Semi-Reliable Periodic Commands

Impromptu

cron is great. Using cron, you can schedule commands to be run at regular intervals or at specific times. One of the major drawbacks of cron is that it doesn't generally keep state regarding job's status. If a job is scheduled to run at midnight, but the system is powered down at midnight, the command will never be run.

There are a few solutions out there to take care of this shortcomings, but many are designed for system-wide use only. What if you don't want to intermingle your personal jobs with the system-wide ones? Here is a fairly simple script that works as a wrapper, providing a little insurance to help make sure jobs get run.

#!/bin/bash

export P_ENV=~/.profile
export P_TRACK=~/.p_track

if [ -e "$P_ENV" ]; then
 . $P_ENV
fi

case "$1" in
 h) 
  export P_NOW=$(date +%Y-%m-%d-%H)
  ;;
 d)
  export P_NOW=$(date +%Y-%m-%d)
  ;;
 w)
  export P_NOW=$(date +%Y-%U)
  ;;
 m)
  export P_NOW=$(date +%Y-%m)
  ;;
 *)
  echo ERROR: Term not specified. Must be one of h, d, w, m .
  exit 1
esac

if [ -z "$2" ]; then
 echo ERROR: Command to run not specified.
 exit 2
fi

export P_TAG=$(echo $2 | sed -e 's/[^A-Za-z0-9]/-/g')
export P_FILE=$P_TRACK/$P_TAG-$P_NOW

if [ -e "$P_FILE" ]; then
 exit 0
else
 rm -f $P_TRACK/$P_TAG*
 echo Executing $2 at $(date)
 $2 $3 $4 $5 $6 $7 $8 $9
 echo Done
 
 if [ "$?" -eq "0" ]; then
  touch $P_FILE
 fi
fi

The script has 4 operating modes. It can help ensure command are run hourly, daily, weekly, or monthly. It uses empty files in a configurable directory (~/.p_track by default) to keep tabs on the last time the command was run. Entries in the tracking directory are only created if the command run returns exit status 0 (no error.)

To use this wrapper, place it and a period (h for hourly, d for daily, and so on) in front of commands in your crontab like so:

*/20 * * * * ~/bin/periodic h ~/bin/command param1 param2

The above will check to see if the command needs to be run every 20 minutes, but will only execute it every hour. The advantage of wrapping individual commands instead using periodic command directories (check out the run-parts command) is less administrative overhead.

Note: The "real" periodic command is generally used to run system-wide periodic commands, often stored in /etc/periodic. UNIX-y systems tend to use other mechanisms and directories to do something similar.

Update 1: Changed the script from a group of if/elif to case (thanks for pointing that out Dave) and added a quick import of .profile or another file to set up environment.