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What is the exact difference between a 'terminal', a 'shell', a 'tty' and a 'console'?

I always see pts and tty when I use the who command but I never understand how they are different? Can somebody please explain me this?

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A tty is a native terminal device, the backend is either hardware or kernel emulated.

A pty (pseudo terminal device) is a terminal device which is emulated by an other program (example: xterm, screen, or ssh are such programs). A pts is the slave part of a pty.

(More info can be found in man pty.)

Short summary:

A pty is created by a process through posix_openpt() (which usually opens the special device /dev/ptmx), and is constituted by a pair of bidirectional character devices:

  1. The master part, which is the file descriptor obtained by this process through this call, is used to emulate a terminal. After some initialization, the second part can be unlocked with unlockpt(), and the master is used to receive or send characters to this second part (slave).

  2. The slave part, which is anchored in the filesystem as /dev/pts/x (the real name can be obtained by the master through ptsname() ) behaves like a native terminal device (/dev/ttyx). In most cases, a shell is started that uses it as a controlling terminal.

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A tty is a regular terminal device (the console on your server, for example).
A pts is a psuedo terminal slave (an xterm or an ssh connection).

man pts has a verbose description of pseudo terminals.

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