How to define hash tables in Bash?

Question

What is the equivalent of Python dictionaries but in Bash (should work across OS X and Linux).

Answer 1

Bash 4

Bash 4 natively supports this feature. Make sure your script's hashbang is #!/usr/bin/env bash or #!/bin/bash so you don't end up using sh. Make sure you're either executing your script directly, or execute script with bash script. (Not actually executing a Bash script with Bash does happen, and will be really confusing!)

You declare an associative array by doing:

declare -A animals

You can fill it up with elements using the normal array assignment operator. For example, if you want to have a map of animal[sound(key)] = animal(value):

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

Or merge them:

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

Then use them just like normal arrays. Use

• animals['key']='value' to set value

• "${animals[@]}" to expand the values

• "${!animals[@]}" (notice the !) to expand 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. Avoid eval like the plague, because it is the plague of shell scripting. The most important reason is that eval treats your data as executable code (there are many other reasons too).

First and foremost: Consider upgrading to bash 4. This will make the whole process much easier for you.

If there's a reason you can't upgrade, declare is a far safer option. It does not evaluate data as bash code like eval does, and as such does not allow arbitrary code injection quite so easily.

Let's prepare the answer by introducing the concepts:

First, indirection.

$ 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 can use associative arrays in the first place, avoiding this workaround.)

Summary:

• Upgrade to bash 4 and use declare -A for associative arrays.
• Use the declare option if you can't upgrade.
• Consider using awk instead and avoid the issue altogether.