ptg10805159286 Process Relationships Chapter 9
common devices on PDP-11swereDH-11sand DZ-11s. A host had a fixed number of
these terminal devices, so therewas a known upper limit on the number of
simultaneous logins.
As bitmapped graphical terminals became available, windowing systems were
developed to provide users with new ways to interact with host computers.
Applications weredeveloped to create ‘‘terminal windows’’ to emulate character-based
terminals, allowing users to interact with hosts in familiar ways (i.e., via the shell
command line).
Today,some platforms allow you to start a windowing system after logging in,
whereas other platforms automatically start the windowing system for you. In the
latter case, you might still have to log in, depending on how the windowing system is
configured (some windowing systems can be configured to log you in automatically).
The procedurethat we now describe is used to log in to a UNIX system using a
terminal. The procedure is similar regardless of the type of terminal we use—itcould
be a character-based terminal, a graphical terminal emulating a simple character-based
terminal, or a graphical terminal running a windowing system.BSD Terminal Logins
The BSD terminal login procedurehas not changed much over the past 35 years. The
system administrator creates a file, usually/etc/ttys,that has one line per terminal
device. Each line specifies the name of the device and other parameters that arepassed
to thegettyprogram. One parameter is the baud rate of the terminal, for example.
When the system is bootstrapped, the kernel creates process ID 1, theinitprocess, and
it isinitthat brings the system up in multiuser mode. Theinitprocess reads the file
/etc/ttysand, for every terminal device that allows a login, does aforkfollowed by
anexecof the programgetty.This gives us the processes shown in Figure9.1.initinitprocess ID 1gettyfork forkper terminalsonceeach child
exec execsgettyFigure 9.1 Processes invoked byinitto allow terminal loginsAll the processes shown in Figure9.1 have a real user ID of 0 and an effective user ID of
0(i.e., they all have superuser privileges). Theinitprocess alsoexecsthegetty
program with an empty environment.