Bash solution for Rain Water kata: https://gist.github.com/knifeofdreams/b6ffe05679e77a7789a6b8921b75e152

rain_water.sh

#!/bin/bash

function max_number() {
    echo $(( $1 > $2 ? $1 : $2 ))
}

function trim_line_breaks() {
    tr -d '\n'
}

function reverse_map() {
    tail -r
}

function remove_elevation() {
    sed 's/X//g'
}

function count_water() {
    wc -c
}

function fill_in_water_after_peaks() {
    local max_elevation=0
    for elevation in $@
    do
        max_elevation="$( max_number $elevation $max_elevation )"
        [[ $elevation -gt 0 ]] && echo -n "$( yes X | head -$elevation | trim_line_breaks )"
        [[ $elevation -lt $max_elevation ]] && echo -n "$( yes W | head -$(( max_elevation - elevation )) | trim_line_breaks )"
        echo ''
    done
}

function reduce_water_before_peaks() {
    local max_elevation=0
    while read -r line
    do
        local elevation="$( echo $line | sed 's/W//g' | trim_line_breaks | wc -c )"
        max_elevation="$( max_number $elevation $max_elevation )"
        [[ $max_elevation -gt 0 ]] && echo -n "$line" | head -c $max_elevation
        echo ''
    done
}

fill_in_water_after_peaks $@ \
    | reverse_map \
    | reduce_water_before_peaks \
    | remove_elevation \
    | trim_line_breaks \
    | count_water