Linux command (+): at command

1. Command format:

at[parameters [TIME]

2. Command function:

Executes a specified task at a specified time, only once, and requires the ATD process to be turned on (

Ps-ef | grep ATD View, open with/etc/init.d/atd start or restart, boot up is required to run Chkconfig--level 2345 ATD on).

3. Command parameters:

-M sends a message to the user when the specified task is completed, even if there is no standard output

-I ATQ aliases

-D ATRM Aliases

-V shows when the task will be executed

-C Print Task content to standard output

-V Display version information

-q< Queue > Use specified queue

-f< file > read from a specified file into a task instead of reading from a standard input

-t< Time parameter > Submit a task to run in the form of a time parameter

At allows you to use a fairly complex set of time-specific methods. He was able to accept the time specified in the hh:mm (hour: minute) of the day. If the time has passed, then it will be executed the next day. Of course, you can use Midnight (late night), noon (noon), Teatime (tea time, usually 4 o'clock in the afternoon) and other vague words to specify the time. The user is also able to use the 12-hour timekeeping system, which is either a AM (morning) or PM (afternoon) After the time to indicate whether it is morning or afternoon. You can also specify a specific date for the execution of the command, specifying the format for Month Day (month) or mm/dd/yy (Month/day/year) or Dd.mm.yy (day, month, year). The specified date must be followed by the specified time. The absolute timing method is described above, and the relative timing method can be used, which is good for arranging orders that will be executed soon. The specified format is: Now + Count Time-units, today is the current time, Time-units is the time unit, here can be minutes (minutes), hours (hours), Days (day), Weeks (week). Count is the amount of time, whether it is a few days, or a few hours, and so on. A more timed method is to specify the time to complete the command directly using today, tomorrow (tomorrow).

Time: Date format, where you can define when to do at this task, in the following format:

hh:mm

Ex> 04:00

At today's hh:mm time, if that moment is over, tomorrow's hh:mm will do the job.

hh:mm YYYY-MM-DD

Ex> 04:00 2009-03-17

To mandate the task at a particular time of day of the month of the year.

HH:MM[AM|PM] [Month] [Date]

Ex> 04pm March 17

Also, forcing a certain day of the year to perform the task at some point in the month

HH:MM[AM|PM] + number [Minutes|hours|days|weeks]

Ex> now + 5 minutes

Ex> 04pm + 3 days

That is, it takes a few more minutes to do that task at a certain point in time.

4. Usage examples:

Example 1: Three days after 5 o'clock in the afternoon Chung execution of/bin/ls

Command:

At 5pm+3 days

Output:

[Email protected] ~]# at 5pm+3 days

At>/bin/ls

At> <EOT>

Job 7 at 2013-01-08 17:00

[Email protected] ~]#

Description

Example 2: Tomorrow 17 o'clock, the output time is within the specified file

Command:

At 17:20 tomorrow

Output:

[Email protected] ~]# at 17:20 tomorrow

At> Date >/root/2013.log

At> <EOT>

Job 8 at 2013-01-06 17:20

[Email protected] ~]#

Description

Example 3: After scheduling a task, we can use the ATQ command to see that the system is not performing a work task until it is executed.

Command:

Atq

Output:

[Email protected] ~]# ATQ

8 2013-01-06 17:20 a root

7 2013-01-08 17:00 a root

[Email protected] ~]#

Description

Example 4: Delete a task that has already been set

Command:

ATRM 7

Output:

[Email protected] ~]# ATQ

8 2013-01-06 17:20 a root

7 2013-01-08 17:00 a root

[Email protected] ~]# ATRM 7

[Email protected] ~]# ATQ

8 2013-01-06 17:20 a root

[Email protected] ~]#

Description

Example 5: Displaying the contents of a task that has been set

Command:

At-c 8

Output:

[Email protected] ~]# At-c 8

#!/bin/sh

# Atrun Uid=0 gid=0

# Mail Root 0

Umask 22 omit n characters here

Date >/root/2013.log

[Email protected] ~]#

Description

Example 6:

Command:

Output:

Description

5. How the ATD starts and at runs:

5.1 ATD Start-up

To use a one-time scheduling task, our Linux system must have a service that is responsible for this program, which is the ATD service. However, not all Linux distributions will be opened by default, so at some point we need to manually activate the ATD service. The activation method is simple:

Command:

/ETC/INIT.D/ATD start

/ETC/INIT.D/ATD restart

Output:

[[email protected]/]#/etc/init.d/atd start

[Email protected]/]#/ETC/INIT.D/ATD

Usage:/etc/init.d/atd {start|stop|restart|condrestart|status}

[[email protected]/]#/etc/init.d/atd stop

Stop atd:[OK]

[Email protected]/]# Ps-ef|grep ATD

Root 25062 24951 0 14:53 pts/0 00:00:00 grep ATD

[[email protected]/]#/etc/init.d/atd start

[OK]td:[OK]

[Email protected]/]# Ps-ef|grep ATD

Root 25068 1 0 14:53? 00:00:00/usr/sbin/atd

Root 25071 24951 0 14:53 pts/0 00:00:00 grep ATD

[Email protected]/]#/etc/init.d/atd restart

Stop atd:[OK]

[OK]td:[OK]

[Email protected]/]#

Description

/ETC/INIT.D/ATD start does not start, the ATD service is started directly

/ETC/INIT.D/ATD Restart Service has been started, restart the ATD service

Note: Configure startup to start this service, so that every restart must be done again

Command:

Chkconfig ATD on

Output:

[Email protected]/]# chkconfig ATD on

[Email protected]/]#

How the 5.2 at operates

Since it is a scheduled task, there should be a way for the task to execute, and these tasks will be queued to the schedule. So how does a planned task occur? In fact, we use the AT command to generate the scheduled task to be run, and to write the scheduled task into the/var/spool/at/directory as a text file, which can wait for the ATD service to be taken and run. It's so simple.

However, not all people are allowed to do at the scheduled task. Why? Because of the system security reasons. Many hosts by the so-called attack, the most often found is that their system has a lot of hacking programs, these programs are likely to use some of the planning tasks to run or collect your system operation information, and regularly sent to the hacker. So, unless you have an approved account, don't let them use the AT command first. How can you control the use of at?

We can use the two files of/etc/at.allow and/etc/at.deny to limit the usage of the at. After adding these two files, at is working like this:

First find/etc/at.allow This file, written in this file users can use at, no user in this file can not use at (even if not written in At.deny);

If the/etc/at.allow does not exist, look for/etc/at.deny this file, if the user written in this at.deny can not use at, but not in this At.deny file users, it is possible to use the AT command.

If none of the two files exist, then only root can use the AT command.

With this explanation, we know that/etc/at.allow is a more rigorous way of managing, while/etc/at.deny is more loosely (because the account is not in the file, it is able to run at). In general distributions, because it is assumed that all users on the system are trustworthy, an empty/etc/at.deny file is usually retained, meaning that all people are allowed to use the AT command (you can check the file yourself). However, in case you don't want some users to use at, write the user's account to/etc/at.deny! One account to write a line.

Linux command (+): at command