Under Linux, how do we get help?

Linux, How do I get help?

1. Review the basic usage of commands in linux

(1) command --help

[root@workstation Desktop]# passwd --help

(2) man command

[root@workstation Desktop]# man passwd

（3）man level target
But the file in the usage of these two commands finding out, if you want to use to view the file command, the introduction of man level target

• First, there are several levels to see passwd

[root@workstation Desktop]# man -k passwd

• From the above chart know, passwd has two levels, namely commands and configuration files, query usage respectively

[root@workstation Desktop]# man  passwd

[root@workstation Desktop]# man 5 passwd

• When the file is too long, you can use the less command page read:

[root@workstation Desktop]# man 5 passwd | less

(4) whatis command
whatis command is used to query a command to perform what functions, and for printing results to the terminal

[root@workstation Desktop]# whatis passwd

(5) man man
to see a total of how many levels, each level represents the meaning

[root@workstation Desktop]# man man

There are nine levels to the levels the following meanings:

   1   Executable programs or shell commands 命令
   2   System calls (functions provided by the kernel)系统调用
   3   Library calls (functions within program libraries)函数调用
   4   Special files (usually found in /dev)特殊文件
   5   File formats and conventions eg /etc/passwd配置文件
   6   Games游戏
   7   Miscellaneous (including macro packages and特殊包 conventions), e.g.
       man(7), groff(7)
   8   System administration commands (usually only for root)管理命令
   9   Kernel routines [Non standard]内核规则

2. Topic Exercise: Using the date command to view the 83 days is a few months a few days

[root@workstation Desktop]# date -d 83day +%Y-%m-%d

Problem-solving process is as follows:
First check the date command usage

[root@workstation Desktop]# whatis date

[root@workstation Desktop]# man 1 date

[root@workstation Desktop]# date -d 83day +%Y-%m-%d
2020-03-25