02

Programming Methods and Techniques I

UNIX files and directories management

Lucjan Janowski

2010-10-08

It is a translation of Michał Grega’s instruction.

Task 1 How to move in the UNIX directory system

1.

The basic information is where we are i.e. in which directory we are executing commands and programs. In
order to get this information

pwd

command is used. It is a very important information especially when you

are removing a group of files!

2.

In order to move into another directory you can use

cd

command. The

cd

command parameter is the

directory where you would like to go.

3. The are some special directories. The main directory is denoted by

/

, the home directory is denoted by

~

and

the upper directory is denoted by

..

.

4. In order to print what is in the current directory we can use

ls

command. Please read carefully what

a

,

l,s,k

options change.

5. As an exercise try to answer below questions:

a. Where is your home directory?
b. What kind of files and directories are located in the main directory

(/)?

c.

What is a difference between command

ls a

and

ls -a

?

d. What kind of information do we get using

ls -l

command?

An example:
After login to the system a user is in his/her home directory (

~

).

Executing command

cd mama

moves the user into the directory mama.

Executing command

cd tata

moves the user into the directory tata that is in mama directory.

Executing command

cd ..

moves the user to the upper directory i.e. into mama directory.

Executing command

cd ~

in any directory moves a user into his/hers home directory.

Task 2 Creating and removing directories and files

1. In order to create a directory use

mkdir

command. Note that you can

create more than one directory at once. Do you know how to do it?

2. Removing directories can be done by

rmdir

command. Please get familiar

with its options.

3. In order to change a file editing date you can use

touch

command.

4. As an exercise:

a. create a directory tree as shown on the figure on the right. How to do it quickly?
b. Create some files in different directories created in the previous point. What are the sizes and time

of the files?

c. Remove all the files and directories. How to do it quickly?

Task 3 Copying, moving, removing and linking files

1. The commands making possible copying, moving, removing and linking files are

cp

,

mv

,

rm

and

ln

respectively. Get familiar with their descriptions and options.

2. As an exercise use all those commands, note the difference between soft and hard linking (see above

example).

An example:
Note a difference in execution of the below commands.

touch zebra.txt
pico zebra.txt

(edit the zebra.txt file to make it not empty)

ln zebra.txt dow_tward
more zebra.txt
more dow_tward
rm zebra.txt

touch zebra.txt
pico zebra.txt

(edit the zebra.txt file to make it not empty)

ln –s zebra.txt dow_miek
more zebra.txt
more dow_miek
rm zebra.txt

more dow_tward

more dow_miek

Task 4 A file permission usage and change

1. By using the

s -l

command we can see a file permission described by characters

d, r, w, x, -

that are

expressed in 10 characters long chain. The interpretation of the characters is given in the below example.

2. A file permission can be changed using

chmod

command. Note that a file permission can be specified by

numbers or letters.

3. As an exercise just play with permission values

a. Execute

chmod 1 [plik], chmod 2 [plik], chmod 3[plik], chmod 4[plik], chmod 5

[plik] chmod 6 [plik], chmod 7 [plik], chmod 17 [plik]

etc. and see what is the

difference.

b. Remove all permissions from the file – can you remove it now?

An example:
After executing

chmod 347 zebra

the file zebra will have permissions:

--wxr--rwx

.

The first character means that it is a file (what will we see if it is a directory?)
The next 3 characters denote the owner permissions i.e. the owner cannot read the file, but he/she can write (

w

) and

execute the file (

x

).

The next 3 characters denote the group permissions i.e. the group members can read (

r

), but they cannot write and

execute zebra file.
The last 3 characters denote other users permissions i.e. they have full permission to zebra file since they can read, write
and execute it.
The numbers in

chmod 347 zebra

command code the file permission. The first number changes the owner permission,

the second the group permission and the last one the other users permission. The value is a sum of possible changes where
1 denotes executing permission, 2 denotes write permission and 4 is a read permission. Therefore, 3 = 1+2 is makes it
possible to execute and write a file.

Task 5 Changing a file owner and group

1. Sometimes an owner of a file has to be changed. To do so you can use

chown

command. If the group has to be

changed you should use

chgrp

command.

2. Can you change a file’s owner or group? Try the same operation after using

su

command.

3. Can you remove a file if you are not an owner? Can the answer vary for different files?

4. Can you change a file’s owner using a reference file?

Additional exercises

1. What to do to make a file size expressed in kilobytes?

2. What are the differences between soft and hard links? Do you know any possible usage of such links?
3. What are permissions of “mama” file after executing command

chmod 427 mama

?

4. Can you change a file’s group using

chmod

command ?

5. What set of permissions should be set to web files? Is it the same for all files?