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Task Scheduling in Linux

Overview

This article discusses some tools used in a Linux system to schedule tasks to run automatically at specified time intervals or at any given point of time in the future. This primer will not cover these commands in-depth; this is just a brief introduction to using these commands. See the individual HOWTOS for each command for an in-depth look at all relevant options and configurations.

Some task-scheduling daemons used in Linux/UNIX are:

  • at – schedule one-time tasks for the future
  • cron – the periodic scheduler most commonly used
  • anacron – anachronistic cron; a periodic scheduler that doesn't rely on the system being left on 24×7

Using at

The at command allows a user to execute commands or scripts at a specified time (required) and date (optional). The commands can be entered via standard input, redirection, or file.

darkstar:~% at

Interactive at

Using the command at with standard input (keyboard) is a little more complicated than typing one line in at the prompt. The command uses an internal “sub-shell” to gather the required information. Once the command information entry is complete, Ctrl+D (EOT) will signify entry completion. The -m flag specifies a mail message will be sent to the user when the job is finished, regardless if any output was created.

darkstar:~% at 12:01 -m
warning: commands will be executed using (in order) a) $SHELL b) login shell c) /bin/sh
at> ./my_script.sh
at> <EOT>
job 4 at 2015-06-22 12:01 
darkstar:~% 

File-driven at

Commands can also be contained within a file and run by at:

darkstar:~% at 12:32 -m -f /usr/local/bin/my_script.sh
warning: commands will be executed using (in order) a) $SHELL b) login shell c) /bin/sh
job 8 at 2015-06-22 12:10

The -m flag will email the user after completion of the command; the -f flag specifies the command will read the job from a file, not from standard input. After the command is typed in (and the appropriate warning is displayed), the at job number1) is displayed.

at Internal Scheduling

The job numbers provided after a command is typed in, or when a file is read, allow the user to know which internal job will be run in sequential order. If a user wants to delete a specific task, all that needs to be known is this internal job number. To remove the job, the command atrm (at remove) is used:

darkstar:~%  at -l
7      2015-06-22 12:10 p tux
8      2015-06-22 12:15 p root

The command atq (at queue) is the same as at -l:

darkstar:~%  atq
7      2015-06-22 12:10 p tux
8      2015-06-22 12:15 p root

To remove the user job, use atrm with the job number:

darkstar:~%  atrm 7

Using cron

cron is a daemon that runs tasks in the background at specific times. For example, if you want to automate downloads of patches on a specific day (Monday), date (2 July), or time (1300), cron will allow you to set this up in a variety of ways. The flexibility inherent in cron can allow administrators and power users to automate repetitive tasks, such as creating backups and system maintenance.

cron is usually configured using a crontab file. The following command will open your user account crontab file:

darkstar:~% crontab -e

To edit the system-level crontab, first log into the root account:

darkstar:~# crontab -e

If your system has sudo installed, type in:

darkstar:~% sudo crontab -e 

The crontab file syntax is:

 # * * * * *  command to execute
 # │ │ │ │ │
 # │ │ │ │ │
 # │ │ │ │ └───── day of week (0 - 6) (Sun(0) /Mon (1)/Tue (2)/Wed (3)/Thu (4)/Fri (5)/Sat (6))
 # │ │ │ └────────── month (1 - 12)
 # │ │ └─────────────── day of month (1 - 31)
 # │ └──────────────────── hour (0 - 23)
 # └───────────────────────── min (0 - 59)

Using an asterisk in any placeholder location, will match any value. For example, the following will run example_script.sh at noon (1200) everyday during the first three months of the year:

#For more information see the manual pages of crontab(5) and cron(8)
#
# min hr day month weekday command
#
#
 
0 11 * 1-3 * /home/user/example_script.sh

Using anacron

anacron is not installed in Slackware by default.2)

anacron is unique from cron in the respect that it does not expect the operating system to be running continuously like a 24×7 server. If the time of execution passes while the system is turned off, anacron executes the command automatically when the machine is turned back on. The reverse is not true for cron - if the computer is turned off during the time of scheduled execution, cron will not execute the job. Another key difference between anacron and cron is the minimum chronological “granularity” - anacron can only execute jobs by day, versus the ability of cron to execute by the minute. Finally, anacron can only be used by root, while cron can be used by root and normal users.

Sources

1) As distinct from a process ID (PID) known to the operating system
2) See Slackbuilds.org for more information on anacron on Slackware

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