How to use the Task Scheduler command at and Crontab on Linux systems

First, summarize the task plan on Linux system at, crontab detailed use method

1. Planning Task Description

Perform a task at a time in the next point: At,batch;

Perform a task periodically: crontab;

2. e-Mail Service

Scheduled tasks send results to users who perform tasks by default in the form of a message

Mail command:

Mail username (@hostname): Enter the interactive send and receive mail interface

Mail-s ' SUBJECT ' [email protected]: Set message subject

</path/from/somefile: Send file content to recipients

echo "XXX" | Mail-s ' SUBJECT ' [email protected]: send echo output to Recipient

3. At command use

At [option] ..... Time

(1) Common options

- q queue:at Job Queue

-f/path/from/somefile: Reads the job to run from the specified file

-L: View the list of jobs that are running in the job queue (equals the ATQ command)

-C At_job_num: Look at the contents of the Run job

-D Number: Deletes the specified job, equivalent at RM

(2) Time format

Absolute time:

hh:mm,
Mmdd[cc]yy, Mm/dd/[cc]yy, dd.mm.[cc]yy or [cc]yy-mm-dd

Relative time:

now+ #UNIT (now+3minute)

Minute, hour, day, week

Blur Time:

midnight,noon,teatime

[Email protected] ~]#/etc/init.d/atd statusatd is stopped[[email protected] ~]#/etc/init.d/atd startstarting ATD: [OK]  [Email protected] ~]# at 17:19at> ls/etcat> cat/etc/fstabat> <eot>job 2 at 2015-09-08 17:19you with Mail In/var/spool/mail/root

4. Batch: The system chooses to run the specified task when the resource is idle.

5. Crontab: Recurring Task plan

Daemon: Crond

[Email protected] ~]#/etc/init.d/crond statuscrond (PID 1977) is running ...

(1) System cron Task: No user identity is run by default, so you need to specify additional runner

Configuration file:/etc/crontab

(2) User Cron Task: Submitted by a user, run as submitter by default (CRONTAB-E recommended)

Configuration file:/var/spool/cron/username

(3) Configuration file field parsing

[[email protected] ~]# cat /etc/crontabshell=/bin/ bashpath=/sbin:/bin:/usr/sbin:/usr/binmailto=roothome=/# for details see man 4  CRONTABS# EXAMPLE OF JOB DEFINITION:#&NBSP---------------- minute  (0 -  59) # |  .------------- hour  (0 - 23) # |  |   .---------- day of month  (1 - 31) # |  |  |  . ------- month  (1 - 12)  OR jan,feb,mar,apr ...# |  |   |  | &NBSP---- day of week  (0 - 6)   (sunday=0 or  7)  or sun,mon,tue,wed,thu,fri,sat# |  |  |  |  |#  *  *  *  *  * user-name command to be  Executed 

*****:5 Time Point: (A few hours and weeks)

Minutes: 0-59

Hours: 0-23

Day: 1-31

Weeks: 0-7

Months and weeks not recommended for simultaneous use

User-name: Run the task as a user

command to be executed: the task to run (command)

Instance:

Annual: 2 3 4 5 *

Monthly: 20 18 6 * *

Daily: 20 9 * * *

Weekly: 17 16 * * *

Per hour: 3 * * * *

Per minute: * * * * * *

Point-in-time notation:

1. *: Each time point within the valid value range of the time point

2.-: Indicates a specific continuous time range (3 * * 1-5 command)

3,,: denotes discrete point of time (2 * * 2,4,6 command)

4./#: one time unit per # in the effective time range; Used to specify the frequency ()

*/3 * * * * command: executed every 3 minutes

* */3 * * Command: Every three hours on arrival, every minute of this hour is executed

6 */3 * * Command: No three hours to execute

Note: It is not accurate to 7 minutes at a time (sleep mechanism can be used)

(4) crontab command

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

-u User: Not managing your own cron task, but specifying the target user's cron task; Root has the right to manage other users ' cron tasks default to manage your own

-l:list, List tasks

-r:remove, remove All Tasks

-e:edi T, edit, open a default editor for the current shell session to edit the Cron task table

Note:   

(1) If you do not want to receive notification messages for task execution results:

COMMAND >/dev/null

COMMAND &>/dev/null

(2) for crontab file,% has a special function, if the command will appear in the%, remember to escape, or use single quotation marks to its reference;

(3) Crontab's path variable is not exactly the same as the user's variable, so it is recommended that the task in cron use an absolute path

/root/bin/a.sh (including commands to be used in the script)

Second, every Monday to Saturday 3:20 A.M., run the CP command to archive the/etc/directory, storage location is/backups/etc-yyyy-mm-dd;

[[email protected] ~]# mkdir-pv/backupsmkdir:created directory '/backups ' [[email protected] ~]# Crontab-eno crontab fo R root-using an empty ONE20 3 * * 1-6/bin/cp-a/etc/backups/etc-' Date +f% '

three, every Sunday 2:30 A.M., run the CP command to backup the/etc/fstab file, the storage location is/backup/fstab-yyyy-mm-dd-hh-mm- SS;

2 * * 7/bin/cp-a/etc/fstab/backup/fstab-' date +f '

12 o'clock every night, get all lines in the/proc/meminfo file that begin with S or M, append to the/statistics/meminfo.txt file, and precede the daily message with a similar =============== separator line

[[email protected] ~]# mkdir-pv/statisticsmkdir:created directory '/statistics ' 0 0 * * */bin/echo-e "===========\n" >>/statistics/meminfo.txt &&/bin/grep "^[s| M] "/proc/meminfo >>/statistics/meminfo.txt

How to use the Task Scheduler command at and Crontab on Linux systems