In Unix, the root file system starts with "/". However, there are other subfilesystems, that are part of the root one. To see the filesystems on your machine, type "df". You'll see something like:
UNIX> df
Filesystem kbytes used avail capacity Mounted on
/dev/sd0a 11167 5952 4099 59% /
/dev/sd0g 140591 84016 42516 66% /usr
/dev/sd0h 171959 141421 13343 91% /blugreen/homes
-memory- 55720 0 55720 0% /tmp
plank:(pid130) 4 4 0 100% /amd
galoob:/export/sun4-sos4_1
335803 313356 0 100% /a/galoob/export/sun4-sos4_1
cs:/var/spool/mail 221351 122607 76609 62% /a/cs/var/spool/mail
alphard:/canary/homes
411687 293685 76833 79% /a/alphard/canary/homes
UNIX>
The last entry on each line in this denotes a different
filesystem. You'll note that all are subdirectories of "/". Each
filesystem resides on a different disk or "disk partition".
When you say "ls -a", it lists all the filenames in the current directory:
UNIX> cd ~huangj/cs360/notes/Rbtree-1 UNIX> ls -a . jh.c makefile mysorti.c mysortu2.c .. lecture mysort.c mysortu0.c mysortu3.c README lecture.html mysort2.c mysortu1.c randfile UNIX>(If you don't say "-a", it will not list filenames beginning with ".") Note there are two filenames that are in every directory: "." and ".."
"." is the current directory. ".." is the parent of the current directory.
The pwd command tells you the complete pathname of the current directory. The cd command moves you between directories:
UNIX> cd ~huangj/cs360/notes/Rbtree-1 UNIX> pwd /home/huangj/cs360/notes/Rbtree-1 UNIX> cd . UNIX> pwd /home/huangj/cs360/notes/Rbtree-1 UNIX> cd .. UNIX> pwd /home/huangj/cs360/notes UNIX> cd ../../.. UNIX> pwd /home UNIX> cd .././././. UNIX> pwd / UNIX> cd .. UNIX> pwd / UNIX>Note that the parent directory of "/" is itself.
When you run a program, its executing instance is called a process.
For example, suppose you type "vi ~huangj/cs360/notes/Rbtree-1/mysort.c" in one window. This executes the vi program, found in /usr/ucb/vi. The executing instance is called a process. If you go to another window, and type "ps x", it will list all of the processes that you are currently executing:
UNIX> ps x 1394 p0 IW 0:00 xclock -geometry 100x100-0-0 -update 60 1416 p0 S 0:01 twm 1436 p1 IW 0:00 -sh (csh) 1427 p2 IW 0:00 -sh (csh) 1428 p3 IW 0:00 -sh (csh) 1443 p5 S 0:01 -sh (csh) 2328 p5 S 0:00 vi /home/huangj/cs360/notes/Rbtree-1/mysort.c 1444 p6 IW 0:00 -sh (csh) 2329 p6 R 0:00 ps x UNIX>Note that we can run more than one vi process at the same time. Go to another window and type "vi /home/huangj/cs360/notes/Rbtree-1/mysort2.c". Now when you type "ps x", you'll see the second process.
UNIX> ps x 1394 p0 IW 0:00 xclock -geometry 100x100-0-0 -update 60 1416 p0 S 0:01 twm 1436 p1 IW 0:00 -sh (csh) 1427 p2 S 0:00 -sh (csh) 2330 p2 S 0:00 vi /home/huangj/cs360/notes/Rbtree-1/mysort2.c 1428 p3 IW 0:00 -sh (csh) 1443 p5 IW 0:01 -sh (csh) 2328 p5 IW 0:00 vi /home/huangj/cs360/notes/Rbtree-1/mysort.c 1444 p6 S 0:00 -sh (csh) 2331 p6 R 0:00 ps x UNIX>You should understand the distinction between process and program. The number in the first column of the ps command is the "process id".
There are two ways that the system deals with identification: User Id's and group id's. We won't talk much about group id's here. Your user id can be gotten by the system call "getuid()" -- read the man page.
To print out your UID, try ch1b.c:
#include < stdio.h >
main()
{
printf("%d\n", getuid());
}
Signals are a way for asynchronous events to occur in a program. For example, compile and run the program ch1c.c:
#include < stdio.h >
main()
{
int i;
i = 0;
while (1) {
i++;
printf("%d\n", i);
sleep(5);
}
}
This program implements a counter that increments itself every 5
seconds. Let it run for a few seconds, and then type < CNTL-Z >. This
sends the "STOP" signal to the program, which stops it. You'll be
back at the shell now. If you type "ps", you'll see something like:
2483 p5 T 0:00 ch1cThe "T" means that the process is not running -- it is stopped. To start it, you can type "fg", which will send it the "START" signal.
Now, while it's running, type < CNTL-C > to kill the program -- this sends it the "INT" signal, which teminates it. Segmentation faults are also signals. You can write your programs to handle signals any way you please. This is a more advanced programming technique, which we will deal with later in the semester.
Yes, signal is a part of the ANSI C standard, it should perform the same (or, very similarly) on all systems claiming to support ANSI'87. The primary differences stem from which additional signals, beyond the basic set of signals, are supported on different systems. The basic set specified by ANSI C standard is rather small.