Crontab: scheduled task execution. Crontab: scheduled task execution.

Crontab: scheduled task execution. Crontab: scheduled task execution.

I recently came into contact with the needs of scheduled execution programs, so I learned about crontab. This article first introduces the syntax knowledge of crontab, and then makes a demo.

I. crontab syntax

1. Basic crontab format

{Minute} {hour} {day-of-month} {day-of-week} {full-path-to-shell-script}

2. crontab syntax example

1 # At. m runs 2 1 0 ***/root/bin/backup. sh 3 # p. m runs 4 59 11 ** 1, 2, 3, 4, 5/root/bin/backup. sh 5 59 11 ** 1-5/root/bin/backup. sh 6 # Run the command every 5 minutes 7 */5 */root/bin/check-status.sh 8 # the first day of each month p. m run 9 10 13 1 **/root/bin/full-backup.sh 10 #11 p per workday. m run 11 0 23 ** 1-5/root/bin/incremental-backup.sh

Ii. crontab instance

1. Start and Stop crontab

1 service crond start/stop/restart

2. Add a crontab task in crontab-e mode

1 crontab-e # Add Task 2 crontab-l # view all tasks 3 crontab-r # delete a task

Preparations:

Vim/home/helloworld. py

Vim/home/helloworld. sh

1 #helloworld.py2 import time3 with open('test.txt','w+') as f:4     f.write(str(time.time()))5 6 7 #helloworld.sh8 cd /home/helloworld/9 python /home/helloworld/helloworld.py

 

Step 1: run the crontab-e command to enter the editing mode and add the task to be executed.

*/1 * * * * bash /home/helloworld/helloworld.sh

Step 2: Start the crontab Service

Step 3: View crontab logs

1 tail-f/var/log/cron 2 # view log file 3 May 12 19:11:01 localhost CROND [12824]: (root) CMD (bash/home/helloworld. sh) 4 May 12 19:11:01 localhost CROND [12823]: (root) MAIL (mailed 14 bytes of output but got status 0x007f #012) 5 May 12 19:12:01 localhost CROND [12865]: (root) CMD (bash/home/helloworld. sh) 6 May 12 19:12:01 localhost CROND [12864]: (root) MAIL (mailed 14 bytes of output but got status 0x007f #012) 7 May 12 19:13:01 localhost CROND [12880]: (root) CMD (bash/home/helloworld. sh) 8 May 12 19:13:01 localhost CROND [12879]: (root) MAIL (mailed 14 bytes of output but got status 0x007f #012) 9 May 12 19:14:01 localhost CROND [12895]: (root) CMD (bash/home/helloworld. sh) 10 May 12 19:14:01 localhost CROND [12894]: (root) MAIL (mailed 13 bytes of output but got status 0x007f #012) 11 12 # view the helloworld folder 13 ll-t14 #15 [root @ localhost helloworld] # ll-t16 total usage 1217-rw-r -- 1 root 13 May 12 19:11 test.txt 18-rw-r -- 1 root 59 May 12 16:45 helloworld. sh19-rw-r -- 1 root 94 May 12 16:41 helloworld. py20 [root @ localhost helloworld] # ll-t21 total usage 1222-rw-r -- 1 root 13 May 12 19:12 test.txt 23-rw-r -- 1 root 59 may 12 16:45 helloworld. sh24-rw-r -- 1 root 94 May 12 16:41 helloworld. py25 [root @ localhost helloworld] # ll-t26 total usage 1227-rw-r -- 1 root 13 May 12 19:13 test.txt 28-rw-r -- 1 root 59 may 12 16:45 helloworld. sh29-rw-r -- 1 root 94 May 12 16:41 helloworld. py30 [root @ localhost helloworld] # ll-t31 total usage 1232-rw-r -- 1 root 12 May 12 19:14 test.txt 33-rw-r -- 1 root 59 may 12 16:45 helloworld. sh34-rw-r -- 1 root 94 May 12 16:41 helloworld. py35 [root @ localhost helloworld] #

3. Add a task using vim/etc/crontab

1 # view/etc/crontab 2 vim/etc/crontab 3 4 # Add Task 5 SHELL =/bin/bash 6 PATH =/sbin:/bin:/usr/sbin: /usr/bin 7 MAILTO = "" 8 HOME =/9 10 # For details see man 4 crontabs 11 # Example of job definition: 12 #. ---------------- minute (0-59) 13 # |. ------------- hour (0-23) 14 # |. ---------- day of month (1-31) 15 # |. ------- month (1-12) OR jan, feb, mar, apr... 16 # |. ---- day of week (0-6) (Sunday = 0 or 7) OR sun, mon, tue, w ed, thu, fri, sat17 # | 18 # ***** user-name command to be executed19 */1 ***** root bash/home/helloworld. sh

4. You can monitor repeated tasks in the same way.