check_lock.sh

#!/bin/bash

check_lock ()
{
    # lock dirs/files
    LOCKDIR="/tmp/statsgen-lock"
    PIDFILE="${LOCKDIR}/PID"

    # exit codes and text
    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, install signal handlers before storing the PID just in case
        # storing the PID fails
        trap 'ECODE=$?;
              echo "[statsgen] Removing lock. Exit: ${ETXT}[${ECODE}](${ECODE})" >&2
              rm -rf "${LOCKDIR}"' 0
        echo "$$" >"${PIDFILE}"
        # the following handler will exit the script upon 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"

    else

        # lock failed, check if the other PID is alive
        OTHERPID="$(cat "${PIDFILE}")"

        # if cat isn't able to read the file, another instance is probably
        # about to remove the lock -- exit, we're *still* locked
        #  Thanks to Grzegorz Wierzowiecki for pointing out this race condition on
        #  http://wiki.grzegorz.wierzowiecki.pl/code:mutex-in-bash
        if [ $? != 0 ]; then
          echo "lock failed, PID ${OTHERPID} is active" >&2
          exit ${ENO_LOCKFAIL}
        fi

        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
}