
map["bar"]="2"

for key in "${array[@]}";do
    printf '%-20s -> %s\n' "$key" "${map["$key"]:-?}"
done

Will produce:

foo                  -> 1
foo                  -> 1
bar                  -> 2
something else       -> ?
also something else  -> ?
bar                  -> 2

Default to another variable

But, if you really want to use map[unknown], you could:

#!/bin/bash

array=("foo" "foo" "bar" "something else" "also something else" "bar")

declare -A map="( ["foo"]="1"  ["bar"]="2"  ["unknown"]="?" )"

for key in "${array[@]}";do
    printf '%-20s -> %s\n' "$key" "${map["$key"]:-${map["unknown"]}}"
done

This will produce same output!

Note: you could even use "default to variable, with default":

echo "${map["$key"]:-${map["unknown"]:-?}}"

This will print

content of map["$key"], or if not exist,
- content of map["unknown"], or if not exist,
  - a single interrogation mark: ?.

You could `map` your array, using `printf`

array=("foo" "foo" "bar" "something else" "also something else" "bar")
declare -A map="( ["foo"]="1"  ["bar"]="2"  ["unknown"]="?" )"

printf -v mapStr '"${map["%s"]:-${map["unknown"]}}" ' "${array[@]}"
declare -a "mappedArray=($mapStr)"
echo "${mappedArray[*]@Q}"

'1' '1' '2' '?' '?' '2'

Then

printf -v mapStr '%-20s -> %%s\n' "${array[@]}"
printf "$mapStr" "${mappedArray[@]}"

foo                  -> 1
foo                  -> 1
bar                  -> 2
something else       -> ?
also something else  -> ?
bar                  -> 2

But using this way could lead to security issues!
See Warning: limitation at end of Array elements in sed operation

Gordon Davisson · Accepted Answer · 2025-11-14 06:03:37Z

3

You can use bash's alternate value expansion feature to accomplish this, e.g. echo "${map[$KEY]-?}". The - indicates that the alternate value ("?" in this case) should be used if the element is not set. If you use :- instead, it'd use the alternate value if the element is unset or set to the null string.

Note: I also added double-quotes around the variable expansion, which is just good scripting hygiene. Without that, the ? might be expanded to a list of single-character filenames in the current directory. shellcheck.net is good at spotting common mistakes like this, so I recommend running your scripts past it and following its suggestions.

answered Nov 14 at 6:03

Gordon Davisson

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Comments

Ed Morton · Accepted Answer · 2025-11-14 16:08:12Z

Don't create fake array elements to handle this, just test for the key being present and use the associated value if so, otherwise use ?:

$ cat tst.sh
#!/usr/bin/env bash

array=("foo" "foo" "bar" "something else" "also something else" "bar")

declare -A map
map["foo"]="1"
map["bar"]="2"

mapval() {
    local idx=$1 val='?'
    [[ -v map[$idx] ]] && val="${map[$idx]}"
    printf '%s' "$val"
}

for key in "${array[@]}"; do
    echo "$key -> $(mapval "$key")"
done

$ ./tst.sh
foo -> 1
foo -> 1
bar -> 2
something else -> ?
also something else -> ?
bar -> 2

or a little more efficiently:

$ cat tst.sh
#!/usr/bin/env bash

array=("foo" "foo" "bar" "something else" "also something else" "bar")

declare -A map
map["foo"]="1"
map["bar"]="2"

mapval() {
    local idx=$1
    local -n _val=$2
    if [[ -v map[$idx] ]]; then
        _val="${map[$idx]}"
    else
        _val='?'
    fi
}

for key in "${array[@]}"; do
    mapval "$key" val
    echo "$key -> $val"
done

$ ./tst.sh
foo -> 1
foo -> 1
bar -> 2
something else -> ?
also something else -> ?
bar -> 2

jared_mamrot · Accepted Answer · 2025-11-14 04:22:35Z

2

One potential option is to conditionally fill in the associative array, e.g. using bash v4.4+:

#!/bin/bash
array=("foo" "foo" "bar" "something else" "also something else" "bar")

declare -A map

for KEY in "${array[@]}"
do
    if [ "$KEY" == "foo" ]; then
        map["$KEY"]="1"
    elif [ "$KEY" == "bar" ]; then
        map["$KEY"]="2"
    else
        map["$KEY"]="?"
    fi
done

for KEY in "${array[@]}"
do
    echo ${map[$KEY]}
done
# 1
# 1
# 2
# ?
# ?
# 2

Would that suit your use-case?

answered Nov 14 at 4:22

jared_mamrot

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Map unknown associative array element

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Default value

Default to another variable

You could `map` your array, using `printf`

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You could map your array, using printf

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You could `map` your array, using `printf`