How to define hash tables in Bash?
There's parameter substitution, though it may be un-PC as well ...like indirection.
#!/bin/bash
# Array pretending to be a Pythonic dictionary
ARRAY=( "cow:moo"
"dinosaur:roar"
"bird:chirp"
"bash:rock" )
for animal in "${ARRAY[@]}" ; do
KEY="${animal%%:*}"
VALUE="${animal##*:}"
printf "%s likes to %s.\n" "$KEY" "$VALUE"
done
printf "%s is an extinct animal which likes to %s\n" "${ARRAY[1]%%:*}" "${ARRAY[1]##*:}"
The BASH 4 way is better of course, but if you need a hack ...only a hack will do. You could search the array/hash with similar techniques.
Bash 4
Bash 4 natively supports this feature. Make sure your script's hashbang is #!/usr/bin/env bash
or #!/bin/bash
so you don't end up using sh
. Make sure you're either executing your script directly, or execute script
with bash script
. (Not actually executing a Bash script with Bash does happen, and will be really confusing!)
You declare an associative array by doing:
declare -A animals
You can fill it up with elements using the normal array assignment operator. For example, if you want to have a map of animal[sound(key)] = animal(value)
:
animals=( ["moo"]="cow" ["woof"]="dog")
Or declare and instantiate in one line:
declare -A animals=( ["moo"]="cow" ["woof"]="dog")
Then use them just like normal arrays. Use
animals['key']='value'
to set value"${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.