What do pty and tty mean?

问题:

I noticed there are many mentions of pty and tty in some opensource projects, could someone can tell me what do they mean and what is the difference between them? Thanks!

回答1:

"tty" originally meant "teletype" and "pty" means "pseudo-teletype".

In UNIX, /dev/tty* is any device that acts like a "teletype", ie, a terminal. (Called teletype because that's what we had for terminals in those benighted days.)

A pty is a pseudotty, a device entry that acts like a terminal to the process reading and writing there, but is managed by something else. They first appeared (as I recall) for X Window and screen and the like, where you needed something that acted like a terminal but could be used from another program.

回答2:

A tty is a terminal (it stands for teletype - the original terminals used a line printer for output and a keyboard for input!). A terminal is a basically just a user interface device that uses text for input and output.

A pty is a pseudo-terminal - it's a software implementation that appears to the attached program like a terminal, but instead of communicating directly with a "real" terminal, it transfers the input and output to another program.

For example, when you ssh in to a machine and run ls, the ls command is sending its output to a pseudo-terminal, the other side of which is attached to the SSH daemon.

回答3:

tty: teletype. Usually refers to the serial ports of a computer, to which terminals were attached.

pty: pseudoteletype. Kernel provided pseudoserial port connected to programs emulating terminals, such as xterm, or screen.

回答4:

If you run the mount command with no command-line arguments, which displays the file systems mounted on your system, you’ll notice a line that looks something like this: none on /dev/pts type devpts (rw,gid=5,mode=620) This indicates that a special type of file system, devpts , is mounted at /dev/pts .This file system, which isn’t associated with any hardware device, is a “magic” file system that is created by the Linux kernel. It’s similar to the /proc file system

Like the /dev directory, /dev/pts contains entries corresponding to devices. But unlike /dev , which is an ordinary directory, /dev/pts is a special directory that is cre- ated dynamically by the Linux kernel.The contents of the directory vary with time and reflect the state of the running system. The entries in /dev/pts correspond to pseudo-terminals (or pseudo-TTYs, or PTYs).

Linux creates a PTY for every new terminal window you open and displays a corre- sponding entry in /dev/pts .The PTY device acts like a terminal device—it accepts input from the keyboard and displays text output from the programs that run in it. PTYs are numbered, and the PTY number is the name of the corresponding entry in /dev/pts .

For example, if the new terminal window’s PTY number is 7, invoke this command from another window: % echo ‘I am a virtual di ’ > /dev/pts/7 The output appears in the new terminal window.