Lock files in bash scripts

Taking inspiration from this wonderful site:

http://www.davidpashley.com/articles/writing-robust-shell-scripts/

I created two reusable bash functions to implement race conditions safe locks:

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me=${0##*/}
lockfile="/var/lock/${me}" # WARNING ! On AIX lock files are in /var/locks/ (note the s)

now() {
        echo `date +%Y%m%d.%H%M%S`
}

lock() {
        waitsecs=$1 # set to "" to have an empty value while being able to set next parameters
        maxtries=$2 # set to 0 to retry undefinetively
        warning=$3 # set to anything to have a warning on standard output
        try=0
        while true ; do
                if ( set -o noclobber ; echo "$$" > "$lockfile") 2> /dev/null ; then
                        trap 'rm -f "$lockfile" ; exit $?' INT TERM EXIT
                        return 0
                else
                        [ ! -z $warning ] && echo "`now`: Failed to acquire lockfile, it's held by pid $(cat $lockfile)"
                        if [ ! -z $waitsecs ] ; then
                                try=$((try+1))
                                [ ${try} -eq ${maxtries} ] && exit 3 || sleep $waitsecs
                        else
                                exit 3
                        fi
                fi
        done
}

unlock() {
        rm -f "$lockfile"
        trap - INT TERM EXIT
}

lock "" 0 yes
# lock 60 5
# lock 1 60 yes

# critical section

unlock

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Share and enjoy :-)