Throttling asynchronous processes in bash

throttle.sh

#!/bin/bash

#Fail fast on errors
set -euo  pipefail


# Arguments
# $1 -> throttlerName
# Initializes a throttler of the given name
initThrottler(){
    rm $1 2>/dev/null|| true
    mkfifo $1
    exec 3<> $1
}

# Arguments
# $1 -> limit
# $2 -> throttlerName
# Called before running any processes
# Will wait while the current number of running processes
# exceed the given limit
throttle(){
    local limit=$1
    local throttlerName=$2
    #We generate a counter name here so that
    #Different throttler's counts dont interact with
    #each other. This makes the access of the variable
    #a little tricky.
    local counterName=${throttlerName}Counter
    if [[ -z ${!counterName:-} ]]; then
        eval "$counterName=0"
    fi
    eval "$counterName=$((${counterName}+1))"
    if [[ ${!counterName} > $limit ]]; then
        while read line < $throttlerName;  do
            echo $line
            eval "$counterName=$((${counterName}-1))"
            if [[ ${!counterName} -le $limit ]]; then 
                break
            fi
        done
    fi
}

#Usage below

randomLongProcess(){
    sleep $((RANDOM % 10))
    echo fin > $throttlerName
}

throttlerName=testThrottler
initThrottler $throttlerName

for x in {1..10}; do
    #Call throttle first
    throttle 3 $throttlerName
    #echo Update
    echo Process number $x
    #Start background process
    randomLongProcess  &
done
wait

This was useful to me for multithreading builds while not totally overloading my machine.