Linux – Difference Between /dev/console, /dev/tty, and /dev/tty0

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.

This book gives a good introduction to Linux ttys, and fgreping for register_console in Linux kernel’s drivers/char/*.c shows what’s going on with Linux console. There are many implementations of the system console available in the Linux kernel. A specific Linux installation can decide which particular device should serve as the console. It can be specified by passing kernel parameters on boot, or possibly other techniques exist. The tty implementation is the default one, and arguably de facto standard, but other exist as well, such as serial-console.

/dev/console has its own dedicated major:minor. Why? Because the kernel itself is a major user of the kernel device when reports emergency messages there. There is struct tty_driver *console_driver kernel object pointing to routines needed. Then why don’t provide an interface to it for userland programs as well? Have no idea whether is /dev/console, for the default setup, really equivalent to /dev/tty0, or there are some differences.

Now about job control at tty0. How could it be possible if tty0 doesn’t really emulate a terminal but only points to a tty that is currently active? Linux kernel prevents processes from attaching to tty0, a “device” that intermittently collides with tty1, … tty7 and so on. Since tty0 can’t control processes, there is no job control possible (by the way, for which legitimate purpose can tty0 feed a shell at all?). But this has nothing to do with possibility to control jobs that run on specific virtual consoles. Other numbered ttys are, in fact, legitimate terminals for any purpose of a console application.