Scheduled automatic execution function (at, crontab) Linux of

concept

In the Linux system, it provides two ways to advance the work arrangements

at only performed once
crontab repeated periodically

By applying these two tools allows us to achieve timing auto-run feature in the operating system level

lab environment

操作系统：CentOS Linux release 7.3.1611 (Core)

IP： 192.168.230.134

at introduction

at command, and will be executed once

This feature uses the atd tool, you need to ensure that the active state

[root@localhost ~]# systemctl status atd
● atd.service - Job spooling tools
   Loaded: loaded (/usr/lib/systemd/system/atd.service; enabled; vendor preset: enabled)
   Active: active (running) since Thu 2017-09-07 18:18:18 CST; 4h 2min ago
 Main PID: 921 (atd)
   CGroup: /system.slice/atd.service
           └─921 /usr/sbin/atd -f

Sep 07 18:18:18 localhost.localdomain systemd[1]: Started Job spooling tools.
Sep 07 18:18:18 localhost.localdomain systemd[1]: Starting Job spooling tools...

at use

Common commands are as follows

[root@localhost ~]# at -help
Usage: at [-V] [-q x] [-f file] [-mMlbv] timespec ...
       at [-V] [-q x] [-f file] [-mMlbv] -t time
       at -c job ...
       atq [-V] [-q x]
       at [ -rd ] job ...
 atrm [-V] job ...  batch

Time format:

format	Examples	significance
HH:MM	01:01	The next execution 1:01
HH:MM YYYY-MM-DD	01:00 2017-09-08	Specify a specific time execution date
HH:MM[am/pm]+number[minutes/hours/days/weeks]	now+3 days	After a certain point in time to put in some time

Examples at

Create a test sh, and modify permissions

[root@localhost ~]# vim test.sh

echo '豆姐威武'

[root@localhost ~]# chmod 755 test.sh [root@localhost ~]# ./test.sh 豆姐威武

View the current time

[root@localhost ~]# date
Thu Sep  7 23:19:51 CST 2017

Use the at command, set 23:21 test.sh execute commands attention with Ctrl + d end

[root@localhost ~]# at 23:21
at> /root/test.sh
at> <EOT>
job 5 at Thu Sep  7 23:21:00 2017

After waiting a minute, receive information system to view the time and the set time

You have new mail in /var/spool/mail/root
[root@localhost ~]# date
Thu Sep  7 23:21:04 CST 2017

View information received by the system

[root@localhost ~]# cat /var/spool/mail/root

From root@localhost.localdomain  Thu Sep  7 23:21:00 2017 Return-Path: <root@localhost.localdomain> X-Original-To: root Delivered-To: root@localhost.localdomain Received: by localhost.localdomain (Postfix, from userid 0) id A139B243D2; Thu, 7 Sep 2017 23:21:00 +0800 (CST) Subject: Output from your job 5 To: root@localhost.localdomain Message-Id: <20170907152100.A139B243D2@localhost.localdomain> Date: Thu, 7 Sep 2017 23:21:00 +0800 (CST) From: root@localhost.localdomain (root) 豆姐威武

A more practical copy of the script:
time off

[root@localhost ~]# at 18:00 2017-09-10
at> /bin/sync
at> /bin/sync
at> /sbin/shutdown -h now
at> <EOT> job 6 at Sun Sep 10 18:00:00 2017

It will shut down automatically at 2017/09/07 18:00

at cancel command

View the current at a command
using the command atq

[root@localhost ~]# atq
6   Sun Sep 10 18:00:00 2017 a root

Cancel command
using the command atrm

[root@localhost ~]# atrm 6

crontab Introduction

crontab command performed periodically according to the configuration

His record will be stored in / var / log / cron in

This feature requires crond service, the need to ensure its normal state

[root@localhost ~]# systemctl status crond
● crond.service - Command Scheduler
   Loaded: loaded (/usr/lib/systemd/system/crond.service; enabled; vendor preset: enabled)
   Active: active (running) since Thu 2017-09-07 18:18:18 CST; 5h 51min ago Main PID: 920 (crond) CGroup: /system.slice/crond.service └─920 /usr/sbin/crond -n Sep 07 18:18:18 localhost.localdomain systemd[1]: Started Command Scheduler. Sep 07 18:18:18 localhost.localdomain systemd[1]: Starting Command Scheduler... Sep 07 18:18:18 localhost.localdomain crond[920]: (CRON) INFO (RANDOM_DELAY will be ...) Sep 07 18:18:19 localhost.localdomain crond[920]: (CRON) INFO (running with inotify ...) Hint: Some lines were ellipsized, use -l to show in full.

use crontab

Common commands are as follows:

[root@localhost ~]# crontab -help
crontab: invalid option -- 'h'
crontab: usage error: unrecognized option
Usage:
 crontab [options] file
 crontab [options]
 crontab -n [hostname]

Options:
 -u <user>  define user
 -e         edit user's crontab
 -l         list user's crontab -r delete user's crontab -i prompt before deleting -n <host> set host in cluster to run users' crontabs -c get host in cluster to run users' crontabs -s selinux context -x <mask> enable debugging

Time format:

*    *    *    *    *
-    -    -    -    -
|    |    |    |    |
|    |    |    |    |
|    |    |    |    +----- day of week (0 - 7) (Sunday=0 or 7) | | | +---------- month (1 - 12) | | +--------------- day of month (1 - 31) | +-------------------- hour (0 - 23) +------------------------- min (0 - 59)

crontab examples

Or use test.sh created before doing the experiment

New plan to use crontab command

To view the current time

[root@localhost ~]# date
Thu Sep  7 23:52:12 CST 2017

Create a plan, set to 23:55 every day script execution /root/test.sh

[root@localhost ~]# crontab -e

55 23 * * * /root/test.sh

no crontab for root - using an empty one crontab: installing new crontab

View the current schedule:

[root@localhost ~]# crontab -l
55 23 * * * /root/test.sh

Wait two minutes:

[root@localhost ~]# date
Thu Sep  7 23:55:07 CST 2017 You have new mail in /var/spool/mail/root

View record:

[root@localhost ~]# cat /var/spool/mail/root 
From root@localhost.localdomain  Thu Sep  7 23:55:01 2017 Return-Path: <root@localhost.localdomain> X-Original-To: root Delivered-To: root@localhost.localdomain Received: by localhost.localdomain (Postfix, from userid 0) id 45D1028EB7; Thu, 7 Sep 2017 23:55:01 +0800 (CST) From: "(Cron Daemon)" <root@localhost.localdomain> To: root@localhost.localdomain Subject: Cron <root@localhost> /root/test.sh Content-Type: text/plain; charset=UTF-8 Auto-Submitted: auto-generated Precedence: bulk X-Cron-Env: <XDG_SESSION_ID=52> X-Cron-Env: <XDG_RUNTIME_DIR=/run/user/0> X-Cron-Env: <LANG=en_US.UTF-8> X-Cron-Env: <SHELL=/bin/sh> X-Cron-Env: <HOME=/root> X-Cron-Env: <PATH=/usr/bin:/bin> X-Cron-Env: <LOGNAME=root> X-Cron-Env: <USER=root> Message-Id: <20170907155501.45D1028EB7@localhost.localdomain> Date: Thu, 7 Sep 2017 23:55:01 +0800 (CST) 豆姐威武

Removing crontab program

If you need to remove one or two programs, the direct use crontab -e to enter the edit delete
If you want to empty the plan, -r can

[root@localhost ~]# crontab -l
55 23 * * * /root/test.sh [root@localhost ~]# crontab -r [root@localhost ~]# crontab -l no crontab for root

crontab supplement

You can also edit / etc / crontab file to achieve planned script

[root@localhost ~]# vim /etc/crontab 

SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin MAILTO=root # For details see man 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

But sometimes this way does not take effect immediately, you need to restart the crond service