A brief summary of Linux recurring Task Scheduler at and Cron

One-time mission plan: At,batch

Recurring Task Scheduler: Cron

At: Simple usage

At time

At>

Submit Task: ^c+d

Job Result: Send the current user on the system as a message

Send and receive mail:

Service start or Not Status View command:

NETSTAT-TNL |grep ": 25"

The default send and receive messages are only made between users on this computer and only through addresses on the loopback interface.

# Mail

Q: Exit

N: View Nth message

Send an email to the specified user (within the system):

# Mail username[@HOSTNAME]

-S "SUBJECT" theme

echo | Mail can also use pipelines

Format of Time

Exact time:

hh:mm

Blur Time:

now+ #UNIT

Noon, Midnight,teatime, tomorrow

Job Queue view:

At-l, ATQ

To delete a job that has not been run:

at-d [Job_num], at RM

You can also put all the tasks at the at to be executed in the file so that at reads this file as the task to be run (for a large amount of jobs)

At-f/path/to/somefile Time

Batch: No time is required and will automatically run at a later point in time for a system resource that is idle

Recurring Task Scheduler: Cron

Rely on background services to view the running status

Service Crond status CentOS {5,6} version view command, you can use cron if you have the following output

Crond (PID 1680) is running

There are two types of cron tasks:

System cron:/etc/crontab

User cron: Defined in a file with the same name as the user name in the/var/spool/cron/directory

Each row defines a separate task:

The following information is available after viewing/etc/crontab

[Email protected] ~]# Cat/etc/crontab

Shell=/bin/bash

Path=/sbin:/bin:/usr/sbin:/usr/bin

Mailto=root

# for details see Mans 4 Crontabs

# 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

Consists of the following 3 parts:

Point in time (recurring task)

User-name: User Identity

Command: The task to run

Note: It is recommended that when you define a cron task, the command you use uses an absolute path, or a custom environment variable that has a full path.

Time notation:

1. Each time is a value in the range of valid values that should be used for the period, * to represent all valid values for the period of use,

2. */#: At the corresponding time bit valid value on every # time unit once

Note: The range of values defined for a minute should be divisible by 60

3. Continuous point of time:-

4. Discrete point-in-time:,

Results after the task is run: The message is sent to the task-related user:

Drop message notification: output redirection

COMMAND >/dev/null

COMMAND &>/dev/null

COMMAND >/dev/null 2>&1

/dev/null: Software-simulated device, bit bucket, data black hole

Definition of User cron task:

crontab command:

-L: Lists all tasks that have been defined

-E: Open editor

-r:remove, remove All Tasks

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

Note: The% in the crontab file must be escaped or referenced in single quotation marks.

Homework::

Every Monday to Saturday 3:20 A.M., run the CP command to save the/etc/directory for archiving, the storage location is/BACKUPS/ETC-YYYY-MM-DD

3 * * 1-6/bin/cp/etc/backups/etc-$ (date+ '%y-%m-%d ')

[[email protected] ~]# mkdir-p/backups/etc[[email protected] ~]# crontab e[[email protected] etc]# CRONTAB-L20 3 * * 1- 6/bin/cp/etc/backups/etc-$ (date + '%y-%m-%d ')

Every Sunday 2:30 A.M., run the CP command to back up the/etc/fstab file with a storage location of/BACKUP/FSTAB-YYYY-MM-DD-HH-MM-SS

2 * * 7/bin/cp/etc/fstab/backup/fstab-$ (date + '%y-%m-%d-%h-%m-%s ')

[[email protected] etc]# mkdir-p/backups/fstab[[email protected] etc]# crontab e[[email protected] etc]# crontab L30 2 * * 7/bin/cp/etc/fstab/backup/fstab-$ (date + '%y-%m-%d-%h-%m-%s ')

Every night 12 o'clock, 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

6 0 * * */bin/egrep/proc/meminfo "^[s,m].*" >>/statistics/meminfo.txt && Echo ========================= = >>/statistics/meminfo.txt

[[email protected] etc]# mkdir-p/backups/fstab[[email protected] statistics]# crontab E[[email protected] statistics]# C RONTAB-L6 0 * * */bin/egrep/proc/meminfo "^[s,m].*" >>/statistics/meminfo.txt && Echo ================ ========== >>/statistics/meminfo.txt

A brief summary of Linux recurring Task Scheduler at and Cron