Linux timed Task crontab command detailed

The Linux system's scheduled tasks are controlled by cron (Crond), the system service. Linux systems have a lot of planned work on them, so this system service is started by default. In addition, because the user can set up scheduled tasks themselves, the Linux system also provides a command for the user to control scheduled tasks: the crontab command.

I. Introduction of Crond

Crond is a daemon that is used to periodically perform certain tasks or wait for certain events under Linux, similar to Scheduled tasks under Windows, when the operating system is installed, the Service tool is installed by default and the Crond process is started automatically. The Crond process periodically checks to see if there is a task to perform and automatically executes the task if there are tasks to perform.

The task scheduling under Linux is divided into two categories, system task scheduling and user task scheduling.

System Task Scheduling: The work to be performed by the system periodically, such as writing cache data to hard disk, log cleanup, etc. In the/etc directory there is a crontab file, this is the System Task Scheduler configuration file.

/etc/crontab文件包括下面几行：cat /etc/crontabSHELL=/bin/bashPATH=/sbin:/bin:/usr/sbin:/usr/binMAILTO=HOME=/# run-parts51 * * * * root run-parts /etc/cron.hourly24 7 * * * root run-parts /etc/cron.daily22 4 * * 0 root run-parts /etc/cron.weekly42 4 1 * * root run-parts /etc/cron.monthly

The first four rows are the environment variables that are used to configure the Crond task to run, the shell variable specifies which shell the system will use, this is bash, and the second line of the path variable specifies the path to the System execution command. The third line of the mailto variable specifies that Crond's task execution information will be emailed to the root user, and if the value of the mailto variable is null, the task execution information is not sent to the user, and the home variable in line fourth specifies the home directory to use when executing the command or script. The meaning of line sixth to Nineth is described in detail in the next section.

User Task Scheduling: users to perform regular work, such as user data backup, scheduled email reminders and so on. Users can use the Crontab tool to customize their own scheduled tasks. All user-defined crontab files are saved in the/var/spool/cron directory. Its file name is the same as the user name.

使用者权限文件：文件：/etc/cron.deny说明：该文件中所列用户不允许使用crontab命令文件：/etc/cron.allow说明：该文件中所列用户允许使用crontab命令文件：/var/spool/cron/说明：所有用户crontab文件存放的目录,以用户名命名crontab文件的含义：用户所建立的crontab文件中，每一行都代表一项任务，每行的每个字段代表一项设置，它的格式共分为六个字段，前五段是时间设定段，第六段是要执行的命令段，格式如下：minute hour day month week command其中：minute： 表示分钟，可以是从0到59之间的任何整数。hour：表示小时，可以是从0到23之间的任何整数。day：表示日期，可以是从1到31之间的任何整数。month：表示月份，可以是从1到12之间的任何整数。week：表示星期几，可以是从0到7之间的任何整数，这里的0或7代表星期日。command：要执行的命令，可以是系统命令，也可以是自己编写的脚本文件。

在以上各个字段中，还可以使用以下特殊字符：星号（*）：代表所有可能的值，例如month字段如果是星号，则表示在满足其它字段的制约条件后每月都执行该命令操作。逗号（,）：可以用逗号隔开的值指定一个列表范围，例如，“1,2,5,7,8,9”中杠（-）：可以用整数之间的中杠表示一个整数范围，例如“2-6”表示“2,3,4,5,6”正斜线（/）：可以用正斜线指定时间的间隔频率，例如“0-23/2”表示每两小时执行一次。同时正斜线可以和星号一起使用，例如*/10，如果用在minute字段，表示每十分钟执行一次。

Second, Crond service

安装crontab：yum install crontabs服务操作说明：CentOS 7.0以前的命令/sbin/service crond start //启动服务/sbin/service crond stop //关闭服务/sbin/service crond restart //重启服务/sbin/service crond reload //重新载入配置/sbin/service crond status //启动服务查看crontab服务是否已设置为开机启动，执行命令：ntsysv加入开机自动启动：chkconfig –level 35 crond onCentOS 7.0以后的命令，修改为 systemctl start/stop/restart/reload/status 服务名.service开机自启动：systemctl enable/disable 服务名.service

Three, crontab command detailed

1. Command format:

crontab [-u user] file

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

2. Command function:

With the crontab command, we can execute specified system instructions or shell script scripts at a fixed interval of time. The units of the time interval can be any combination of minutes, hours, days, months, weeks, and more. This command is very useful for periodic log analysis or data backup.

3. Command parameters:

-u user: Used to set a user's crontab service, for example, "-u ixdba" means to set IXDBA user's crontab service, this parameter is usually run by the root user.

file: File is the name of the command file, which represents the task list file as crontab and loaded into crontab. If this file is not specified on the command line, the crontab command will accept the commands typed on the standard input (keyboard) and load them into crontab.

-e: Edits the contents of a user's crontab file. If you do not specify a user, the crontab file for the current user is edited.

-l: Displays the contents of a user's crontab file, or displays the contents of the current user's crontab file if no user is specified.

-r: Deletes a user's crontab file from the/var/spool/cron directory, or deletes the current user's crontab file by default if no user is specified.

-i: Give confirmation prompt when deleting user's crontab file.

4. Common methods:

1). Create a new crontab file

The first thing to do before considering submitting a crontab file to the cron process is to set the environment variable editor. The cron process depends on it to determine which editor to use to edit the crontab file. 9 9 of UNIX and Linux users use VI, and if you do, then you edit the $ home directory. Profile file in which to add such a line:

Editor=vi; Export EDITOR

Then save and exit. You might want to create a file named <user> Cron, where <user> is the user name, for example, Davecron. Add the following to the file.

# (put your own initials here)echo the date to the console every

# 15minutes between 6pm and 6am

0,15,30,45 18-06 */bin/echo ' date ' >/dev/console

Save and exit. Make sure that the previous 5 fields are separated by spaces.

In the example above, the system will output the current time to the console every 1 to 5 minutes. If the system crashes or hangs, you can see at what time the system stopped working at the last displayed time. In some systems, the use of tty1 to represent the console, according to the actual situation of the above example can be modified accordingly. In order to submit the crontab file you just created, you can use this newly created file as a parameter to the cron command:

$ crontab Davecron

Now that the file has been submitted to the Cron process, it will run every 1 5 minutes.

At the same time, a copy of the newly created file has been placed in the/var/spool/cron directory, and the file name is the user name (that is, Dave).

2). List crontab Files

In order to list crontab files, you can use:

$ crontab -l

0,15,30,45,18-06 * * * /bin/echoDate> dev/tty1

You will see something similar to the above. You can use this method to make a backup of the crontab file in the H O M e directory:

$ crontab -l > $HOME/mycron

This way, once the crontab file is accidentally deleted, it can be recovered quickly using the method described in the previous section.

3). Edit the Crontab file

If you want to add, delete, or edit an entry in the Crontab file, and the E D I to r environment variable is set to V I, then you can edit the crontab file with V I, and the corresponding command is:

$ crontab-e

You can modify the Crontab file and exit as if you were editing any other file using V I. If some entries are modified or a new entry is added, C R o N will perform the necessary integrity checks on the file when it is saved. If one of the fields has a value that exceeds the allowable range, it will prompt you.

When we edit the crontab file, we may not be adding a new entry. For example, add one of the following:

# DT:delete core files,at 3.30am on 1,7,14,21,26,26 days of each month

3 1,7,14,21,26 /bin/find-name "Core '-exec rm {} \;

Save and exit now. It is best to add a comment above each entry in the crontab file, so that you can know its function, run time, and, more importantly, what user's job it is.

Now let's use the previous crontab-l command to list all of its information:

$ crontab-l

# (crondave installed on Tue May 4 13:07:43 1999)

# DT:ech the date to the console every 30 minites

0,15,30,45 18-06 * * * /bin/echoDate> /dev/tty1

# DT:delete core files,at 3.30am on 1,7,14,21,26,26 days of each month

30 3 1,7,14,21,26 * * /bin/find -name “core’ -exec rm {} \;

4). Delete the crontab file

To delete a crontab file, you can use:

$ crontab -r

5). Recover the Lost crontab file

If you accidentally delete the crontab file, assuming you have a backup in your own $ H O M directory, you can copy it to/var/spool/cron/<username>, where <username> is the user name. If the copy cannot be completed due to a permissions issue, you can use:

$ crontab <filename>

Where,<filename> is the file name of your copy in the $ H O M e directory.

I recommend that you save a copy of the file in your own $ H O M directory. I have had a similar experience, several times accidentally deleted the crontab file (because the R key is close to the E key to the right). This is why some system documentation does not recommend editing the crontab file directly, but instead edits a copy of the file and then resubmit the new file.

Some variants of crontab are somewhat bizarre, so be careful when using the crontab command. If you omit any of the options, crontab may open an empty file, or it might look like an empty file. Then hit the delete key to exit, do not press <ctrl-d>, otherwise you will lose the crontab file.

5. Working with instances

  Instance 1: Execute command command every 1 minutes: * * * * * Command Instance 2:3rd and 15 minutes per hour execution of command: 3,15 * * * = Command instance 3: At 8 o'clock in the morning to 11, 3rd and 15 minutes execution of commands: 3,  8-11 * * * Command Example 4: Every two days from 8 o'clock in the morning to 11 points 3rd and 15 minutes execution command: 3,15 8-11 */2 * Command Instance 5: Every Monday 8 o'clock in the morning to 11 points 3rd and 15 minutes Execute command: 3,15 8-11 * * 1 Command Instance 6:21:30 restart per night SMB command: * * * * * * */ETC/INIT.D/SMB Restart Instance 7:1, 10, 22nd-month restart SMB command: 4:45 1,10,22 * */ETC/INIT.D  /SMB Restart Instance 8:1:10 restart SMB command per Saturday, Sunday: 1 * * 6,0/ETC/INIT.D/SMB Restart Instance 9: Restart SMB command every 30 minutes from 18:00 to 23:00 daily: 0,30 18-23 * * */ETC/INIT.D/SMB Restart instance 10: Every Saturday night at 11:00am restart SMB command: 0 * * 6/ETC/INIT.D/SMB Restart Instance 11: Restart SMB command every hour: * */1 * * */ ETC/INIT.D/SMB Restart Instance 12: From 11 o'clock to 7 in the morning, restart the SMB command every hour: * 23-7/1 * * */ETC/INIT.D/SMB Restart Example 13:4th for each month and 11 points for every Monday to Wednesday restart SMB command: 0 4 * MON-WED/ETC/INIT.D/SMB Restart instance 14: January 1 4-point restart SMB command: 0 4 1 Jan */etc/init.d/s MB Restart Instance 15: Execute script commands within/etc/cron.hourly directory per hour: * * * * * * Root run-parts/etc/ Cron.hourly Description: Run-parts This parameter, if you remove this parameter, you can write a script name to run, instead of the directory name  

Iv. Precautions for use

Pay attention to the environment variable problem
Sometimes we create a crontab, but this task cannot be executed automatically, but it is not a problem to perform this task manually, which is usually caused by not configuring environment variables in the crontab file.

When defining multiple dispatch tasks in a crontab file, one of the issues that needs special attention is the setting of environment variables, because when we perform a task manually, it is done in the current shell environment, the program can certainly find the environment variable, and the system will not load any environment variables when it automatically executes the task schedule. Therefore, you need to specify all the environment variables that are required for the task to run in the crontab file, so that the system does not have a problem when it executes the Task Scheduler.

Don't assume that Cron knows the special circumstances you need, and it doesn't really know. So you have to make sure that you provide all the necessary path and environment variables in the shelll script, except for some auto-set global variables. So note the following 3 points:

1) Write the global path when the file path is involved in the script;

2) When script execution is used in Java or other environment variables, the environment variables are introduced through the source command, such as:

cat start_cbp.sh

#!/bin/sh

source /etc/profile

export RUN_CONF=/home/d139/conf/platform/cbp/cbp_jboss.conf

/usr/local/jboss-4.0.5/bin/run.sh -c mev &

3) When the script is executed manually, but crontab is not executed. At this point, we must boldly suspect that environmental variables are the bane, and can try to directly introduce environmental variables in crontab to solve the problem. Such as:

0 . /etc/profile;/bin/sh/var/www/java/audit_no_count/bin/restart_audit.sh

Note Clean up the message log for system users
Each task is scheduled to execute, the system will send the task output information in the form of e-mail to the current system users, so the cumulative, log information will be very large, may affect the normal operation of the system, so it is important to redirect each task.

For example, you can set the following form in the crontab file, ignoring the log output:

0 /3 /usr/local/apache2/apachectl restart >/dev/null 2>&1

"/dev/null 2>&1" means that the standard output is redirected to/dev/null and then the standard error is redirected to standard output, and standard errors are redirected to/dev/null because the standard output has been redirected to/dev/null. This will solve the problem of log output.

System-level task scheduling and user-level task scheduling
System-level task scheduling is mainly to complete some maintenance operations of the system, user-level task scheduling is mainly to complete the user-defined tasks, you can put the user-level task scheduling to the system-level task scheduling to complete (not recommended), but in turn, the root user's task scheduling operation can be through the "crontab– Uroot–e "To set, you can also write the dispatch task directly to the/etc/crontab file, it is important to note that if you want to define a scheduled restart of the system task, you must put the task into the/etc/crontab file, even under the root user to create a The task of restarting the system regularly is also invalid.

Other precautions
The newly created cron job will not execute immediately, at least 2 minutes. If you restart Cron, it will be executed immediately.

When the crontab suddenly fails, you can try/etc/init.d/crond restart solve the problem. Or check the log to see if a job has execution/error tail-f/var/log/cron.

Don't run crontab-r. It removes the user's crontab file from the crontab directory (/var/spool/cron). All crontab of the user have been deleted.

In Crontab, the% has a special meaning, indicating the meaning of the line break. If you want to use the words must be escaped \%, such as the frequently used date ' +%y%m%d ' in the crontab will not be executed, should be replaced by the date ' +\%y\%m\%d '.

Linux timed Task crontab command detailed