Task Management¶

In this chapter you will learn how to manage scheduled tasks.

Objectives : In this chapter, future Linux administrators will learn how to:

Linux deals with the tasks scheduling;
restrict the use of cron to certain users;
schedule tasks.

crontab, crond, scheduling, linux

Knowledge:
Complexity:

阅读时间: 15 分钟

概论¶

The scheduling of tasks is managed with the cron utility. It allows the periodic execution of tasks.

It is reserved to the administrator for system tasks but can be used by normal users for tasks or scripts that they have access to. To access the cron utility, we use: crontab.

The cron service is used for:

Repetitive administration operations;
Backups;
Monitoring of system activity;
Program execution.

crontab is short for cron table, but can be thought of as a task scheduling table.

!!! Warning To set up a schedule, the system must have the correct time set.

How the service works¶

The cron service is run by a crond daemon present in memory.

To check its status:

[root] # systemctl status crond

!!! Tip If the crond daemon is not running, you will have to initialize it manually and/or automatically at startup. Indeed, even if tasks are scheduled, they will not be launched.

Initialization of the crond daemon in manual:

[root]# systemctl {status|start|restart|stop} crond

Initialization of the crond daemon at startup:

[root]# systemctl enable crond

Security¶

In order to implement a schedule, a user must have permission to use the cron service.

This permission varies according to the information contained in the files below:

/etc/cron.allow
/etc/cron.deny

!!! Warning If neither file is present, all users can use cron.

The `cron.allow` and `cron.deny` Files¶

File /etc/cron.allow

Only users contained in this file are allowed to use cron.

If it exists and is empty, no users can use cron.

!!! Warning If cron.allow is present, cron.deny is ignored.

File /etc/cron.deny

Users in this file are not allowed to use cron.

If it is empty, all users can use cron.

By default, /etc/cron.deny exists and is empty and /etc/cron.allow does not exist.

Allowing a user¶

Only user1 will be able to use cron.

[root]# vi /etc/cron.allow
user1

Prohibit a user¶

Only user2 will not be able to use cron.

[root]# vi /etc/cron.deny
user2

cron.allow must not be present.

Scheduling tasks¶

When a user schedules a task, a file with his name is created under /var/spool/cron/.

This file contains all the information the crond needs to know regarding all tasks created by this user, the commands or programs to run, and when to run them (hour, minute, day ...).

Cron tree

The `crontab` command¶

The crontab command is used to manage the schedule file.

crontab [-u user] [-e | -l | -r]

Example:

[root]# crontab -u user1 -e

Option	Description
`-e`	Edit the schedule file with vi
`-l`	Displays the contents of the schedule file
`-u`	Name of the user whose schedule file is to be manipulated
`-r`	Delete the schedule file

!!! Warning crontab without option deletes the old schedule file and waits for the user to enter new lines. You have to press ctrl + d to exit this editing mode.

Only `root` can use the `-u user` option to manage another user's schedule file.

The example above allows root to schedule a task for user1.

Uses of `crontab`¶

The uses of crontab are many and include:

Modifications to the crontab files taken into account immediately;
No need to restart.

On the other hand, the following points must be taken into account:

The program must be autonomous;
Provide redirections (stdin, stdout, stderr);
It is not relevant to run commands that use input/output requests on a terminal.

!!! Note It is important to understand that the purpose of scheduling is to perform tasks automatically, without the need for external intervention.

The `crontab` file¶

The crontab file is structured according to the following rules.

Each line of this file corresponds to a schedule;
Each line has six fields, 5 for the time and 1 for the order;
Each field is separated by a space or a tab;
Each line ends with a carriage return;
A # at the beginning of the line comments it.

[root]# crontab –e
10 4 1 * * /root/scripts/backup.sh
1  2 3 4 5       6

Field	Description	Detail
1	Minute(s)	From 0 to 59
2	Hour(s)	From 0 to 23
3	Day(s) of the month	From 1 to 31
4	Month of the year	From 1 to 12
5	Day(s) of the week	From 0 to 7 (0=7=sunday)
6	Task to execute	Full command or script

!!! Warning The tasks to be executed must use absolute paths and if possible use redirects.

In order to simplify the notation for the definition of time, it is advisable to use special symbols.

Wildcards	Description
`*`	All possible values of the field
`-`	Indicates a range of values
`,`	Indicates a list of values
`/`	Defines a step

Examples:

Script executed on April 15 at 10:25 am:

25 10 15 04 * /root/scripts/script > /log/…

Run at 11am and then at 4pm every day:

00 11,16 * * * /root/scripts/script > /log/…

Run every hour from 11am to 4pm every day:

00 11-16 * * * /root/scripts/script > /log/…

Run every 10 minutes during working hours:

*/10 8-17 * * 1-5 /root/scripts/script > /log/…

For the root user, crontab also has some special time settings:

Setting	Description
@reboot	Run command on system reboot
@hourly	Run command every hour
@daily	Runs daily just after midnight
@weekly	Runs command every Sunday just after midnight
@monthly	Runs command on the first day of the month just after midnight
@annually	Runs January 1st just after midnight

Task execution process¶

A user, rockstar, wants to edit his crontab file:

1) crond checks to see if he is allowed (/etc/cron.allow and /etc/cron.deny).

2) If he is, he accesses his crontab file (/var/spool/cron/rockstar).

Every minute crond reads the schedule files.

3) It executes the scheduled tasks.

4) It reports systematically in a log file (/var/log/cron).