call function at specific time intervals in Linux

Question

What is the best way to call a function inside a Linux daemon at specific time intervals, and specific time (e.g. at 12am every day, call this function). I'm not referring to calling a process with crontab, but a function within a long running daemon.

Thanks

Answer 1

Compare the current time to the time you should be running, and if it's later than run and then reset the time.

Answer 2

use settimer with ITIMER_REAL and have your function be called by the handler for SIGALARM.

Answer 3

My favourite trick is:

• sleep and wake every Nth seconds (I personally prefer either every second or every 10 seconds)
• when woken up check the time and check to see if I need to run anything
• rinse, repeat...

How you do this depends on your language. In tcl I would do something like:

proc scheduler {} {
    global task_list
    set now [clock format [clock seconds] -format "%H:%M"]

    foreach task $task_list {
       lassign $task_list time task
       if {$time == $now} {
           apply $task
       }
    }
    after 1000 scheduler ;# repeat after 1 second
}

scheduler ;# GO!

Answer 4

From your question tags I understand you are running a shell daemon. So my suggestion is to use crontab, as it is already waiting to be used, to signal your daemon.

In your shell daemon you need a signal handler

   handler() {
      echo "$(date): doing something special" >> $LOG
   }

you have to trap a signal, SIGALRM in this example

    trap handler ALRM

and in your crontab send the signal, assuming your daemon is daemon.sh

   0 0 * * * killall -s SIGALRM daemon.sh