Linux Command Classification and priority

I. LIUNX classification
Linux commands can be broadly divided into three types of built-in commands, external commands and alias commands (aliases) (which can be queried by type comment)
1 Built-in commands (BUILTIN), the built-in commands are actually part of the shell program and contain some of the more concise Liunx system commands that are identified by the shell program and run inside the shell program.
（ Note: This is an internal command for type queries）

    查询所有激活的内置命令 enable*   [[email protected] ~]# enable enable .enable :

Enable [
Enable CD
... *
to disable built-in commands, such as enable-n comment ( comment refers to an instruction, the following )

Show all disabled internal commands Enable-n

If you want to re-enable a built-in command that has been banned, enable comment

2 external command , which is a utility part of the Linux system, Because the included program is too large, it will not be loaded into the system memory as soon as it is turned on, but will be transferred into memory when needed. (The shell program manages the path lookup, loading and storage of external commands, and Controls command execution.)
( Note: This is the external command for type query )
The external command is divided into hash cache command and path execution, and all executed commands are called into the cache, and the cache is hash. The hash records the most recent external command that you have executed.
Show all cached command path hash

Empty all cache Hash-r

Execute by Path and can be divided into direct input related path execution and automatic search path execution after input command, directory available for automatic search path Echo￥path view

as shown, search from left to right separated by a colon, and if the same command exists within two folders, the first search command is stored in a hash and executed.
3 alias , alias command refers to the user set command aliases, with shorthand command, improve operational efficiency.
Show all aliases alias

Set an alias alias = ' command-option parameter '

Delete a set alias Unalias alias

Second, priority
If a command is assumed to be present in the built-in command, external command, alias



Direct input comment for testing

You can have the first alias, remove the command from the aliases, and test

The second is a built-in command, disable it and then test


The third is a hash, the cache is deleted after the last Test

It is shown that the priority displayed in the system based on the same command is alias> built-in Commands > External commands (Hash>path)

Linux Command Classification and priority