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

I really liked Al P's answer but wanted uniqueness enforced cheaply so I took it one step further - use a directory. There are some obvious limitations (directory file limits, invalid file names) but it should work for most cases.

hinit() {
    rm -rf /tmp/hashmap.$1
    mkdir -p /tmp/hashmap.$1
}

hput() {
    printf "$3" > /tmp/hashmap.$1/$2
}

hget() {
    cat /tmp/hashmap.$1/$2
}

hkeys() {
    ls -1 /tmp/hashmap.$1
}

hdestroy() {
    rm -rf /tmp/hashmap.$1
}

hinit ids

for (( i = 0; i < 10000; i++ )); do
    hput ids "key$i" "value$i"
done

for (( i = 0; i < 10000; i++ )); do
    printf '%s\n' $(hget ids "key$i") > /dev/null
done

hdestroy ids

It also performs a tad bit better in my tests.

$ time bash hash.sh 
real    0m46.500s
user    0m16.767s
sys     0m51.473s

$ time bash dirhash.sh 
real    0m35.875s
user    0m8.002s
sys     0m24.666s

Just thought I'd pitch in. Cheers!

Edit: Adding hdestroy()

Answer 2

Prior to bash 4 there is no good way to use associative arrays in bash. Your best bet is to use an interpreted language that actually has support for such things, like awk. On the other hand, bash 4 does support them.

As for less good ways in bash 3, here is a reference than might help: http://mywiki.wooledge.org/BashFAQ/006

Answer 3

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.

Answer 4

hput () {
  eval hash"$1"='$2'
}

hget () {
  eval echo '${hash'"$1"'#hash}'
}
hput France Paris
hput Netherlands Amsterdam
hput Spain Madrid
echo `hget France` and `hget Netherlands` and `hget Spain`

---

$ sh hash.sh
Paris and Amsterdam and Madrid