ICS54
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Solutions to Assignment #1
There are several ways to do each question. I have listed several solutions
for each.
1) What is your home directory? How did you find out?
There are several ways to find out.
- "echo $HOME"
- login, and before typing any "cd" command, type "pwd".
- type "cd" by itself, and type "pwd"
- Read the first few lines of "man 4 passwd", then grep in /etc/passwd:
$ grep Hayes /etc/passwd
wayne:x:666:114:Wayne Hayes:/u/wayne:/var/shell/tcsh
^^^^^^^^ this is my home directory.
2) What is your UID? How did you find out?
Your UID is not the same as your user name; the former is an integer,
the latter is a word.
- use type "id", and read the output.
- read the first few lines of "man 4 passwd" and grep /etc/passwd...
- "/bin/ls -n" on any file you own, and look at the uid.
3) What is your group name(s)? How did you find out?
The group name is not the same as the group id; the former is a word, the
latter is an integer. The solutions are the same as #2, except look at
different fields.
4) What is your "kill" character used for?
The "kill" character erases the current line that you are typing. It
is usually '^U'. It is NOT the same as the kill command (type "man 1 kill"
to read about that), nor is it the character that you press to interrupt
a program. That's called the "interrupt" character, and is usually '^C'.
It's mentioned on page 6 of K&P. Type "stty -a" to see all the "magic"
characters that are currently set on your terminal.
5) If you remove execute permission for yourself from a directory, are
you still allowed to create files in that directory? Can you cd into
it? Can you "ls" it?
This question was pure experimentation. You could answer it in 2 minutes
if you simply tried it. Here's my output and comment:
$ mkdir foo
$ chmod u-x foo
$ echo hi >foo/bar
ksh: foo/bar: cannot create # that answers the first question...
$ cd foo
ksh: foo: permission denied # that answers the second...
$ ls foo
$ ls -l foo
total 0 # hmm, but there are no files. Let's try...
$ chmod u+x foo # make it executable again...
$ echo hi >foo/bar # put a file there
$ ls foo
bar
$ ls -l foo # OK, it's there.
total 0
-rw-r--r-- 1 wayne 3 Sep 26 18:05 bar
$ chmod u-x foo # take away execute permission again...
$ ls foo
bar # works...
$ ls -l foo
ls: foo/bar: Permission denied # AHA!
total 0
The last line tells us that we can read the directory file (which makes
sense, because it's just a file) and so we can see what file names are in
there. But "ls -l" needs to get information that's stored in the i-nodes
of the files in the directory, and THAT is not allowed. That is called
"searching" a directory, and that's not allowed without execute permission.
6) If you make a hard link to a file, and then remove the original file,
are the contents of the file actually removed? Why? How about if you
make a symbolic link? Why?
No, and yes, respectively. A hard link is just as strong a connection
to the inode (and thus the data) as the original filename, so the inode
and the file data stay around if you remove the original file. However,
a soft link is only a pathname to the original filename, not the original
i-node. So if you remove the original file (and assuming it was the only
hard link to the file), then the data is erased and the soft link points
nowhere.
7) Are you allowed to "mv" a directory? Are you allowed to "cp" a
directory? If so, how?
Since a directory is just a file, there is no problem moving it to
somewhere else on the *same* mounted file system. The directory still
has the same i-node, and the filenames in the directory still have the
same inodes; it is really just a name change. You cannot move a directory
across file systems, though, because the inodes and data reside on a
specific mounted file system. You can copy a directory with the "-r"
option. (You may need to use "-R" option if named pipes are present;
and you may want to use "-p" to preserve date-time stamps.) You can
also use tar(1):
$ (cd src-dir; tar cf - .) | (cd dest-dir; tar vxf -)
8) How many mounted file systems are there on eddie.cdf?
First you have to get onto eddie; to do that, use "rlogin eddie" or
"rsh eddie", or "telnet eddie".
Typing "mount | wc" gives 16 lines; "df | wc" gives 17, but that includes
a header, so you need to subtract 1.
9) Using only "ls", can you find out the name of your login terminal? How?
This was alot harder than I intended. Here's how I did it:
$ cd /dev
$ ls -l | grep wayne # no output... Hmmmm
$ who am i # cheating...
wayne /dev/pts/82 Sep 26 18:47
$ ls -l /dev/pts/82 # the symbolic link isn't any help
lrwxrwxrwx 1 root 29 Jun 20 15:34 /dev/pts/82 -> (long name)
$ ls -lL /dev/pts/82 # ah, much better
crw--w---- 1 wayne 24, 82 Sep 26 18:48 /dev/pts/82
$ cd # back to the home directory with knew knowledge...
$ ls -lL /dev/pts | grep wayne # I'm logged in 4 times.
crw-rw-rw- 1 wayne 24, 2 Sep 26 15:19 2
crw--w---- 1 wayne 24, 46 Sep 26 18:49 46
crw--w--w- 1 wayne 24, 52 Sep 26 18:49 52
crw--w---- 1 wayne 24, 82 Sep 26 18:50 82
$ date # it must be /dev/pts/82, since that's the most recent one,
Thu Sep 26 18:50:19 EDT 1996
$ # and that's the one I'm typing on right now.
10) In slide 6 from lecture #1, assume binary 00000000 represents
register D0. Then the address translation from "FFFFDC0x" to
binary is incorrect. Ignore the leading FFFF, then correct the
binary representation of the DC0x addresses. What is the machine
language (binary) representation of the MOVE instruction? What
about the MUL instruction?
There are three lines of machine language, and three lines of assembly
language, so they go one-to-one to each other. Here is the corrected
version:
1100101 11011100 00000000 00000000
MOVE DC 00 D0
1010001 00000010 01011101 00000000
MUL #2 garbage D0
1100101 00000000 11011100 00000100
MOVE D0 DC 04
The MOVE instruction is represented by 1100101, MUL by 1010001.
There are several ways to convert from hexadecimal to binary. The easiest
way (not including using a calculator that can do it directly) is to realize
that 16 = 2^4, so that you can build the following table (note: by convention
in Unix & C, base-8 numbers have a leading 0, and base-16 have a leading 0x):
base 10 base 8 base 16 base 2
0 00 0x0 0000
1 01 0x1 0001
2 02 0x2 0010
3 03 0x3 0011
4 04 0x4 0100
5 05 0x5 0101
6 06 0x6 0110
7 07 0x7 0111
8 010 0x8 1000
9 011 0x9 1001
10 012 0xA 1010
11 013 0xB 1011
12 014 0xC 1100
13 015 0xD 1101
14 016 0xE 1110
15 017 0xF 1111
16 020 0x10 10000
So, for example, 0xDC = 1101 1100, as I have in the solution above.