Linux task scheduler (at, crontab)

Linux task scheduler (at, crontab)

In Linux, task plans are scheduled to execute certain tasks or periodically execute some tasks.

Task Plan:

Execute a task at a time in the future: at, batch

Periodically execute a task: crontab

After the task is executed, liunx will send an email to the current user.

Email Service: smtp: simple mail transmission protocol

Path in linux:/var/spool/mail/USERNAME

View the MAIL command: yum install mailx to install the mail service

Mail command:

Mail: Enter the interactive mail sending and receiving interface;

Mail-s 'subobject' username @ hostname

Mail-s 'subobject' username @ hostname </PATH/FROM/SOMEFILE

COMMAND | mail-s 'subobject' username @ hostname

At command:

A job running in the future:

Job Queue is supported:

The default value is queue;

Ctrl + d: Submit the task

At [option]... TIME

TIME:

(1) absolute time

HH: MM,

MMDD [CC] YY, MM/DD/[CC] YY, DD. MM. [CC] YY or [CC] YY-MM-DD

Tomorrow

(2) Relative time

Now + # UNIT

Minute, hour, day, week

(3) fuzzy time

Midnight

Noon

Teatime

Common options:

-Q QUEUE: at Job QUEUE;

-F/PATH/FROM/SOMEFILE: reads the job to be run FROM the specified file;

-L: view the list of other running jobs in the Job Queue. It is equivalent to using the atq command;

-C AT_JOB_NUM: view the content of the job to be run;

-D: deletes a specified job, which is equivalent to atrm.

Batch:

The system selects the task to run when the resource is idle;

Crontab: periodic Task Plan

Daemon: crond

There are two types of periodic tasks:

(1) The system cron task does not have the default running user identity, so you need to specify another operator;

/Etc/crontab

Vim command

# Example of job definition:

#. ---------------- Minute (0-59)

# |. ----------- Hour (0-23)

# |. ---------- Day of month (1-31)

# |. ------- Month (1-12) OR jan, feb, mar, apr...

# |. ---- Day of week (0-6) (Sunday = 0 or 7) OR sun, mon, tue, wed, thu, fri, sat

# |

# ***** User-name command to be executed

Seven fields:

First five fields: Time Point

User-name: the user ID used to run the task.

Command to be executed: to run a task

(2) User cron task: a user submits a task. By default, the task runs as a submitter. Therefore, you do not need to specify another operator;

/Var/spool/cron/USERNAME

Vim command; not recommended

Crontab command: Recommended

# Example of job definition:

#. ---------------- Minute (0-59)

# |. ----------- Hour (0-23)

# |. ---------- Day of month (1-31)

# |. ------- Month (1-12) OR jan, feb, mar, apr...

# |. ---- Day of week (0-6) (Sunday = 0 or 7) OR sun, mon, tue, wed, thu, fri, sat

# |

# ***** Command to be executed

Five time points:

Minute: valid value range: 0-59;

Hour: 0-23

Day: 1-31

Month: 1-12

Weeks: 0-7

Note: It is not recommended that you use both the number of months and the number of weeks;

For example:

6 ****

Time Representation:

*: Each time point within the valid value range of the corresponding time point;

-: A specific continuous time range of 3-7

,: A discrete time point, 3, 5, 7

/#: The amount of time within the effective time range; used to specify the frequency;

1-30/4, */4

5 */3 ***/bin/echo "howdy"

5 7 ** 1-5/bin/echo "howdy"

Crontab command:

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

-Uuser: it is a cron task of the specified target user instead of a cron task. Only root has the permission to manage cron tasks of other users. By default, it manages its own cron tasks;

-L: list: Lists tasks;

-R: remove: removes all tasks;

-E: edit, edit. Open the Default Editor of the Current shell session to edit the cron task table;

Note:

(1) If you do not want to receive the notification email for the task execution result:

COMMAND>/dev/null

COMMAND &>/dev/null

(2) For the crontab file, % has a special function. If % appears in the command, remember to escape it or use single quotes to reference it;

(3) the PATH variable of crontab is not exactly the same as the user variable. Therefore, we recommend that you use the absolute PATH for the task in cron.

/Root/bin/a. sh

Exercise:

1. Every Monday to Saturday 03:20, run the cp command to archive the/etc/directory for another storage, the storage location is/backups/etc-YYYY-MM-DD;

~] # Crontab-l

20 3 ** 1-6/bin/cp/etc-rpf/backups/etc-'file \ % Y-\ % M-\ % d'

2. Run the cp command to back up the/etc/fstab file at 02:30 every Sunday. The storage location is/backup/fstab-YYYY-MM-DD-hh-mm-ss.

~] # Crontab-l

30 2 ** 7/bin/cp/etc/fstab/backup/fstab-'file \ % Y-\ % M-\ % D-\ % H-\ % M- \ % s'

3. at every night, get all the lines starting with S or M in the/proc/meminfo file and append them to the/statistics/meminfo.txt file, add a separator line like =====================;

~] # Crontab-l

00 0 **** echo "====================">/proc/meminfo | egrep "^ S | ^ M"/proc /meminfo>/statistics/meminfo.txt

Use crontab in Linux to create scheduled tasks

Routine scheduling of crontab in Linux

Linux crontab does not run troubleshooting

Ubuntu uses crontab for scheduled tasks

Linux scheduled task (at batch crontab anacron)

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