Examples for batch on linux

Linux has a few ways to schedule jobs to be executed. I am sure most are familiar with crontab and at, but batch is lesser known.

"batch" can be used to: (from the man-page on batch)
executes commands when system load levels permit; in other words, when the load average drops below 0.8, or the value specified in the invocation of atrun.

So:

  • crontab is used for periodic scheduling.
  • at is used for executing something once at a specific time.
  • batch can be used to execute commands when your system has resourses.

You can also combine crontab and batch. Imagine you need to run a sequence of command in a specific order every hour. crontab does not guarantee one command is finished when it executs the next command.
batch can be used from crontab like so:

crontab -l
0 * * * * /usr/bin/batch now /usr/local/bin/prepare-something.sh
1 * * * * /usr/bin/batch now /usr/local/bin/process-something.sh
2 * * * * /usr/bin/batch now /usr/local/bin/report-something.sh

This batches these three commands in a specific order, one after the other, when the systemload is not too high.

One specific situation where I use this; Drupal needs to run a program (cron.php) every hour. crontab would be perfect for that, but when the load is too high, it's not a problem that this program is executed a little later. This is what I have setup:

0 * * * * /usr/bin/batch now /usr/bin/wget -o /dev/null -O /dev/null http://1.example.com/cron.php
1 * * * * /usr/bin/batch now /usr/bin/wget -o /dev/null -O /dev/null http://2.example.com/cron.php
2 * * * * /usr/bin/batch now /usr/bin/wget -o /dev/null -O /dev/null http://3.example.com/cron.php

This ensures that every hour cron.php is ran, but not if the systemload is too high (0.8 or more). One disadvantage of this solution; when your system is overloaded for a long period of time, these batch jobs pile up, then when the load drops below 0.8, all batched commands will be executed. Happily Drupals cron.php will not consume that much resources when it's ran twice.