A detailed explanation of Linux terminal types

Unix is ​​a multi-user and multi-tasking operating system. In the early days, computers were expensive, so cheap devices were connected to the computers (there were no keyboards and monitors at that time, and paper tapes and cards were used for input and output) to use the operating system. This cheap device was a terminal, and a terminal can also be considered a console. So you can think of the computer itself as the console terminal, and the cheap connection device as the physical terminal pty.

Linux is a Unix-like system, so it also inherits the characteristics of the terminal. But then computers became cheaper and monitors and keyboards appeared, so you could use the keyboard as an input terminal and the monitor as an output terminal. These terminals were virtual terminals, and the virtual terminal was actually a virtual console, or a virtual device.

Linux provides many kinds of virtual terminals, which are represented by ttyN. You can use Ctrl+Alt+F[1-6] to switch virtual terminals. These terminal devices are recorded in the /dev/ directory.

[root@xuexi ~]# ls /dev/tty
tty    tty12  tty17  tty21  tty26  tty30  tty35  tty4   tty44  tty49  tty53  tty58  tty62  ttyS0 
tty0   tty13  tty18  tty22  tty27  tty31  tty36  tty40  tty45  tty5   tty54  tty59  tty63  ttyS1 
tty1   tty14  tty19  tty23  tty28  tty32  tty37  tty41  tty46  tty50  tty55  tty6   tty7   ttyS2 
tty10  tty15  tty2   tty24  tty29  tty33  tty38  tty42  tty47  tty51  tty56  tty60  tty8   ttyS3 
tty11  tty16  tty20  tty25  tty3   tty34  tty39  tty43  tty48  tty52  tty57  tty61  tty9

The tty plus the value is the virtual terminal. CTRL+ALT+F1 means switching to the tty1 terminal, ctrl+alt+f2 means switching to the tty2 terminal. Generally, Linux only provides the function of switching between the six terminals ctrl+alt+f[1-6]. The two special terminals are tty and tty0. tty represents the terminal currently in use, and tty0 represents all virtual terminals that are currently activated. There are also ttySN, these represent serial terminals.

There are also terminals connected to the computer from the network through ssh or telnet, or command line terminals opened from the graphical virtual terminal, which are called pseudo terminals, represented by pts/N, and the corresponding device is / Numerical N files in the dev/pts directory.

[root@xuexi ~]# ls /dev/pts/0     ptmx

0 represents the first pseudo terminal, 1 represents the second pseudo terminal.

The management method of pseudo terminal is different from that of all other terminals. It is managed through the program that connects to the computer. For example, for ssh connection, ssh is responsible for applying for pseudo terminal resources and requires input of user name and password. If the ssh connection process is killed, the pseudo terminal will exit accordingly.

In addition, some authentication programs may not necessarily allocate a terminal to the connection slave program. For example, when executing sudo ssh, sudo may not necessarily allocate a pseudo terminal for ssh.

On modern Linux, the console terminal is no longer the same as its original meaning. Its device is mapped on /dev/console. All information output by the kernel is output to the console terminal, while other user programs output Information is output to a virtual terminal or pseudo terminal.

To summarize:

/dev/console: console terminal

/dev/ttyN: virtual terminal, ctrl+alt+f[1-6] switches Virtual terminal

/dev/ttySN: Serial terminal

/dev/pts/N: Pseudo terminal. The command line terminal that is connected to the past by ssh and other tools or opened under the graphical terminal is a pseudo terminal. .

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