February 13, 2006

Coverage: [USP] Chapters 5 and 6, and [UPE] Chapter 2

Disk Statistics

total line in ls -l output gives the #blocks in the directory
- a block in most UNIX systems = 1/2k (512 bytes)
du (disk usage) command display disk usage statistics
- displays the amount of space (1/2k blocks) occupied by a directory plus all of its files and subdirectories
- -a option displays an entry for each file in a file hierarchy
```
$ du -a  /usr/users/guest
1047    ./eps
3       ./permsrec.p
76      ./printscreen.ps
93      ./menu.ps
1       ./.Xdefaults
1247    .
```
- du -k shows usage in 1k blocks

df (disk free) command displays amount of free disk space on all disks:

$ df
Filesystem          Total    kbytes   kbytes     
node               kbytes      used     free   used    Mounted on
/dev/rz2a           19743     10387     7382    58%    /
/dev/rz2g          294518    248466    16601    94%    /usr

notice that each file system is mounted starting from / (root)
- makes the presence of multiple file systems (and devices) transparent to the user

`od` (octal dump) Command

display an ascii, decimal, hexadecimal, or octal dump of a file
takes each set of 16-bits words and converts each to octal

examples:

$ od -d # decimal
$ od -x # hex
$ od -c # as a print character
$ od -b # bytewise, but in octal
$ od -bc

7-digit #s down the lhs are positions in the file, i.e., the ordinal # (starting from 0) of the next character shown, in octal
emphasis on octal is remnant of the PDP-11
also see xxd command

File `Types' and `Names'

no such concept as a file type in UNIX

how does the file command do its job?

uses clues, e.g., binary `magic' #

examples:

$ od /bin/ls | head -2
0000000 077505 046106 000402 000400 000000 000000 000000 000000
0000020 000002 000002 000000 000001 000001 007010 000000 000064
$ od /bin/od | head -2
0000000 077505 046106 000402 000400 000000 000000 000000 000000
0000020 000002 000002 000000 000001 000001 006014 000000 000064
$ od 20060125/v3cat | head -2
0000000 077505 046106 000402 000400 000000 000000 000000 000000
0000020 000002 000002 000000 000001 000001 002320 000000 000064

0000000 077505 046106 000402 000400 000000 000000 000000 000000 on sun131-32

ls -i will display inode #s
if filename is not stored in the inode, where is a file's name located?
- try an od on a directory
- notice anything interesting?
- compare ordering of filenames to that in output of ls -l
- a directory consists of 16-byte chunks
  - first 2 hold the inode #
    - the only connection between name of a file and its contents
    - this is why each filename in a directory is called a `link'
      - links name of file to inode, and thus data
    - rm removes directory entries, not inodes
    - an inode # of 0 of a file in a directory indicates that the link been removed, but not necessarily the contents of the file
  - last 14 hold the filename, padded with ASCII NUL's
    - filename size limits?

Questions to Investigate

why cannot we od or xxd on directories we own?
places to look for information:
- section 4 of the UNIX manual covers `File Formats'
- dir_ufs(4), dir(4)

Set-uid Program

e.g., passwd
how can you change your password when you do not have write permission on the password file?
```
$ ls -l /etc/passwd 
-rw-r--r--   1 root     sys          580 Aug  1  2005 /etc/passwd
```
set-uid: grant permission to use file and while doing so to have all of owner's privileges
- sounds dangerous
- indicated with s bit(s) in permission list

$ ls -l /bin/passwd
-r-sr-sr-x   1 root     sys        27220 Jan 22  2005 /bin/passwd*

bin/passwd is said to be `set-uid to root'

root is superuser on the system
su command allows you to become superuser or another user
- passing a - changes the environment to what would be expected if the user actually logged in as the specified user; otherwise environment inherited
- root assumed if no user specified

Login Process

your login name is just a mnemonic for your uid
when you enter your password during login, the system encrypts it (e.g., using crypt) and compares the encrypted form to value (which is encrypted there) of the appropriate field in your entry in the password file
if they agree, you are permitted to access the system

Things to Do

find the entry in the passwd file which corresponds to your account
because of the shared file systems you will have to do some exploring
once your find your entry, record your uid and gid
see the following manpages for help:
- passwd(1)
- passwd(4)
- finger(1)
- ypcat(1)
- ypmatch(1)
- getent(1)

References

The UNIX Programming Environment

UNIX Systems Programming: Concurrency, Communication, and Threads