while (1) {
printf ("$");
readcommand (command, args); // parse user input
if ((pid = fork ()) == 0) { // child?
exec (command, args, 0);
} else if (pid > 0) { // parent?
wait (0); // wait for child to terminate
} else {
printf ("Unable to fork\n");
}
}
The split of creating a process with a new program in fork and exec is mostly a historical accident. See the assigned paper for today.
$ ls
read (1, buf, bufsize)
write (1, "hello\n", strlen("hello\n")+1)
$ls > tmp1
just before exec insert:
close (1);
fd = open ("tmp1", O_CREAT|O_WRONLY); // fd will be 1!
The kernel will return the first free file descriptor, 1 in this case.
$ls 2> tmp1 > tmp1
replace last code with:
close (1);
close (2);
fd1 = open ("tmp1", O_CREAT|O_WRONLY); // fd will be 1!
fd2 = dup (fd1);
both file descriptors share offset
$ sort file.txt | uniq | wc
or
$ sort file.txt > tmp1 $ uniq tmp1 > tmp2 $ wc tmp2 $ rm tmp1 tmp2or
$ kill -9
int fdarray[2];
if (pipe(fdarray) < 0) panic ("error");
if ((pid = fork()) < 0) panic ("error");
else if (pid > 0) {
close(fdarray[0]);
write(fdarray[1], "hello world\n", 12);
} else {
close(fdarray[1]);
n = read (fdarray[0], buf, MAXBUF);
write (1, buf, n);
}
int fdarray[2];
if (pipe(fdarray) < 0) panic ("error");
if ((pid = fork ()) == 0) { child (left end of pipe)
close (1);
tmp = dup (fdarray[1]); // fdarray[1] is the write end, tmp will be 1
close (fdarray[0]); // close read end
close (fdarray[10); // close fdarray[1]
exec (command1, args1, 0);
} else if (pid > 0) { // parent (right end of pipe)
close (0);
tmp = dup (fdarray[0]); // fdarray[0] is the read end, tmp will be 0
close (fdarray[0]);
close (fdarray[1]); // close write end
exec (command2, args2, 0);
} else {
printf ("Unable to fork\n");
}
The shell creates processes for the other commands in the pipeline and hook up their stdin and stdout correctly.
$ compute &