Linux periodically cleans up expired files using the Find and Crontab commands _linux

crontab command

The crontab command is a tool used in Linux to set up repeated execution of commands or scripts. It is able to execute commands or scripts at a certain time interval for a specified period of time, as required.

Basic usage of crontab

crontab [-u <user>] [-e|-l|-r]
crontab <filename>

crontab <filename>You can read a file written in crontab syntax and perform a timed task according to the instructions in the file. At the same time, it is crontab -l possible to list all scheduled tasks for the current user on standard output. Therefore, we can use crontab -l > <filename> to save the current crontab situation, when necessary (such as mistakenly delete) with crontab <filename> read into the recovery.

crontab -e The system default editor is started. This editor is specified by the system environment variable EDITOR, for example export EDITOR=vi , the crontab -e VI is started to edit the crontab timed task file for the current user. crontab -rall crontab Scheduled tasks for the user are deleted.

The option to specify the user -u <user> will change the behavior of the -e/-l/-r options. If you do not -u specify a user, the default action is for the current user or, conversely, for the specified user. For example crontab -u Liam -r , you would delete the Crontab timed task of the user Liam--of course, you need to have sufficient permissions to do so.

Format of crontab files

Whether you are crontab <filename reading a timed task with > from a file, or crontab -e editing a timed task, we will be in touch with the crontab format command.

The crontab command format is as follows:

From the above we can see:

1. A crontab command is divided into six columns, written on a single line;
2. The first 5 columns are used for timing, indicating when the execution begins;
3. The 6th column specifies the command or script that needs to be executed regularly;

In addition, as with Shell scripts, we can use # to represent annotations in crontab files.

Fancy timing

In the first 5 columns used for timing in the crontab command, the following symbols are supported for "Fancy timing":

1. Star Flower (*): represents all possible values;
2. Commas (,): Separating the different ranges of the same field with commas;
3. Hyphen (-): Connects two integers with hyphens, representing an integer range;
4. Slash (/): is actually a division slash, which is used to specify the time interval frequency.

Some examples

With this knowledge, we can look at some examples.

# Execute once per minute * * *
command
# Execute every 5 minutes */5 * * * *
command
# 15 minutes and 45 minutes per hour
15,45 * * * * * * * COMM and # Night 20:00-20:15 0-15 * * * * * * * *
command
# every Monday 10 o'clock in the morning
0 * * 1 command

find--, get some fun.

The general form of the find command given by man is:

Find [-h] [l] [P] [D-debugopts] [-olevel] path ... [Expression]

Few of them [-H] [-L] [-P] [-D debugopts] [-Olevel] will be used. Therefore, the general form of the find command can be reduced to f ind path ... [expression]； , where the appropriate file is found in some paths (the default recursively include Subpath), and then the corresponding action is performed according to expression.

path ...: findPath for command lookup

Expression: the concrete form is-parameters [-exec -ok]


-parametersThere are a lot of the next section to describe the commonly used parameters

-exec command {}\; Execute a Shell command

-ok command {} \; with-exec, but before executing each command, prompts the user to confirm

Commonly used parameters

-name: Find files by file name, accept wildcard characters

Find. -name "*.cpp": recursively find all files with the suffix. cpp in the current directory (.) and subdirectories, and print in standard output

-perm: Find files by file permissions

Find. -perm 755: Recursively Find all files (directories) with permission 755 in the current directory (.) and subdirectories and print them in standard output

Find. -perm 644-name "*.cpp": recursively find all. cpp files with permission 644 in the current directory and subdirectories, and print them in standard output

-group: Search by file's owning group

-user: Find by Owner of file

Find. -user search: In the current directory (.) and subdirectories to recursively find all search accounts belong to the file, and print in the standard output

-atime, -mtime, -amin, -mmin: "A representation that starts with the last access time to look up the",m "to find";-n in the last modified time indicates n time, +n represents n time before

Find. -atime + 7: Recursively find all files that were last accessed before 7 days in the current directory (.) and subdirectories and print them in standard output

Find. -MMIN-10: Recursively find all files that were last modified within 10 minutes in the current directory (.) and subdirectories and print them in standard output

-newer file1 ! file2: Find files that are newer than file1 but older than file2

-type [b|d|c|p|l|f]: Find by File type

B: Block device files

D: Table of Contents

C: Character device files

P: Pipe File

L: Symbolic Link file

F: Ordinary Documents

-follow : If the find command encounters a symbolic link file, it tracks to the file that the link points to

-delete: Deletes a found file or directory

Regular cleanup

At this point, it becomes easy to clean up useless files regularly. We only need to combine the Find command option in the Crontab -delete .

0 8 * * * find/home/s/coredump-user search-type f-mtime +7-delete

Here, we execute the find command at 8 o'clock in the morning every day, and the command will search the/home/s/coredump directory for an ordinary 7-day file created by the search user and delete it.

Summarize

The above is the entire content of this article, I hope the content of this article for everyone's study or work can bring certain help, if you have questions you can message exchange.