use flock to create an exclusive file lock like a pinch on the neck from mr. spock

flock_lock_spock

http://www.kfirlavi.com/blog/2012/11/06/elegant-locking-of-bash-program/

Simple
#########################################
#!/bin/bash

# exit if flock fails.
set -e

# Wait for lock on /var/lock/.myscript.exclusivelock (fd 200) for 10 seconds
( flock -n 200 || exit 1 ) 200>/var/lock/.myscript.exclusivelock

#########################################
ADVANCED
#########################################
#!/bin/bash

readonly PROGNAME="$(basename "$0")"
readonly LOCKFILE_DIR='/tmp'
readonly LOCK_FD='200'

# exec attempts to secure a file descriptor 
# 

Lock_Flock_FD() {
    local prefix="${1:+spockflocklock}"
    local fd="${2:-$LOCK_FD}"
    local lock_file="$LOCKFILE_DIR/${prefix}.lock"

    eval 'exec "$fd">"$lock_file"'
     [[ 1 -eq $? ]] \
       && echo "error : unable to acquire file descriptor $fd " && return 1

    flock -n "$fd" \
        && return 0 \
        || return 1
}

# echo parameter as error message and exit - use " " or ' '
Err_Exit() {
    local error_str="$@"
    echo "error  :  $error_str  :  " >&
    exit 1
}

main() {
    lock $PROGNAME \
        || Err_Exit "one $PROGNAME at a time"

    do_A
    do_B
}

main

I think i improved on the original a little