Linux Certification Study Group
A A set of LTEX presentations
Angus Lees
gus@inodes.org

Granville College of TAFE Sydney, Australia

Linux Certification Study Group – p. 1/13

111.4 Scheduling jobs [4]
Objective
Candidate should be able to use cron or anacron to run jobs at regular intervals and to use at to run jobs at a specific time. Tasks include managing cron and at jobs and configuring user access to cron and at services.

Linux Certification Study Group – p. 2/13

111.4 Scheduling jobs [4]
Key files, terms and utilities
crontab /etc/anacrontab /etc/crontab /etc/cron.allow /etc/cron.deny /var/spool/cron/* at atq /etc/at.deny /etc/at.allow

Linux Certification Study Group – p. 3/13

Basically

at – Run a command once cron – Run a command periodically

Linux Certification Study Group – p. 4/13

The at command
at takes a time and a list of commands to run. Any output to STDOUT or STDERR will be mailed to the user running at. $ at 2pm ← warning: commands will be executed using /bin/sh at> date ← at> ˆD ← job 3 at 2002-05-08 14:00

Linux Certification Study Group – p. 5/13

The at command
The current umask, working directory and environment (except for TERM, DISPLAY and _) are saved and restored before running the job (unlike cron). The commands to run will be read from STDIN or from a file given with -f .

Linux Certification Study Group – p. 6/13

Example at time specifications
at allows a very flexible time format. 17:36 Run at 5:36pm today or tomorrow. 9pm May 8 Run at 9pm on May 8th. noon tomorrow Run at 12pm tomorrow. now + 2 hours Run in 2 hours.

See at(1) for more details.

Linux Certification Study Group – p. 7/13

Queued jobs
atq lists a user’s pending jobs. $ atq ← 3 2002-05-08 14:00 a gus $ atrm 3 ← removes the queued job. $ at -c 3 ← dumps the job on STDOUT.

Linux Certification Study Group – p. 8/13

Queued jobs
atq lists a user’s pending jobs. $ atq ← 3 2002-05-08 14:00 a gus $ atrm 3 ← removes the queued job. $ at -c 3 ← dumps the job on STDOUT.

Linux Certification Study Group – p. 8/13

crontab
cron is a daemon that reads everyone’s crontab information, spawning new tasks at the appropriate times. crontab file Replace your crontab file with file. crontab -l List your crontab. crontab -r Remove your crontab. crontab -e Edit your crontab (with $EDITOR).

Linux Certification Study Group – p. 9/13

crontab file format
A sample crontab file: 0 7 1 jan * echo "sleep in, you dont feel so good"
# gratuitous noise 0 17 * * mon,wed,fri wall%meeting in 5 minutes% 0 9-18/2 * * mon-fri $HOME/bin/cron.bihourly

Linux Certification Study Group – p. 10/13

crontab file format
A sample crontab file: 0 7 1 jan * echo "sleep in, you dont feel so good"
# gratuitous noise 0 17 * * mon,wed,fri wall%meeting in 5 minutes% 0 9-18/2 * * mon-fri $HOME/bin/cron.bihourly

Line based, hash comments, ignored blank lines, etc

Linux Certification Study Group – p. 10/13

crontab file format
A sample crontab file: 0 7 1 jan * echo "sleep in, you dont feel so good"
# gratuitous noise 0 17 * * mon,wed,fri wall%meeting in 5 minutes% 0 9-18/2 * * mon-fri $HOME/bin/cron.bihourly

Minute (0-59) Hour (0-23)

Linux Certification Study Group – p. 10/13

crontab file format
A sample crontab file: 0 7 1 jan * echo "sleep in, you dont feel so good"
# gratuitous noise 0 17 * * mon,wed,fri wall%meeting in 5 minutes% 0 9-18/2 * * mon-fri $HOME/bin/cron.bihourly

Day of month (1-31) Month (1-12 or jan-dec) Day of week (0-7 or sun-sat)

Linux Certification Study Group – p. 10/13

crontab file format
A sample crontab file: 0 7 1 jan * echo "sleep in, you dont feel so good"
# gratuitous noise 0 17 * * mon,wed,fri wall%meeting in 5 minutes% 0 9-18/2 * * mon-fri $HOME/bin/cron.bihourly

Step Wildcard Ranges Lists
Linux Certification Study Group – p. 10/13

crontab file format
A sample crontab file: 0 7 1 jan * echo "sleep in, you dont feel so good"
# gratuitous noise 0 17 * * mon,wed,fri wall%meeting in 5 minutes% 0 9-18/2 * * mon-fri $HOME/bin/cron.bihourly

See crontab(5) for: Environment variables Providing STDIN

Linux Certification Study Group – p. 10/13

cron from root
A few extra issues arise when editing /etc/crontab (and similar “system” crontab files): Don’t use crontab -e, edit /etc/crontab directly. A new column (after timespec, before command) gives the user to the command run as. Distributions often create directories for “common” frequencies. It usually makes much more sense to place a script in there, rather than adding your own crontab lines. Debian (for example) runs any scripts in /etc/cron.{daily,weekly,monthly} – but these are triggered from normal entries in /etc/crontab, so there’s no real mystery here. (Debian specific?) /etc/cron.d/* is read in addition to /etc/crontab (they also have the extra user field).
Linux Certification Study Group – p. 11/13

anacron
Apparently some people turn their machines off. If your computer is always turned off at night (for example), then daily jobs which are usually scheduled to run in the wee hours, will never be run. This is a problem.
anacron fixes this by running any missed jobs after a reboot (or other times, like AC-on for laptops).

Linux Certification Study Group – p. 12/13

anacron
Since anacron can’t use the crontab files, it has its own simplified /etc/anacrontab. If you only use the standard /etc/cron.daily,monthly,weekly, then no further configuration will be necessary. Otherwise, see anacrontab(5). Note that the frequency of anacron jobs can only be specified in days.

Linux Certification Study Group – p. 13/13