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.

You can further modify the hput()/hget() interface so that you have named hashes as follows:

hput() {
    eval "$1""$2"='$3'
}

hget() {
    eval echo '${'"$1$2"'#hash}'
}

and then

hput capitals France Paris
hput capitals Netherlands Amsterdam
hput capitals Spain Madrid
echo `hget capitals France` and `hget capitals Netherlands` and `hget capitals Spain`

This lets you define other maps that don't conflict (e.g., 'rcapitals' which does country lookup by capital city). But, either way, I think you'll find that this is all pretty terrible, performance-wise.

If you really want fast hash lookup, there's a terrible, terrible hack that actually works really well. It is this: write your key/values out to a temporary file, one-per line, then use 'grep "^$key"' to get them out, using pipes with cut or awk or sed or whatever to retrieve the values.

Like I said, it sounds terrible, and it sounds like it ought to be slow and do all sorts of unnecessary IO, but in practice it is very fast (disk cache is awesome, ain't it?), even for very large hash tables. You have to enforce key uniqueness yourself, etc. Even if you only have a few hundred entries, the output file/grep combo is going to be quite a bit faster - in my experience several times faster. It also eats less memory.

Here's one way to do it:

hinit() {
    rm -f /tmp/hashmap.$1
}

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

hget() {
    grep "^$2 " /tmp/hashmap.$1 | awk '{ print $2 };'
}

hinit capitals
hput capitals France Paris
hput capitals Netherlands Amsterdam
hput capitals Spain Madrid

echo `hget capitals France` and `hget capitals Netherlands` and `hget capitals Spain`