mutex.sh

#!/bin/env bash

# The MIT License (MIT)
#
# Copyright (c) 2016 Anton Semjonov
#
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to deal
# in the Software without restriction, including without limitation the rights
# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in
# all copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
# THE SOFTWARE.


# This is an example script on how to use mutexes in bash.
# There is no 'real' mutex available in bash, because a mutex would need
# a really atomic operation with support from the cpu itself. The closest
# thing to an atomic operation appears to be 'mkdir' which succeeds and
# creates a folder or fails if it exists. Additionally, use signals and
# traps to avoid busy waiting. This is the result ...

trap "kill -SIGTERM -$$" SIGINT

iterations=5

function mutex {
# mutex (new|acquire|release) [$lock, iff !new]

    subcommand=${1:?(new|acquire|release)}
    case $subcommand in

        new)
            mktemp --tmpdir --directory mutex-XXXXXXXXXXXXXXXXXXX;;

        acquire)
            lock=${2:?specify lock!}
            while ! mkdir "$lock" &>/dev/null; do
            kill -SIGSTOP $BASHPID; done;;

        release)
            lock=${2:?specify lock!}
            rm -r "$lock";
            kill -SIGCONT -$$;;

        *)
            return 1;;

    esac

}

function say { echo -e "${colorspec:+\e[${colorspec}m}${name:+$name }${child:+$(progress $iteration/$iterations) }\e[0m$@"; }

function progress { #give: 5/7 or 99/100 etc
    have=${1%%/*}
    length=${1##*/}
    let left="length - have"

    L="▐"; R="▌";
    F="▓"; E="░";

    printf "$L%${have}s" | sed "s, ,$F,g"
    printf "%${left}s$R" | sed "s, ,$E,g"
}

function child {
    name="${1:?'I need a name!'}"
    colorspec="${2:-1}"
    child=yes
    iteration="0"

    say "new child. PID $BASHPID, ProcessGroup $$"

    until test "$iteration" -ge "$iterations"; do
    
        mutex acquire "$uselock"

        let "iteration++"
        say working ...
        say "$iteration/$iterations$(test "$iteration" = "$iterations" && echo " DONE!")"

        mutex release "$uselock"

    done
}

#######################################################################

name="MASTER:"
colorspec="1"
uselock=$(mutex new)
say "Set mutex: '$uselock'"

# SPAWNING ...
say "spawning children ..."
childcolorspec=31
for childname in CHILD_{A..F}; do
    child "$childname" "$childcolorspec" &
    let "childcolorspec++"
done

sleep 1
for sec in 3 2 1; do say "$sec ..."; sleep 0.5; done
say "releasing mutex ..."
mutex release "$uselock"

wait
say "all children exited. FINISHED!"
mutex release "$uselock" &>/dev/null