ptmx, pts - pseudoterminal master and slave
/dev/ptmx is a character file with major number 5 and minor number 2,
usually of mode 0666 and owner.group of root.root. It is used to
create a pseudoterminal master and slave pair.
When a process opens /dev/ptmx, it gets a file descriptor for a
pseudoterminal master (PTM), and a pseudoterminal slave (PTS) device
is created in the /dev/pts directory. Each file descriptor obtained by
opening /dev/ptmx is an independent PTM with its own associated PTS,
whose path can be found by passing the descriptor to ptsname(3).
Before opening the pseudoterminal slave, you must pass the master's
file descriptor to grantpt(3) and unlockpt(3).
Once both the pseudoterminal master and slave are open, the slave
provides processes with an interface that is identical to that of a
Data written to the slave is presented on the master descriptor as
input. Data written to the master is presented to the slave as input.
In practice, pseudoterminals are used for implementing terminal
emulators such as xterm(1), in which data read from the pseudoterminal
master is interpreted by the application in the same way a real
terminal would interpret the data, and for implementing remote-login
programs such as sshd(8), in which data read from the pseudoterminal
master is sent across the network to a client program that is
connected to a terminal or terminal emulator.
Pseudoterminals can also be used to send input to programs that
normally refuse to read input from pipes (such as su(1), and
They're usually xterminal/gnome-terminal/terminator sessions.