crontab timer
Introduction: scheduled tasks under Linux
1,vi /etc/crontab
(1) First, write a script under the clone session, vi an x.sh script (whatever it is called), echo a hello, and add a redirect folder
[root@hadoop61 ~]# cd /usr/local/shellDemo [root@hadoop61 shellDemo]# ll Total consumption 36 -rw-r--r--. 1 root root 125 10 June 14-16:34 a.txt -rw-r--r--. 1 root root 60 10 December 16:53 b.sh -rw-r--r--. 1 root root 39 10 December 16:37 c.sh -rwxr-xr-x. 1 root root 53 10 October 11:26 demo.sh -rw-r--r--. 1 root root 46 10 December 16:55 d.sh -rw-r--r--. 1 root root 48 10 December 20:17 for.sh -rw-r--r--. 1 root root 62 10 December 21:45 if.sh -rw-------. 1 root root 0 10 November 14:17 nohup.out -rw-r--r--. 1 root root 47 10 November 14:26 while2.sh -rw-r--r--. 1 root root 45 10 December 20:59 while.sh [root@hadoop61 shellDemo]# vi x.sh #!/bin/bash echo "hello" >> /usr/local/shellDemo/hello.txt
(2) Assign an execution permission to the script
[root@hadoop61 shellDemo]# chmod +x x.sh [root@hadoop61 shellDemo]#
(3) Execute the vi /etc/crontab command
Execution:
[root@hadoop61 shellDemo]# vi /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
Note:
<1> The front # is full of notes
<2> You can see five * signs, which represent minute, hour, day, month and day of the week from left to right
(4) Write a timer script on the next line
Format: write all * means to execute once every minute, starting from zero minute
The bin/sh path is fixed, followed by the command path
Note: the user name must be written. If it is not written, an error will be reported (the following user name is root)
[root@hadoop61 shellDemo]# vi /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 #* * * * * root /bin/sh /usr/local/shellDemo/x.sh * * * * * root bin/sh /usr/local/shellDemo/x.sh
2. tail -f: monitor the new data in the file (used to monitor the real-time changing file)
(1) Note: in the following case, the document has not been executed yet, because it is executed from zero every minute
[root@hadoop61 shellDemo]# tail -f hello.txt tail: Cannot open"hello.txt" Read data: There is no such file or directory tail: No files left
(2) Check it again and you'll find it
[root@hadoop61 shellDemo]# tail -f hello.txt hello
(3) Check the latest 1 line and the latest 3 lines of the file execution
[root@hadoop61 shellDemo]# tail -1 hello.txt hello [root@hadoop61 shellDemo]# tail -3 hello.txt hello hello hello
3. tail -f /var/log/cron: view the log monitoring file of crontab
Note: the rsyslog service must be opened before the execution log (service rsyslog status) can be found in the cron file
<1> Service crond status: View cron service status
Execute: you can see that running is enabled
[root@hadoop61 shellDemo]# service crond status
Redirecting to /bin/systemctl status crond.service
● crond.service - Command Scheduler
Loaded: loaded (/usr/lib/systemd/system/crond.service; enabled; vendor preset: enabled)
Active: active (running) since IV 2021-10-14 07:57:43 CST; 12h ago
Main PID: 641 (crond)
CGroup: /system.slice/crond.service
└─641 /usr/sbin/crond -n
<2> Service crond start: starts the cron service
<3> Tail - F / var / log / cron: view the log monitoring file of crontab
[root@hadoop61 shellDemo]# tail -f /var/log/cron Oct 14 17:56:01 hadoop61 CROND[6400]: (root) CMD (/bin/sh /usr/local/shellDemo/x.sh) Oct 14 17:57:01 hadoop61 CROND[6404]: (root) CMD (/bin/sh /usr/local/shellDemo/x.sh) Oct 14 17:58:01 hadoop61 CROND[6408]: (root) CMD (/bin/sh /usr/local/shellDemo/x.sh) Oct 14 17:59:01 hadoop61 CROND[6413]: (root) CMD (/bin/sh /usr/local/shellDemo/x.sh) Oct 14 18:00:01 hadoop61 CROND[6419]: (root) CMD (/bin/sh /usr/local/shellDemo/x.sh) Oct 14 18:01:01 hadoop61 CROND[6426]: (root) CMD (/bin/sh /usr/local/shellDemo/x.sh) Oct 14 18:01:01 hadoop61 CROND[6427]: (root) CMD (run-parts /etc/cron.hourly) Oct 14 18:01:01 hadoop61 run-parts(/etc/cron.hourly)[6427]: starting 0anacron
Note: it can be seen from one of these monitoring files in the above table
Oct 14 17:56:01 hadoop61 CROND[6400]: (root) CMD (/bin/sh /usr/local/shellDemo/x.sh)
(1) Who executed: root
(2) What is the command: / bin/sh /usr/local/shellDemo/x.sh
4. Question: how to write timer script if it is set to execute every 5 branches
(1) Find the * corresponding to minute and write it as * / 5, which is executed every 5 minutes
# 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 */5 * * * * root /bin/sh /usr/local/shellDemo/x.sh
Note:
(1) You can write multiline tasks. There is no relationship between multiline tasks
(2) If you don't want to use the task, you can add # comments
#*/5 * * * * root /bin/sh /usr/local/shellDemo/x.sh
(3) It starts at 9 a.m. every day
0 9 * * * root /bin/sh /usr/local/shellDemo/x.sh