13.3.2 An Alarm Clock Program

Nothing cures insomnia like a ringing alarm clock.
Arnold Robbins

The following program is a simple “alarm clock” program. You give it a time of day and an optional message. At the specified time, it prints the message on the standard output. In addition, you can give it the number of times to repeat the message as well as a delay between repetitions.

This program uses the gettimeofday function from Gettimeofday Function.

All the work is done in the BEGIN rule. The first part is argument checking and setting of defaults: the delay, the count, and the message to print. If the user supplied a message without the ASCII BEL character (known as the “alert” character, "\a"), then it is added to the message. (On many systems, printing the ASCII BEL generates an audible alert. Thus when the alarm goes off, the system calls attention to itself in case the user is not looking at the computer or terminal.) Here is the program:

     
     # alarm.awk --- set an alarm
     #
     # Requires gettimeofday library function
     
     
     # usage: alarm time [ "message" [ count [ delay ] ] ]
     
     BEGIN    \
     {
         # Initial argument sanity checking
         usage1 = "usage: alarm time ['message' [count [delay]]]"
         usage2 = sprintf("\t(%s) time ::= hh:mm", ARGV[1])
     
         if (ARGC < 2) {
             print usage1 > "/dev/stderr"
             print usage2 > "/dev/stderr"
             exit 1
         } else if (ARGC == 5) {
             delay = ARGV[4] + 0
             count = ARGV[3] + 0
             message = ARGV[2]
         } else if (ARGC == 4) {
             count = ARGV[3] + 0
             message = ARGV[2]
         } else if (ARGC == 3) {
             message = ARGV[2]
         } else if (ARGV[1] !~ /[0-9]?[0-9]:[0-9][0-9]/) {
             print usage1 > "/dev/stderr"
             print usage2 > "/dev/stderr"
             exit 1
         }
     
         # set defaults for once we reach the desired time
         if (delay == 0)
             delay = 180    # 3 minutes
         if (count == 0)
             count = 5
         if (message == "")
             message = sprintf("\aIt is now %s!\a", ARGV[1])
         else if (index(message, "\a") == 0)
             message = "\a" message "\a"
     

The next section of code turns the alarm time into hours and minutes, converts it (if necessary) to a 24-hour clock, and then turns that time into a count of the seconds since midnight. Next it turns the current time into a count of seconds since midnight. The difference between the two is how long to wait before setting off the alarm:

     
         # split up alarm time
         split(ARGV[1], atime, ":")
         hour = atime[1] + 0    # force numeric
         minute = atime[2] + 0  # force numeric
     
         # get current broken down time
         gettimeofday(now)
     
         # if time given is 12-hour hours and it's after that
         # hour, e.g., `alarm 5:30' at 9 a.m. means 5:30 p.m.,
         # then add 12 to real hour
         if (hour < 12 && now["hour"] > hour)
             hour += 12
     
         # set target time in seconds since midnight
         target = (hour * 60 * 60) + (minute * 60)
     
         # get current time in seconds since midnight
         current = (now["hour"] * 60 * 60) + \
                    (now["minute"] * 60) + now["second"]
     
         # how long to sleep for
         naptime = target - current
         if (naptime <= 0) {
             print "time is in the past!" > "/dev/stderr"
             exit 1
         }
     

Finally, the program uses the system function (see I/O Functions) to call the sleep utility. The sleep utility simply pauses for the given number of seconds. If the exit status is not zero, the program assumes that sleep was interrupted and exits. If sleep exited with an OK status (zero), then the program prints the message in a loop, again using sleep to delay for however many seconds are necessary:

     
         # zzzzzz..... go away if interrupted
         if (system(sprintf("sleep %d", naptime)) != 0)
             exit 1
     
         # time to notify!
         command = sprintf("sleep %d", delay)
         for (i = 1; i <= count; i++) {
             print message
             # if sleep command interrupted, go away
             if (system(command) != 0)
                 break
         }
     
         exit 0
     }