Story of TTY and PTS
2025-02-09
A little bit backgroud
As a Linux user, I am using terminal every day. One day I find the term "terminal" should be precisely named as "terminal emulator", I decide to dig more into the mystery of TTY. TTY stands for teletype and it has a long history all the way back to 1800s even before ASCII was invented. It was introduced into UNIX in 1900s for multiple users to talk to the operating system. By then the physical teletype is equipped with ticker tape (for input) and the paper printer (for output). It was gradually evolved to hardware emulated teletype. CRT and keyboard was made to replace paper printer and ticker tape. Now days keyboard and monitor are equipped with every Laptop and PC. People use software emulated terminal to talk to kernel or applications. But the legacy from the old cast-iron is till lurking beneath the surface. How kernel treats physical teletype and software emulated teletype doesn’t vary as much as hardware evolves.
Physical teletype
Ken Thompson developped UNIX on PDP7 with it.
Teletype Model 33
Hardware emulated teletype
DEC VT05
DEC VT100
Software emulated teletype
Now days physical and hardware emulated teletypes are practically kept in computer museum. People install software emulated terminal in graphical desktop environment to manage day to day jobs. Some well known software terminal emulators: urxvt, xterm, konsole, iterm2, st, kitty. In modern UNIX or Linux distros, mutiple software emulated terminal processes can run concurrently by a single user or multiple users, but all of them are attached to same tty (who(1)) which usually assigned for graphical desktop environment. How to distinguish different terminal processes as a logical tty? Here comes PTS - pseudo-teletypes. In Linux there is a master multiplexor ptmx(4) /dev/ptmx manages all master PTM slave PTS /dev/pts/* pair. The slave PTS number can be checked with tty command.
IO control commands
termios(3), tty_ioctl(4), stty(1)
Print TTY device setting
$ stty -a
speed 38400 baud; rows 75; columns 254; line = 0;
intr = ^C; quit = ^; erase = ^?; kill = ^U; eof = ^D; eol = <undef>; eol2 = <undef>; swtch = <undef>; start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R; werase = ^W; lnext = ^V; discard = ^O; min = 1; time = 0;
-parenb -parodd -cmspar cs8 -hupcl -cstopb cread -clocal -crtscts
-ignbrk -brkint -ignpar -parmrk -inpck -istrip -inlcr -igncr icrnl ixon -ixoff -iuclc -ixany -imaxbel iutf8
opost -olcuc -ocrnl onlcr -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0 ff0
isig icanon iexten echo echoe echok -echonl -noflsh -xcase -tostop -echoprt echoctl echoke -flusho -extproc
Current PTS
$ tty
/dev/pts/1
$ who
hjl tty1 2025-02-09 13:18
$ tty -s && echo "In a tty"
In a tty
Interesting read
- https://www.linusakesson.net/programming/tty/