Software Tools
UNIX Files
and Security
Slide 2
File Systems

What is a file system?
A means of organizing information on the computer. A file
system is a logical view, not necessarily a physical view.

What does the file system provide:




ways to create, move, and remove files
ways to order files
security
Examples of file systems:

DOS, Macintosh, CD-ROM, UNIX, NFS (networked file
system)
Slide 3
UNIX File Systems



Hierarchical Organization
Root of tree is at top denoted by ‘/’
Kinds of files:

Directory files
(the branches in the tree)

/
Regular files
(leaves in the tree)
bin
.mailrc
homes
lib
...
horner
jbond
...
111
top10
...
Slide 4
Home and
Working Directories

Home directory





The directory you are in when you first login in
This is your space; you control security
Place to put your personalized .startup files
Your working directory after typing cd with no arguments
Working directory




Can access files in your working directory by simply typing
the filename
To access files in other directories, must use a pathname
pwd command prints the working directory
cd command changes the working directory
Slide 5
Directory Shorthands




“.” is the directory itself
“..” is the parent directory
In most shells “~” means your home directory)
~user means user’s home directory, so:
$ more ~jbond/.plan
looks at the file .plan in /homes/jbond, which
is jbond’s home directory.
Slide 6
Special Directories






“/” (pronounced “slash” and also called “the “root”)
is the ancestor of all files in the file system
/bin and /usr/bin contain UNIX utilities (e.g.,
cat)
/dev contains files which describe “devices” such
as terminals and printers
/etc has administrative programs like password
files
/tmp is for temporary files; periodically deleted
Every directory has at least two entries: “.” is the
directory itself, and “..” is the directory’s parent
Slide 7
Naming Files


Files in the same directory can’t have the same
name
Case sensitive: secret and Secret are different

Files are sometimes named with an extension (e.g.,
bond.cpp, 007.jpg) to show the file’s content.

You cannot create a file named “.” or “..”
“Invisible” files and directories (those that don’t
appear using ls) have a period as the first character
(e.g., .plan). Some programs use invisible files to
store information.

Slide 8
Pathnames

Simple filenames
Can only be used if files are in
working directory

Relative pathname
A string of directory references, beginning with the
working directory. Examples:
./secret1
../007/names
top10/LG7soBad

Absolute pathname
A pathname beginning at the root. e.g.,:
/homes/jbond/.plan
/etc/passwd
Slide 9
Directory Commands

mkdir makes a new directory (if you have permission
to do so). With a simple pathname, mkdir makes a
new directory in your working directory.
$ pwd
/homes/jbond/111
$ ls -l
total 6
-rw-r--r-1 jbond
-rw-r--r-1 jbond
drwxr-xr-x
2 jbond
$ mkdir newdir
$ ls -l
total 8
-rw-r--r-1 jbond
-rw-r--r-1 jbond
drwxr-xr-x
2 jbond
drwxr-xr-x
2 jbond
cs
cs
cs
154 Feb
64 Feb
512 Feb
4 15:00 letter3
4 15:00 names
4 15:00 secret/
cs
cs
cs
cs
154
64
512
512
4
4
4
4
Feb
Feb
Feb
Feb
15:00
15:00
15:26
15:00
letter3
names
newdir/
secret/
Slide 10
Directory Commands

rmdir deletes a directory
(if you have permission).
$ rmdir newdir
$ ls -l
total 6
-rw-r--r--rw-r--r-drwxr-xr-x
$ rmdir
1 jbond
1 jbond
2 jbond
cs
cs
cs
154 Feb
64 Feb
512 Feb
4 15:00 letter3
4 15:00 names
4 15:00 secret/
/usr
rmdir: directory "/usr": Search or write permission needed
Slide 11
Directory Commands

mv can be used to move a file to another directory.
$ ls
letter3
names
$ mv letter3 secret
$ ls
names
newdir/
$ ls secret
letter3

newdir/
secret/
secret/
mv can be used to move a directory into a directory.
$ ls
names
$ mv newdir
$ ls
names
$ ls secret
letter3
newdir/
secret
secret/
newdir/
secret/
Slide 12
Directory Commands

You can also move several files at once using mv
$ ls
letter1 letter2
$ mv letter* secret
$ ls
names
secret/
$ ls secret
letter1 letter2
names
secret/
Slide 13
Security and Access Permissions

There are three types of users:




The owner of the file (user)
The group of the file (group)
Anyone else (other)
There are three types of permission
(independent of each other):

Read permission
 Write permission
 Execute permission
Slide 14
Security and Access Permissions

Use ls -l to see file permissions
-rw-r--r--
1 jbond
cs
#links
Group
User
Permissions

154 Feb
4 15:00 letter3
Byte size
Name
Last modification
There are four sets of items in the permissions:
-rw-r--r-

type
user group other
The type is:
“-” regular files, “d” directories , “l” symbolic links.
The next nine characters indicate if the file is readable,
writable, or executable for the file owner, the file group, or
other users, respectively.
Slide 15
Security and Access Permissions

Examples:
$ ls -l
total 34
-r-xr-xr-x
1 jbond
cs
9388 Feb
4 16:31 cat*
-rw-r--r--
1 jbond
cs
154 Feb
-rw-------
1 jbond
cs
64 Feb
drwxr-xr-x
2 jbond
cs
512 Feb
4 15:41 newdir/
drwxr-xr-x
2 jbond
cs
512 Feb
4 16:38 secret/
d---------
2 jbond
cs
512 Feb
4 16:39 secret1/
dr--r--r--
2 jbond
cs
512 Feb
4 16:39 secret2/
d--x--x--x
2 jbond
cs
512 Feb
4 16:38 secret3/
4 15:00 letter1
4 15:00 names
Slide 16
Directory Permissions

Can use ls -ld to lists a directory’s information
(instead of its contents):
$ ls -l secret
total 4
-rw-r--r--
1 jbond
cs
154 Feb
4 16:38 letter1
-rw-r--r--
1 jbond
cs
34 Feb
4 15:00 letter4
cs
512 Feb
4 16:38 secret/
$ ls -ld secret
drwxr-xr-x
2 jbond
Slide 17
Directory Permissions
Directory read permission means that you can
see what files are in the directory.
 Directory write permission means that you can
add/remove/rename files in the directory.
 Directory execute permission means that you
can search the directory (i.e., you can use the
directory name when accessing files inside it).

Slide 18
Directory Permissions
$ ls -ld secret*
drwxr-xr-x
2 jbond
d--------2 jbond
dr--r--r-2 jbond
d--x--x--x
2 jbond
cs
cs
cs
cs
512
512
512
512
$ ls -l secret*
secret:
total 2
-rw-r--r-1 jbond
cs
1054 Feb
secret1 unreadable
ls: secret2/letter1: Permission denied
secret2:
total 0
secret3 unreadable
Feb
Feb
Feb
Feb
4
4
4
4
16:38
16:39
16:39
16:38
secret/
secret1/
secret2/
secret3/
4 16:38 letter1
Slide 19
Directory Permissions

Directory execute permission means that you can do ls and
cp on individual files in the directory.
drwxr-xr-x
secret/
d--------dr--r--r-d--x--x--x
$ ls -l secret*/letter1
-rw-r--r-1 jbond
cs
-rw-r--r-1 jbond
cs


154 Feb
154 Feb
secret1/
secret2/
secret3/
4 16:38 secret/letter1
4 16:39 secret3/letter1
Real-life Example: What if you want your friend to get a file and no one
else?
Solution: Set the directory execute permission to “on” and read
permission to “off” (like directory secret3), and the file read permission
to “on”. Tell your friend the filename (the complete path). This allows your
friend to access the file by typing the exact filename. Others will not
know that the file exists.
Slide 20
Changing Permissions

The chmod command is used to modify permissions.
chmod can only be used by the owner of a file/dir.

The arguments are:

chmod [ugoa] [+-=] [rwx] [file/dir]
In other words:
 Optionally, one of the characters: u (user/owner), g (group),
o (other), or a (all).
 Optionally, one of the characters: + (add permission),
- (remove permission), or = (set permission).
 Any combination of the characters r (read), w (write), or
x (execute).
Slide 21
Permission Example

To let everybody read or write the file letter1
$ chmod a+rw letter1
$ ls -l letter1
-rw-rw-rw1 jbond

4 15:00 letter1
cs
154 Feb
4 15:00 letter1*
To not let “other” to read or write file letter1
$ chmod o-rw letter1
$ ls -l letter1
-rwxrw---1 jbond

154 Feb
To allow user to execute file letter1
$ chmod u+x letter1
$ ls -l letter1
-rwxrw-rw1 jbond

cs
cs
154 Feb
4 15:00 letter1*
To let “group” only read the file letter1
$ chmod g=r letter1
$ ls -l letter1
-rwxr----1 jbond
cs
154 Feb
4 15:00 letter1*
Slide 22
Permission Shortcut



chmod allows you to use 3 decimal digits to set the
permissions, where user is the 1st digit, group is the 2nd digit,
and other is the 3rd digit.
Each of these decimal digits represents a 3-digit binary
number for read permission (1st binary digit), write permission
(2nd binary digit), and execute permission (3rd binary digit).
For example, with the file letter1, to allow



user to read, write, and execute (binary 111 = decimal 7),
group to read and write (110=6),
other to read only (100=4):
$ chmod 764 letter1
$ ls -l letter1
-rwxrw-r-1 jbond

cs
154 Feb
4 15:00 letter1
As another example, with the file letter1, to allow



user to execute only (001=1),
group to write and execute (011=3),
other to read and execute (101=5):
$ chmod 135 letter1
$ ls -l letter1
---x-wxr-x
1 jbond
cs
154 Feb
4 15:00 letter1