Summary of some related operations of Linux scheduled tasks

I have searched various major websites and tested the operation of related scheduled tasks to facilitate everyone's reference and operation.

1. Introduction to cron

The crontab command we often use is the abbreviation of cron table. It is the configuration file of cron, and it can also be called job list. We can find the relevant configuration files in the following folders.

1.1. Cron related directories

• The /var/spool/cron/ directory stores crontab tasks for each user, including root. Each task is named after the creator.
• /etc/crontab This file is responsible for scheduling various management and maintenance tasks.
• /etc/cron.d/ This directory is used to store any crontab files or scripts to be executed.
• We can also put the script in the /etc/cron.hourly, /etc/cron.daily, /etc/cron.weekly, /etc/cron.monthly directories to have it executed every hour, day, week, or month.
  

1.2. Common commands of crontab

• crontab [-u username] //Omitting the user table means operating the crontab of the current user
• crontab [-u username] -e //Edit worksheet)
• crontab [-u username] -l //List the commands in the worksheet)
• crontab [-u username] -r //Delete job)
  

1.3. Writing specifications

1. The parameters of each position are as shown below, and there are also notes for details.

Remark:

1) * means that any time (minute, hour, day, month, week) will be executed

2) - Indicates a time range, such as 5-7 o'clock

3) , represents the time interval, such as 6,0,4 represents Saturday, Sunday, and Thursday.

4) /1 means every n time units, such as */10 every 10 minutes

2. Set up scheduled tasks

2.1. Globally set up scheduled tasks crontab --- when the task is to link, curl or write files

(1) Execute the command crontab -e

(2) Write a scheduled task

If the scheduled task is to link

*/1 * * * * /usr/local/curl (your own curl path) www.baidu.com >/dev/null 2>$1

If you need to write the content to the file

*/1 * * * * echo "hello" >> abc.log

I would also like to share a few points

• Standard input 0 gets input from the keyboard /proc/self/fd/0
• Standard output 1 is output to the screen (i.e. console) /proc/self/fd/1
• Error output 2 is output to the screen (ie console) /proc/self/fd/2
• /dev/null represents the empty device file of Linux. All the contents written to this file will be lost, commonly known as the "black hole"
• >/dev/null means to output errors to the "black hole"
• >/dev/null 2>&1 The default is 1, which is equivalent to 1>/dev/null 2>&1. This means redirecting the standard output to the "black hole" and redirecting the error output 2 to the standard output 1, that is, both the standard output and the error output go into the "black hole".
• 2>&1 >/dev/null means redirecting error output 2 to standard output 1, that is, the screen, and the standard output goes into a "black hole", that is, the standard output goes into a black hole, and the error output is printed to the screen
• Regarding the role of "&" here, we can understand it this way: 2>/dev/null redirects to a file, then 2>&1, if & is removed here, the error output is sent to file 1, and using & indicates that 1 is the standard output.
  

(3) Save scheduled tasks

1. Press i to insert the task and write the scheduled task
2. Press Esc to exit and enter :wq , then press Ctrl + C to save successfully.
   

2.2. .sh method implementation

(1) Create a Shell Script

• Create a file with the suffix .sh in the project
• Add sufficient permissions to this shell file in this directory
  

chmod -R 777 you create the file name

(2) Write the Shell script into the scheduled task

Write the corresponding shell file into the scheduled task

*/1 * * * * The absolute path of your .sh file>/dev/null 2>$1

Write the corresponding shell file to the log when executing the scheduled task

*/1 * * * * The absolute path of your .sh file >> a.log >/dev/null 2>$1

(3) Restart crond

service crond restart

(4) Check whether it is written into the project

crontab -l

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