bash case stmt - pattern not matching

Question

This conundrum is stumping me. Why can't bash's case stmt match the pattern variable and properly assign the array's index value to the command variable?

Script Code:

#!/usr/bin/env bash

function run() {
    local       command SearchPattern="a2522866d08d848dd99b5997ad08eb936f68ce91"
    local -A    shaCommands=(
                    ['md5sum']=32
                    ['sha1sum']=40
                    ['sha224sum']=56
                    ['sha256sum']=64
                    ['sha384sum']=96
                    ['sha512sum']=128
                )

    printf "\${#SearchPattern}=[%s]\n" "${#SearchPattern}"
    declare -p shaCommands
    case ${#SearchPattern} in
        ${shaCommands['md5sum']} ) command="${!shaCommands['md5sum']}" ;;
        ${shaCommands['sha1sum']} ) command="${!shaCommands['sha1sum']}" ;;
        ${shaCommands['sha224sum']} ) command="${!shaCommands['sha224sum']}" ;;
        ${shaCommands['sha256sum']} ) command="${!shaCommands['sha256sum']}" ;;
        ${shaCommands['sha384sum']} ) command="${!shaCommands['sha384sum']}" ;;
        ${shaCommands['sha512sum']} ) command="${!shaCommands['sha512sum']}" ;;
        * ) printf "unrecognized checksum length=[%d] for [%s]\n" \
                "${#SearchPattern}" "${SearchPattern}"
            return $Fail
        ;;
    esac
    printf "command=[%s]\n" "${command}"
}

run

The output shows the case patterns are never matched - it should match on the ${shaCommands['sha1sum']} ) pattern - so $command is never assigned ...

${#SearchPattern}=[40]
declare -A shaCommands=([sha256sum]="64" [sha512sum]="128" [sha1sum]="40" [sha384sum]="96" [md5sum]="32" [sha224sum]="56" )
command=[]

Answer 1

The case statement matches (you can confirm with set -x), but the assignment to command after the match is empty. "${!shaCommands['sha1sum']}" does not get the index of the array element like you want it to, because that expansion to get the indices of the array only works for ${!name[@]} or ${!name[*]} according to the manual.

${!name[@]}
${!name[*]}
If name is an array variable, expands to the list of array indices (keys) assigned in name. 
If name is not an array, expands to 0 if name is set and null otherwise. 
When ‘@’ is used and the expansion appears within double quotes, each key 
expands to a separate word.

In other cases, ${!var} is an indirect reference (see Parameter Expansion) where the value of var is used as the name of a variable to reference. ${shaCommands['sha1sum']} has a value of 40, so ${!shaCommands['sha1sum']} is interpreted as ${40} which is not set in your command, so you get an empty string from the expansion.

You can simply use command=sha1sum instead. You're already hardcoding the key that you use to access the array, so your attempted solution isn't really any more robust than just hardcoding the command value.