About tty, PPy, and console

Reproduced source http://blog.chinaunix.net/space.php? Uid = 8116903 & Do = Blog & id = 334708

Tty is the abbreviation of teletype)
A terminal is a type of terminal device. It has multiple types and generally uses tty for short. Tty is short for teletype. Teletype is one of the earliest terminal devices, such as a telex typewriter (OR), produced by teletype. The device name is stored in the special file directory/dev/. The special device files on the terminal generally include the following types:
1. Serial Port Terminal (/dev/ttysn)
The serial port terminal (serial port terminal) is a terminal device connected by a computer serial port. The computer regards each serial port as a character device. For some time, these serial port devices are usually called terminal devices, because at that time they were used to connect terminals. The device names corresponding to these serial ports are/dev/TTS/0 (or/dev/ttys0),/dev/TTS/1 (or/dev/ttys1), etc, the device numbers are (), (), and so on, respectively, corresponding to COM1 and com2 in the DOS system. To send data to a port, You can redirect the standard output to these special file names on the command line. For example, at the command line prompt, type: Echo
Test>/dev/ttys1 sends the word "test" to the device connected to the ttys1 (com2) port.
2. Pseudo Terminal (/dev/PtY /)
Pseudo Terminal is a pair of logical terminal devices, for example,/dev/ptyp3 and/dev/ttyp3 (or/dev/PtY/m3 and/dev/PtY/S3 in the device file system ). They are not directly related to physical devices. If a program regards ttyp3 as a serial port device, its read/write operations on the port will be reflected in another logical terminal device pair (ttyp3 ). Ttyp3 is the logical device used by another program for read/write operations. In this way, two programs can communicate with each other through this logical device, and one of the programs using ttyp3 thinks they are communicating with a serial port. This is like pipeline operations between logical device pairs.
For ttyp3 (S3), any program designed to use a serial port device can use this logical device. However, for programs that use ptyp3, a dedicated design is required to use the ptyp3 (m3) logical device.
For example, if someone uses a telnet program to connect to your computer online, the telnet program may start to connect to the device ptyp2 (m2) (a Pseudo Terminal port ). At this time, a Getty program should run on the corresponding ttyp2 (S2) port. When Telnet obtains a character from the remote end, the character is passed to the Getty program through m2 and S2, the getty program returns the "login:" string information to the network through S2, m2, and telnet. In this way, the login program communicates with the telnet program through a "Pseudo Terminal. By using appropriate software, you can connect two or more Pseudo Terminal devices to the same physical serial port.
Before using device filesystem, HP-UX AIX and others used complicated naming methods to get a large number of special files for pseudo-terminal devices.
3. control terminal (/dev/tty)
If the current process has a control terminal (controlling terminal),/dev/tty is the device special file of the control terminal of the current process. You can run the "PS-Ax" command to check which control terminal the process is connected. For the shell you log on to,/dev/tty is the terminal you are using, and the device number is (5, 0 ). Run the "tty" command to check which actual terminal device it corresponds. /Dev/tty is similar to a connection to the actually used terminal device.
4. Console terminal (/dev/ttyn,/dev/console)
In Unix systems, a computer monitor is usually called a console ). It simulates a Linux terminal (term = Linux), and some special files of the device are associated with it: tty0, tty1, tty2, and so on. When you log on to the console, tty1 is used. With Alt + [F1-F6] keys, we can switch to tty2, tty3 and so on. Tty1-tty6 is called a virtual terminal, while tty0 is an alias of the currently used virtual terminal. The information generated by the system is sent to the terminal. Therefore, no matter which virtual terminal is being used, the system information will be sent to the console terminal.
You can log on to different virtual terminals, so that the system can have several different sessions at the same time. Only the system or Super User Root can write data to/dev/tty0,
5. Other Types
There are many other types of special files for terminal devices for many different character devices. For example,/dev/ttyin terminal devices for ISDN devices. I will not repeat it here