pollcond

#!/bin/sh

usage()
{
    cat >&2 <<__EOF__
Usage: $(basename $0) [-l] [-m <addr>] -a <action -c <condition> -t <interval>

Options:
  -a <action>           -- The action to be executed when the condition
  -c <condition>        -- The condition to be periodically evaluated.
  -l                    -- If specified, the script will continue to run
                           after the condition has been detected to be true.
                           Otherwise the script will exit with a status of 0.
  -m <addr>             -- Send an email to <addr> when the condition becomes
                           true.
  -t <interval>         -- Sleep for <interval> seconds in between each check.
                           Use an interval of 0 to busy-wait on the condition.
__EOF__
    exit ${1-1}
}

log()
{
    echo "$(basename $0): $1" >&2
}

INFINITE=false

while [ $# -gt 0 ]; do
    case $1 in
    -a)
        shift; ACTION=$1
        ;;
    -c)
        shift; COND=$1
        ;;
    -l)
        INFINITE=true
        ;;
    -m)
        shift; MADDR=$1
        ;;
    -t)
        shift; INTERVAL=$1
        ;;
    *)
        usage
        ;;
    esac
    shift
done

if [ -z "$ACTION" -o -z "$COND" -o -z "$INTERVAL" ]; then
    usage
elif ! [ $(expr "$INTERVAL" : [1-9][0-9]*) -eq ${#INTERVAL} ]; then
    log "Invalid interval \"${INTERVAL}\""
    usage
fi

while true ; do
    eval "$COND" >/dev/null 2>&1

    if [ $? -eq 0 ]; then
        if [ -n "$MADDR" ]; then
            mail -s pollcond "$MADDR" <<__EOF__
$(basename $0): condition "$COND" evaluated to true. Executing "$ACTION":

$(eval "$ACTION")
__EOF__
        else
            log "condition \"$COND\" evaluated to true. Executing \"$ACTION\":"
            eval "$ACTION"
        fi

        [ $INFINITE = false ] && exit 0
    fi

    [ $INTERVAL -gt 0 ] && sleep $INTERVAL
done