Getting help,File and Directory Permissions In Linux

Getting Help:

The command that are used to get the help are discussed as :

1. Whatis

Display a short description of command , it uses a database that is updated nightly. Often not available immediately after installation.

Syntax:

# Whatis cal

2. Help

Display usage summary and argument list

Syntax:

<command> --help

Example:

#Date –help

3. Man and Info:

Both provide documentation for command. Almost every command has a “man” page. Collection of pages are called linux manual.

# man date

# info date

Viewing Text Page

Syntax:

#less [option] [filename]

Example:

# less abc.txt

scroll with arrows/PgUp /PgDown

/text    :           search for text

n          :           Next Match

Option:

-c         :           Clear before displaying

-s         :           Squeeze multiple blank lines into a single blank line

Simply we can also use “less” along with pipe | as

# ll |less

File and Directory:

‘ll’ is used to display the information about the files and directory including date, time, users,group, size, name and permission.

Four symbols are used when displaying permission.

R          :           Read

W        :           Write

X          :           Execute

-           :           no permission

-rwxrwxrwx :             files

drwxrwxrwx :            directory

files and directory permission are symbolized by ten character.

If we want to change permission, then there are two methods:

1. symbolic
2. Numeric

1. Symbolic Method:

Syntax:

Chmod mode directory/filename

Mode Option:

1. u,g,o
2. w,r,x
3. +,-
4. =

1. # chmod u+rwx file or directory : in case of user only
2. # chmod ug+rwx file or directoty : in case of user and group
3. # chmod u+w,g+r,o+x directory/file
4. # chmod u+rw,g+rw directory/file
5. # chmod u-r, g-w,o-rw directory/file
6. # chmod ugo+rwx file/directory
7. # chmod ugo-rwx file/directory

• + is used to add permission
• - is used remove permission

chmod ugo=rw directory/file

this command will assign read/write permission to u,g,o

suppose we have one file as

test.txt

permission : -r- - r- -r- -

chmod u=w,g=wx,o=w test.txt

this command will assign write to user, write/execute to group and write to other while remove the previous permission.

The main difference between +,= are + operator simply add the new permission with previous one and = assign the new permission while removing old (new permission overwrite an old)

2. Numeric Method:

In this method, calculation are based on following numbers

       r = 4        

      w = 2        

       x = 1        

      0 = no permission

Example:

#chmod 777 file/directory

in this case user get 7 means that user has permission of read/write/execute, group get 7 means read/write/execute and ame for other

# chmod 531 file/directory

in this case user get 5 means that user has permission of read/execute, group get 3 means write/execute and other get 1 means that other has permission to execute.

#chmod 742 file/directory

7 : User : rwx

4 :Group : r

2 : Other : w

Thanks ,