Simple bash script parallelization using semaphores

semlock.sh

#!/bin/bash

SEMPATH="/tmp"
SEMNAME=""

semtake() {
	local name="$1"
	[ -z "$name" ] && echo "Missing semaphore name!" && return 1
	local j="$2"
	[ -z "$2" ] && j=$(nproc)
	[ -n "$SEMNAME" ] && echo "Already have $SEMNAME" && return 1
	while true; do
		for i in $(seq 1 $j); do
			SEMNAME=".semlock-$name-$j-$i"
			mkdir "$SEMPATH/$SEMNAME" 2>/dev/null && break 2
		done
		sleep 1
	done
	trap semgive EXIT
}

semgive() {
	[ -z "$SEMNAME" ] && return
	rmdir "$SEMPATH/$SEMNAME" &>/dev/null || true
	SEMNAME=""
}

semnotify.sh

#!/bin/bash

[ -z "$(command -v inotifywait)" ] && echo "inotify-tools need to be installed for $0 to work!" >&2 && return 1

SEMPATH="/tmp"
[ ! -d "$SEMPATH" ] && echo "$SEMPATH is not a valid directory" >&2 && return 1

! (return 0 2>/dev/null) && echo "$0 can only be sourced, not executed" >&2 && exit 1

#SEMNAME=""
#SEMNAMEID=""

semtake_pool() {
	local SEMNAME="$1"
	local j="$2"
	for i in $(seq 1 "$j"); do
		SEMNAMEID="$i"
		mkdir "$SEMPATH/$SEMNAME-$SEMNAMEID" 2>/dev/null || continue
		return 0
	done
	unset SEMNAMEID
	return 1
}

semtake() {
	local name="$1"
	[ -z "$name" ] && echo "Missing semaphore name!" >&2 && return 1
	local j="$2"
	[ -z "$2" ] && j=$(nproc)
	[ -n "$SEMNAMEID" ] && echo "Already have $SEMNAME" >&2 && return 1
	SEMNAME=".semlock-$name"
	until semtake_pool "$SEMNAME" "$j"; do
		local i
		i="$(find "$SEMPATH" -maxdepth 1 -type d -name "$SEMNAME-wait-*" 2>/dev/null | sed 's/^.*-\([[:digit:]]*\)$/\1/' | sort -n | tail -1)"
		[ -z "$i" ] && i=0
		local SEMWAITNAME
		while true; do
			SEMWAITNAME="$SEMNAME"-wait-$i
			i=$((i+1))
			mkdir "$SEMPATH"/"$SEMWAITNAME" &>/dev/null || continue
			break
		done
		inotifywait --quiet --quiet --event delete_self "$SEMPATH"/"$SEMWAITNAME"
		rmdir "$SEMPATH"/"$SEMWAITNAME" &>/dev/null || true
	done
	trap semgive EXIT
}

semgive() {
	[ -z "$SEMNAME" ] && return
	[ -z "$SEMNAMEID" ] && return
	rmdir "$SEMPATH"/"$SEMNAME"-"$SEMNAMEID" &>/dev/null || true
	unset SEMNAMEID
	local i
	i="$(find "$SEMPATH" -maxdepth 1 -type d -name "$SEMNAME-wait-*" 2>/dev/null | sed 's/^.*-\([[:digit:]]*\)$/\1/' | sort -n | head -1)"
	[ -z "$i" ] && return
	local SEMWAITNAME
	local waiter
	for waiter in "$SEMPATH"/"$SEMNAME"-wait-*; do
		SEMWAITNAME="$SEMNAME"-wait-$i
		i=$((i+1))
		rmdir "$SEMPATH"/"$SEMWAITNAME" &>/dev/null || continue
		break
	done
	unset SEMNAME
}

semnotify.sh

Another implementation of semlock.sh.
It features the exact same usage as semlock.sh, so the instructions and documentation from semlock.sh apply.

This variant uses inotify-tools to notify the next waiting process that the semaphore is available.
This way we achive two additional points:

1. No busy waiting required, as the processes are passively waiting on filesystem changes.
2. Ordered execution, because the waiting line is now numbered and semaphores will be distributed "first come, first serve"

This can be used as a drop-in replacement to semlock.sh.
If you have inotify-tools installed, simply download semnotify.sh, rename it to semlock.sh and replace the other implementation.

Reserved