How to define hash tables in bash?

Bash 4

Bash 4 natively supports this feature. Make sure your script's hashbang is #!/usr/bin/env bash or#!/bin/bash or anything else that references bash and not sh. Make sure you're executing your script, and not doing something silly like sh script which would cause your bash hashbang to be ignored. This is basic stuff, but so many keep failing at it, hence the re-iteration.

You declare an associative array by doing:

declare -A animals

You can fill it up with elements using the normal array assignment operator:

animals=( ["moo"]="cow" )

Or merge them:

declare -A animals=( ["moo"]="cow" )

Then use them just like normal arrays. "${animals[@]}" expands the values, "${!animals[@]}"(notice the !) expands the keys. Don't forget to quote them:

echo "${animals["moo"]}"
for sound in "${!animals[@]}"; do echo "$sound - ${animals["$sound"]}"; done

Bash 3

Before bash 4, you don't have associative arrays. Do not use eval to emulate them. You must avoid eval like the plague, because it is the plague of shell scripting. The most important reason is that you don't want to treat your data as executable code (there are many other reasons too).

First and foremost: Just consider upgrading to bash 4. Seriously. The future is now, stop living in the past and suffering from it by forcing stupid broken and ugly hacks on your code and every poor soul stuck maintaining it.

If you have some silly excuse why you "can't upgrade", declare is a far safer option. It does not evaluate data as bash code like eval does, and as such it does not allow arbitrary code injection quite so easily.

Let's prepare the answer by introducing the concepts:

First, indirection (seriously; never use this unless you're mentally ill or have some other bad excuse for writing hacks).

$ animals_moo=cow; sound=moo; i="animals_$sound"; echo "${!i}"
cow

Secondly, declare:

$ sound=moo; animal=cow; declare "animals_$sound=$animal"; echo "$animals_moo"
cow

Bring them together:

# Set a value:
declare "array_$index=$value"

# Get a value:
arrayGet() { 
    local array=$1 index=$2
    local i="${array}_$index"
    printf '%s' "${!i}"
}

Let's use it:

$ sound=moo
$ animal=cow
$ declare "animals_$sound=$animal"
$ arrayGet animals "$sound"
cow

Note: declare cannot be put in a function. Any use of declare inside a bash function turns the variable it creates local to the scope of that function, meaning we can't access or modify global arrays with it. (In bash 4 you can use declare -g to declare global variables - but in bash 4, you should be using associative arrays in the first place, not this hack.)

Summary

Upgrade to bash 4 and use declare -A. If you can't, consider switching entirely to awk before doing ugly hacks as described above. And definitely stay the heck away from eval hackery.

http://stackoverflow.com/questions/1494178/how-to-define-hash-tables-in-bash