Cirrus Logic CS485 Manual do Utilizador descarregar pdf (Página 44)

CS485G Spring 2015 44

2. Lecture 27, 4/6/2015

3. login starts a shell for each logged-in user.

(a) sh: original shell, Stephen Bourne, 1977

(b) csh: BSD Unix C shell. tcsh: enhanced at CMU.

(d) ash: Almquist shell, 1997 dash: POSIX standard.

4. The shell runs a read-eval loop.

(a) Primarily it runs commands as processes (using fork() and

exec().

(b) It waits for commands to ﬁnish (using wait()), or not, if they

have been started in the background.

mand)

50 Signals

1. When a process terminates, the kernel sends its parent a software

interrupt.

2. Software interrupts are called signals.

3. There are about 30 possible signals, each with a different purpose.

see signal(7).

4. Each signal has its default action. For instance, SIGSEGV terminates

with a dump ﬁle; SIGCHLD (a child has stopped or terminated) is

ignored.

5. A process may explicitly send a signal to another process by the

kill(2) system call. The sending and target processes must be

owned by the same user, or the sending process must be owned by

root.

6. The /bin/kill command sends a given signal to a given target process

(or process group).

7. The user can send signals to processes by some keyboard methods:

ctrl-c sends SIGINT; ctrl-z sends SIGSTOP.

8. For most signals, a process can choose in advance what action to take

when receiving them, but using signal(2).

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Cirrus Logic CS485 Manual do Utilizador Página 44