gistfile1.rbx

#!/bin/bash


if test $# -lt 1
then
  echo "Usage: $0 LOCKDIR cmd"
  echo "Example: $0 /var/lock/my_process"
  exit
fi

# lock dirs/files
LOCKDIR=$1
shift

#LOCKDIR=/Users/nathan/Desktop/my_lock
PIDFILE="${LOCKDIR}/PID"

# exit codes and text for them - additional features nobody needs :-)
ENO_SUCCESS=0;  ETXT[0]="ENO_SUCCESS"
ENO_GENERAL=1;  ETXT[1]="ENO_GENERAL"
ENO_LOCKFAIL=2; ETXT[2]="ENO_LOCKFAIL"
ENO_RECVSIG=3;  ETXT[3]="ENO_RECVSIG"

###
### start locking attempt
###

trap 'ECODE=$?; echo "[statsgen] Exit: ${ETXT[ECODE]}($ECODE)" >&2' 0
echo -n "[statsgen] Locking: " >&2

if mkdir "${LOCKDIR}" &>/dev/null; then

    # lock succeeded, store the PID and install signal handlers
    echo "$$" >"${PIDFILE}"
    trap 'ECODE=$?;
          echo "[statsgen] Removing lock. Exit: ${ETXT[ECODE]}($ECODE)" >&2
          rm -rf "${LOCKDIR}"' 0
    # the following handler will exit the script on receiving these signals
    # the trap on "0" (EXIT) from above will be triggered by this trap's "exit" command!
    trap 'echo "[statsgen] Killed by a signal." >&2
          exit ${ENO_RECVSIG}' 1 2 3 15
    echo "success, installed signal handlers"
    echo $@
    $@
else

    # lock failed, now check if the other PID is alive
    OTHERPID="$(cat "${PIDFILE}")"
    if ! kill -0 $OTHERPID &>/dev/null; then
        # lock is stale, remove it and restart
        echo "removing stale lock of nonexistant PID ${OTHERPID}" >&2
        rm -rf "${LOCKDIR}"
        echo "[statsgen] restarting myself" >&2
        exec $0 "$@"
    else
        # lock is valid and OTHERPID is active - exit, we're locked!
        echo "lock failed, PID ${OTHERPID} is active" >&2
        exit ${ENO_LOCKFAIL}
    fi

fi