File Permission in Linux/Unix

chmod Command  in Linux/Unix
                                          Assigning file permission is another security feature provided by Linux/Unix.

Why File permission??
File permission helps you to restrict the access of  you files from others (Group member and other user) in Multi-user environment. By using permissions you can specify who can interact with your files and how (read,write or execute).



Linux/Unix has three user classes

1. User (u) : The owner of the file.

2. Group (g) : The group that the owner belongs to. This can be useful when you are involve in a project , and group member require to access file which other can't.

3. Others (o) : Everyone else.



Access Permission In Linux/Unix

 1. Read (r) 
 Files : If r bit is set for file then file is Readable.
 Directory : If r bit is set for directory then you can list the content of directory .                    

2. Write (w) 
Files : If w bit is set you can modify that file i,e write data to the file , delete data from file.
Directory : If w bit is set for directory then you are permitted to create or remove files in it. if w bit is not set , you can't create , copy or delete a files in it.

3. Execute (x) 
Files : If x bit is set you can Execute that file i,e Executing any shell script file or c/c++file.
Directory : For directory it is also refer as search bit , It allows to search for a sub- directory and files in a directory if search bit is set. If directory doesn't have execute permission , then it can't searched for the name of the next directory.



File Permission
You can use ls -l command to list content of current directory with their access permissions.

For file abc.lst
1. It belongs to user "sandeep"
2. It belongs to a group "sandeep"

Lets closely interpret the meaning of first field        

                  
chmod
          chmod command is used to change the permission of files and directory.

Syntax
         chmod mode file/directory

Permission can be changed in two ways
1. Symbolic mode
2. Numeric mode

Symbolic mode

category         operation                             Permission
u-user            + assign permission                 r  read
g-group          -  Remove permission            w write
o-others         =  Absolute permission           x  execute
a-all(ugo)

Example
Suppose we have file abc.lst and it has permission -rw-r--r--.

1. Give read permission to all users and remove all other permission.

2. Give write and execute  permission to user and execute permission to group members.

3. Remove execute permission from both user and group.

Numeric Mode

Pemission in Numeric mode is represented by

1. Read permission - 4 ( 100)

2. Write Permission - 2 ( 010)

3. Execute permission - 1 ( 001)

4. No permission - 0 ( 000)

If command is like this  chmod  754  abc.lst , then this means

1. 7 - 111 -  rwx  -      All permission for user
2. 5 - 101 -  r-x    -     Readable and executable by group.
3. 4 - 100 -  r--     -    Only Readable by others

Different combination of Permission

Binary             octal            permission                 Significance

000                    0                  ---                              No permission
001                    1                  --x                             Executable Only
010                    2                  -w-                            Writable only
011                    3                  -wx                           Writable and Executable
100                    4                   r--                             Readable only
101                    5                   r-x                            Readable and Executable
110                    6                   rw-                           Readable and writable
111                    7                   rwx                           Readable,Writable and Executable

*They are also Known as absolute permission. In this you don't have to remember what a file's current permission , you have to set all permission explicitly.

Example


Suppose we have file abc.lst and it has permission -rw-r--r-- i,e 644 in octal.

1. Give read permission to all users and remove all other permission.

2. Give write and execute permission to user and execute permission to group members and remove all other permission.

3. Set all Permission for all users.

Using chmod Recursively
chmod can be used to apply same permission for directory and all other sub-directory with in it.This can be done using -R option

$ chmod -R 755 dir_name

* This will apply 755 permission to all files and sub-directory with in dir_name directory.

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