For my final entry on termios, I will be looking at job control in the
shell (i.e. backgrounding and foreground jobs) and the very closely
related topic of signal generation by termios, in response to
Sessions and Process Groups
For the purposes of termios, processes are organized into two hierarchical groups, process groups and sessions. Every process belongs to exactly one process group and one session, and each process group is contained entirely within a session.
Process groups and sessions are both named by the process ID of the
process to create the group. This process is known as the process
group leader or session leader. A process creates a new session
setsid(2), or a new process group using
On Linux, you can inspect the process group and session of a process
stat field in
/proc/$PID. The first several fields in
that file are:
pid (name) state ppid pgid sid …
or, in more words:
[process id] ([name]) [state] [parent process id] [process group id] [session id] …
Sessions are the fundamental group of terminal management. Every
session may have an associated controlling terminal, which is
treated specially. A process may open and talk to any number of
terminals, but the special behaviors related to access control and job
control only apply to a process’s controlling terminal. Each terminal
may be the controlling terminal of at most one session. It follows
setsid(2) to create a new session causes a process to
lose its previous controlling terminal. Acquiring a controlling
terminal is OS-specific, but can usually be accomplished by opening a
terminal device without the
O_NOCTTY flag, while not already having
a controlling terminal.
Generally, all your processes within a single login session, or within
a single instance of your terminal emulator, are within the same
session, with the
pty allocated by your terminal emulator or
or whatever as their controlling terminal.
Process groups are the unit of control for signal generation by a terminal. A terminal never sends a signal to a specific process, but always to all processes within a process group.
Access to a terminal is also mediated in terms of process groups. In addition to having an associated session, every terminal has exactly one foreground process group. Every other process in that terminal’s session is a background process group.
The foreground process group is awarded special access to its
controlling terminal. It is allowed unrestricted access to read from
and write to the terminal, as well as to call various control
functions, such as
tcsetattr on it.
In addition, if any signal-generating character is read by a terminal, it generates the appropriate signal to the foreground process group.
Background process groups are restricted in their access to their
controlling terminal. If any process in a background process group
attempts to read from its controlling terminal, it will result in
SIGTTIN being sent to its process group. Background processes may
write to their controlling terminal, unless
TOSTOP is set in
c_lflag, in which case doing so will generate
SIGTTOU to its
process group. Calling terminal control functions such as
is treated like a write operation with
TOSTOP set (i.e.
sent unless the process is blocking or ignoring it).
The foreground process group for a terminal may be set by the
tcsetpgrp(3) function, which may be called by any process in the
terminal’s session, but is treated in the same way as
the previous paragraph.
We’ve now got most of what we need to understand job control in your shell.
When processing each command line, the shell uses
setpgid to place
all of the programs executed by the line into the same process group,
and then calls
tcsetpgrp to make that job the foreground job, and
waitpid to wait on that process.
Thus, when you run a shell pipeline (
foo | bar | grep baz), all the
programs in the pipeline are in the same process group, and in the
foreground, which is why a
^C kills all of them.
^Z a jobs, all the processes in the process group are
stopped, and the shell’s
waitpid returns, informing it of the status
change. The shell restores itself to the foreground process group, and
marks the job as backgrounded.
When you use
bg to background a stopped job, the shell just uses
SIGCONT the group. If a job in the background tries
to read from the terminal, the
SIGTTIN stops it and the shell’s
wait detects the state change and adjusts the job’s state
If you launch a job in the background, the shell simply doesn’t
tcsetpgrp it into the foreground, nor
wait on it.
That’s probably all I want to say about termios. I could talk more about terminal emulation, ncurses,
$TERM and friends, but it’s less interesting to me – I think I’m a kernel hacker at heart, and that stuff is just userspace programs talking to each other at this point. I hope you found this series interesting and/or informative, and I’m always happy to answer questions.