2004.12_Admin Workshop-Scheduling Tasks with Cron, at, and Anacron.pdf

SYSADMIN

Admin Workshop: Scheduling

The Scheduling Game

Unix systems need to handle a multitude of repetitive tasks:

rotating logfiles, creating backups, updating indices, and

managing databases. The traditional approach is to assign

these tasks to cron and at . Another tool called anacron takes

care of sleepy heads. BY MARC ANDRÉ SELIG

active processes on a Unix or Linux

system, you will probably discover

that it includes the atd and cron dae-

mons (see Figure 1). Both daemons are

used to schedule program launches.

Admins need to tell cron what to do, at

what time, and how often, although

most Linux distributions provide ready-

to-run cron jobs.

cron is particularly useful for time-con-

suming jobs that would stress even the

most modern computer temporarily and

thus might interfere with a user’s every-

day work. locate is a typical example.

This program is used to find files more

quickly. To do so it creates an index of all

the files it discovers on a system.

If an index is to be useful, it needs to

be updated regularly. And it is just as

important to keep the whatis and apro-

pos databases up to

date. Depending on

the configuration,

these jobs can be

extremely time-con-

suming and really

interfere with a

user’s daily chores.

So it makes sense for

admins to run them during the night.

cron can also take care of important

maintenance work that admins tend to

forget. For example, admins use cron to

rotate protocol files at regular intervals.

crontab. In typical Unix style, the lines

that start with a pound sign, # , are com-

ments.

Each job instruction comprises a time

and the program to run. The time is sub-

divided into five fields for minute, hour,

day, month, and weekday. Weekdays are

simply numbered, with 0 standing for

Sunday, but if you prefer to start the

week on a Monday, you can use 7 for

Sunday instead.

Don’t worry if this is getting you con-

fused, and you can’t remember what

order the fields are in: man 5 crontab

will give you details of the crontab syn-

tax. Even experienced sysadmins tend to

consult this manpage before setting up a

new job.

Instead of configuring static times, you

can also specify a scope. For example, an

asterisk, * , in a field means any value

from the beginning to the end, for exam-

ple, every minute. A value of * * * * *

would mean running a program every

minute of every hour every day. 37 * * *

* would launch the program 37 minutes

after the full hour.

Fixed Schedule

There are any number of programs that

can handle scheduled job execution, but

cron is the great grandpa of them all, and

many of the other scheduling tools are

actually derived from the original cron .

cron is launched when the system

boots and immediately starts looking for

jobs to run. The daemon also checks

if the schedule has changed. This

means you can modify the schedule

without relaunching cron . Figure

2 shows an exam-

ple of a schedule,

also known as a

Figure 1: The atd and cron daemons sit in the background waiting to perform

scheduled tasks at predefined times.

Figure 2: The /etc/crontab file contains the master crontab, which is typically

provided by the distribution.

December 2004

www.linux-magazine.com

Insider Tips: Cron , at , anacron

I f you take a close look at the list of

Admin Workshop: Scheduling

SYSADMIN

Incidentally, you might pre-

fer to use unusual time

values, such as 37 or 21,

rather than zero. This reduces

the risk of jobs running at the

same time, and causing

unnecessary load. The fields

for day and weekday have a

special feature: if both fields

are set to a value (or range),

cron will combine the values to create an

either/or condition. 37 2 1 * 0 will thus

launch a command at 02:37 on the first

of the month, and every Sunday.

Users wanting to start time-

consuming jobs should add

an ampersand ( &) to the at

prompt to tell the program to

run in the background. This

said, if 20 users have the

same idea, the system load

may reach a point where nor-

mal work becomes im-

possible. To handle this kind

of scenario, there is a special version of

at called batch .

Like at , batch accepts one or multiple

commands. However, batch will only

actually run the commands once the sys-

tem load falls below a certain threshold

value. This threshold is hard-coded into

the at daemon, but admins can change it

by launching atd with the -l value flag.

On legacy Unix systems, at jobs are

launched by a crontab entry that runs

the atrun program once a minute, but

most current Linux distributions provide

a genuine daemon called atd (see Figure

1).

removes it from the list. This is useful,

for example, if you need to send a mes-

sage to remind someone to do

something.

It only makes sense that at is an inter-

active program. The program expects

you to specify the job time, although it is

not too strict on the format. So to run a

program at midnight, you can just say at

midnight , or in five hours at now +5

hours . Of course, this does not mean that

you can’t be more precise if you need to

be: at 05:45 11 Jun 2005 .

After typing the at command, you are

shown a prompt where you can enter

shell commands. [Ctrl]+[D] takes you

back to the normal shell, and tells at to

save the job. Figure 3 shows an example,

where at will be sending an email that

says “Time to go!” with a subject line of

“Ring ring” in five hours from now.

at handles job execution in an intelli-

gent way. It “remembers” what shell the

user was using (as this could affect the

command syntax!), the current working

directory, and the current environmental

variables. In other words, users just need

to type commands exactly as they would

when running them in the shell.

Your Own crontab

In addition to the global /etc/crontab ,

every user can create their own crontab.

A crontab file is installed onto the system

using the crontab command. To create a

crontab, type crontab -e (for “edit”). This

will launch an editor and load your

crontab in a temporary file.

After modifying the file, quit the edi-

tor, and crontab will copy the file to the

cron spool directory. This is the directory

that stores the contabs for local users

(typically /var/spool/cron/crontabs/ ).

crontab -l outputs a job list on your

screen. Users who prefer to store their

crontab in their home directory need to

point to the file to update the spool

directory : crontab ~/filename.

Of course, the admin is responsible for

updating the global crontab. As Figure 2

shows you, this file is typically quite

small, and only calls programs from sub-

directories. Most distributions store bash

scripts or even binaries in the cron.daily ,

cron.weekly , and cron.monthly directo-

ries below /etc/ .

Assigning cron jobs to subdirectories

helps to keep the master crontab read-

able. Each time an admin installs a new

package that needs a cron job, your dis-

tribution’s installation system will

typically copy the relevant scripts to one

of the cron directories. This removes the

need to edit the master file.

The global crontab has an additional

job field that stores the user ID with

which cron should run the job. The field

is directly behind the time specification.

anacron for Sleepy Heads

When cron and at were developed,

programmers simply assumed that com-

puters would be running 24x7. These

programs date back to the days when

Unix typically ran on heavy iron.

Home and office users are quite likely

to switch their computers off, and lap-

tops often power down to save batteries.

cron and at jobs are disabled when a

computer is down – and if the computer

remains switched off for a significant

time, important jobs just might never be

performed.

anacron allows for more imprecise

scheduling. The anacron application can

handle scheduling such as “round about

once a day.”

When you boot your computer,

anacron will check to see if a task needs

to be performed and, if so, will launch

the task. To prevent the computer taking

ages to boot, anacron adds a delay,

ensuring that each job will wait for the

previous job to finish.

If a user who normally switches his or

her computer off at night happens to

leave it switched on instead, anacron

will simply run the outstanding jobs at

their normal times. And that takes some

of the headaches out of system adminis-

tration.

Avoiding Load

Commands that need a graphical inter-

face are exceptions to the rule, as at will

not store the $DISPLAY variable. This

said, there is an easy workaround to the

display problem: instead of typing xeyes ,

simply type DISPLAY=:0.0 xeyes . If the

server is running on a different display,

you will need to modify the variable to

reflect your setup. Mechanisms such as

SSH agents will take effect as soon as a

user logs out.

The at spool directory (to which only

root has access) stores the jobs as simple

shell scripts. If you do not have root priv-

ileges, typing atq will give you an

overview of the outstanding jobs. atrm

removes jobs from the queue.

Play it Once with at !

Whereas cron reads its crontabs to coor-

dinate recurring tasks, at takes care of

one-off jobs. at performs a job and then

■

www.linux-magazine.com December 2004

Figure 3: The at program performs one-off jobs. In this case, a user wants at

to send an email reminder in five hours time.

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