Built-in and external commands in Linux
This article focuses on "built-in commands and external commands in Linux". Interested friends may wish to take a look. The method introduced in this paper is simple, fast and practical. Let's let the editor take you to learn "built-in commands and external commands in Linux".
Linux commands are divided into internal commands and external commands:
1. Internal commands are called into memory when the system is started, and are resident in memory, so the execution efficiency is high.
two。 The external command is the software function of the system, and the user only reads the memory from the hard disk when needed.
Type can be used to determine whether a command is a built-in command
The code is as follows:
Type: usage: type [- afptP] name [name...]
The code is as follows:
[root@linuxeye ~] # type type
Type is a shell builtin
[root@linuxeye] # type-p type
[root@linuxeye] # type-t type
Builtin
[root@linuxeye ~] # type type
Type is a shell builtin
[root@linuxeye] # type-t type
Builtin
[root@linuxeye ~] # type pwd
Pwd is a shell builtin
[root@linuxeye ~] # type whiptail
Whiptail is / usr/bin/whiptail
[root@linuxeye] # type-t whiptail
File
Enable can not only view internal commands, but also determine whether they are internal commands.
The code is as follows:
[root@linuxeye ~] # enable-a # View internal commands
[root@linuxeye ~] # enable whiptail # non-internal command
-bash: enable: whiptail: not a shell builtin
[root@linuxeye ~] # enable pwd # is an internal command
Internal command user input when the system call rate is fast, not built-in commands, the system will read the environment variable file .bash _ profile, / etc/profile to find the PATH path.
Then when you mention the invocation of the command, some historical commands will exist in the hash table after they have been used, and the call to the command will be such a process when you type the command again.
The invocation of the hash-- > built-in command-- > PATH command should actually be such a process.
The code is as follows:
[root@linuxeye ~] # type pwd
Pwd is a shell builtin
[root@linuxeye ~] # type cat
Cat is / usr/bin/cat
[root@linuxeye ~] # pwd
/ root
[root@linuxeye ~] # ls linuxeye*
Linuxeye.pem linuxeye.txt
[root@linuxeye ~] # cat linuxeye.txt
Linuxeye
[root@linuxeye ~] # hash-l # displays the hash table
Builtin hash-p / usr/bin/cat cat
Builtin hash-p / usr/bin/ls ls
[root@linuxeye ~] # type cat
Cat is hashed (/ usr/bin/cat)
[root@linuxeye ~] # hash-r # clear the hash table
[root@linuxeye ~] # type cat
Cat is / usr/bin/cat
As can be seen from the above operation. The hash table does not hold system built-in commands.
At this point, I believe you have a deeper understanding of the "built-in commands and external commands in Linux". You might as well do it in practice. Here is the website, more related content can enter the relevant channels to inquire, follow us, continue to learn!
