Crontab scheduled execution of task commands under Linux

Transferred from: https://www.cnblogs.com/longjshz/p/5779215.html

In Linux, the tasks performed by the cycle are typically handled by the cron Daemon (ps-ef|grep cron). Cron reads one or more configuration files that contain the command line and its invocation time.
The cron configuration file is called "crontab" and is shorthand for "cron table".

First, cron service
Cron is a timed execution tool under Linux that can run a job without human intervention.
Service Crond Start//Startup services
Service Crond stop//Shutdown services
Service Crond restart//Restart services
Service Crond Reload//Reload Configuration
Service Crond Status//view services statuses

Second, cron find the configuration file in 3 places:
1,/var/spool/cron/This directory is stored in each user including root crontab task, each task is named after the creator, such as Tom built crontab task corresponding to the file is/var/spool/cron/tom. Generally, a user has only one crontab file at most.

/etc/crontab This document is responsible for arranging the crontab of maintenance systems and other tasks developed by the system administrator.

Shell=/bin/bash
Path=/sbin:/bin:/usr/sbin:/usr/bin
Mailto=root
home=/

# 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

• mailto=root: Does it mean that when an error occurs in the/etc/crontab of a routine command in this file, the error message or the message that is displayed on the screen is passed to whom? Because root and no longer user-side to receive the letter, therefore, I usually will this e-mail to my account, so that I always know the status of the system!
• * * * * * root run-parts/etc/cron.hourly : After #run-parts this line of command, we can find that five digits followed by Root, this line represents the Yes "level of execution is root" of course, you can also change this line into other identities! The/etc/cron.hourly that  run-parts represents is "all executables within a directory (/etc/cron.hourly)", that is, 01 points per hour, the system will be/etc/as root. Cron.hourly execute all executable files in this directory! The following three lines are also similar in meaning! You can go to the bottom of the/etc/to see, the system is originally preset these 4 directories! You can write the commands you need to execute every day directly to/etc/cron.daily, and you don't need to use the CRONTAB-E program!

Four,/etc/cron.d/this directory is used to hold any crontab files or scripts to be executed.

Five, permissions (?
Crontab permissions issue to/var/adm/cron/Next, the file Cron.allow and Cron.deny exist
Usage:  
1, if none of the two files exist, Only the root user can use the crontab command.  
2, if Cron.allow exists but Cron.deny does not exist, only the user listed in the Cron.allow file can use the crontab command, and if the root user is not inside, the root user cannot use crontab.  
3, if the Cron.allow does not exist, Cron.deny exists, only the users who are listed in the Cron.deny file cannot use the crontab command, which can be used by other users.  
4, if two files are present, Users who are listed in the Cron.allow file and are not listed in Cron.deny can use crontab, if the same user is in two files, to cron.allow whether the user is in the file, or if there is a user in Cron.allow, you can use the crontab command.  

Normal users in AIX have crontab permissions by default, and if you want to restrict users from using crontab, you need to edit/var/adm/cron/cron.deny 
Hp-unix The default normal user does not have crontab permissions, to let go of the normal user's crontab permissions can be compiled

Six, create cron script
The first step: Write the cron script file named Crontest.cron.
15,30,45,59 * * * * echo "xgmtest ..." >> xgmtest.txt  , every 15 minutes, perform a print once command  
Step Two: Add a scheduled task. Executes the command "Crontab Crontest.cron". Take care of  
Step Three: "Crontab-l" to see if the scheduled task is successful or to detect if the corresponding cron script is generated under/var/spool/cron

Note: This is a direct replacement for the user under the crontab, not the new

Vii. usage of crontab
The crontab command is used to install, delete, or list tables used to drive a cron background process. The user places the sequence of commands that need to be executed in the crontab file for execution.
Each user can have their own crontab file. The crontab file under/var/spool/cron cannot be created directly or modified directly. The crontab file was created with the crontab command

How to enter the command and time to execute in the crontab file. Each row in the file contains six fields, the first five of which are the time the specified command was executed, and the last field is the command to be executed.
Spaces or tabs are used to separate each field. The format is as follows:
Minute hour day-of-month month-of-year day-of-week commands
Legal value 00-59 00-23 01-31 01-12 0-6 (0 is Sunday)
In addition to the numbers there are several special symbols that are "*", "/" and "-", ",", * represent all the values within the range of the number, "/" for each meaning, "/" for each of the 5 units, "-" represents from a number to a number, "," separate several discrete numbers.

-L Displays the current crontab on the standard output.
-R Deletes the current crontab file.
-e Edit the current crontab file using the editor referred to in visual or editor environment variables. When you finish editing, the edited file is automatically installed.

Viii. Examples:
Every morning at 6.
0 6 * * echo "Good morning." >>/tmp/test.txt//Note simply Echo, no output is visible from the screen, because cron emails any output to root.

Every two hours
0 */2 * * echo "has a break now." >>/tmp/test.txt

11 o'clock to 8 a.m. every two hours and eight a.m..
0 23-7/2,8 * * * echo "a good Dream" >>/tmp/test.txt

Every month, number 4th and Monday to Sunday, three a.m., 11.
0 4 * 1-3 command line

January 1 morning, 4.
0 4 1 1 * command line Shell=/bin/bash path=/sbin:/bin:/usr/sbin:/usr/bin mailto=root//If an error occurs or there is data output, the data is sent to this account as an email home= /

Execute scripts within/etc/cron.hourly per hour
* * * * * Root run-parts/etc/cron.hourly
Execute scripts within/etc/cron.daily daily
4 * * * Root run-parts/etc/cron.daily

Execute scripts within/etc/cron.weekly every week
4 * * 0 root run-parts/etc/cron.weekly

Every month to execute scripts within the/etc/cron.monthly
4 1 * * Root run-parts/etc/cron.monthly

Note: "Run-parts" This parameter, if you remove this parameter, you can later write to run a script name, not the folder name.

Every day 4 o'clock in the afternoon, 5, 6 points of 5 min, min, min, min, min, min, when the execution of orders.
5,15,25,35,45,55 16,17,18 * * command

Every Monday, three, five 3:00 the system enters the maintenance state and restarts the system.
* * 1,3,5 shutdown-r +5

10 minutes per hour, 40 minutes execute the innd/bbslin command in the user directory:
10,40 * * * * Innd/bbslink

1 minutes per hour execute the bin/account command in the user directory:
1 * * * * bin/account

Execute two instructions (each instruction is delimited) below the user directory every day 3:20 A.M.:
3 * * * (/bin/rm-f expire.ls logins.bad;bin/expire$#@62;expire.1st)

Each year in January and April, 4th to 9th of 3:12 and 3:55 execute/bin/rm-f expire.1st This command, and add the results after mm.txt this file (mm.txt file is located in the user's own directory location).
12,55 3 4-9 1,4 */bin/rm-f expire.1st$#@62;$#@62;mm.txt

Crontab scheduled execution of task commands under Linux