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Why do we use file permission in Linux/Unix?
Linux is a clone of UNIX operating system, it is a multi-user operating system, which can be accessed by many users at a time.
Because of this , it raises the question on its security, that it can be easily corrupted or any one can change or remove the crucial data.
So, taking care of the security.
Linux divides authorisation level into 2 levels
1…Ownership
2…Permissions
Ownership
In linux the system is allocated 3 types of owner
→ First one is “User”
Owner of the file.The person who creates the file is called the owner.
→ Second one is “Group”
In a group there can be multiple users we can put multiple users in a group and assign the same file permission.
→ Third one is “other”
Other can be anybody except user & group
Permissions
For every user, group & other there their is set permission
- Read
- Write
- Execute
User → rwx
Group → rwx (-) → file -rw-rwxrw-
Other → rwx (d) → directory drw-rwxrwxrw-
Read – Gives the permission to read the file,
Write – Gives the permission to modify the file it gives the authority to add ,remove ,rename the file which is stored in the directory.
Execute – we cannot open the file if the execute permission is not given to the file.
Example:- 
| (d)/(-) | Show directory or file if directory (d) sign shows in starting If file (-) hyphen sign show in starting |
| first (rwx) | Permission of user (read,write,execute) |
| Second (r-x) | Permission of group (read,-,execute) |
| Third (r-x) | Permission of other (read,-,execute) |
| r | r refer to read permission |
| w | w refer to write permission |
| X | x refer to execute permission |
Give permission to file and directory has two way
1. Symbolic method
2. Octal method / numerical method
(chmod) command use to change permission
Default permission of file :-
Symbolic method
Symbolic method is used alphabets to set permission.
| u | u refer to user |
| g | g refer to group |
| o | o refer to other |
| (+) | Use to add permission |
| (-) | Use to remove permission |
| (=) | Use to overwrite permission |
Examples:-
Create “mydir” directory
1. add write permissions to group on mydir (# chmod g+w mydir)
2. Give only execute permission to other (# chmod o=x mydir)
3. Remove write permissions to user (# chmod u-w mydir)
Octal or numeric method
Octal method use numbers to set permission
| 0 | (—) | No permission |
| 1 | (–x) | Execute permission |
| 2 | (-w-) | Write permission |
| 3 | (-wx) | Write and execute permission |
| 4 | (r–) | Read-only permission |
| 5 | (r-x) | Read & execute permission |
| 6 | (rw-) | Read & write permission |
| 7 | (rwx) | All permission |
example:- use octal method to give permission
chmod user|group|other file/directory path
1. # chmod 755 mydir (this command give permission to mydir user have all permissions, group & other have only read and execute permission.)
Special permission
1. Setuid (set user id)
2. Setgid (set group id)
3. Sticky bit
1. setuid :- setuid is the permission bit that allows the user to execute a command/ program with the permission of its owner. Setuid permission are used to tell the system to run an executable file as the owner with the owner’s permission.
Symbolic method to setuid —># chmod u+s file_name (This command use to setuid bit on file)
Octal method to setuid —># chmod 4766 file_name (This command use to setuid bit using octal method)
Remove setuid bit —># chmod u-s file_name (This command use to remove the setuid bit form file)
2. Setgid :- setgid is a bit that allows the user to execute a program with the permission of the group owner.
Symbolic method to setgid —># chmod g+s file_name (This command use to setgid bit on file)
Octal method to setgid —># chmod 2766 file_name (This command use to setgid bit using octal method)
Remove setgid bit —> # chmod g-s file_name (This command use to remove the setuid bit form file)
3. Sticky bits :- A sticky bit is a permission bit that is set on a directory that allows only the owner of the file within that directory or the root user to delete or rename the file. No other user has the needed privileges to delete the file created by some other user.
Symbolic method to set sticky bit —># chmod +t file_name (This command use to set the sticky bit on file and directory)
Octal method to set sticky bit —># chmod 7660 file_name (This command use to set stickybit using octal method)
Remove sticky bit —> # chmod -t file_name (This command use to remove the sticky bit)
Conclusion:
By following this tutorial you will learn what are the permissions as well as special permission in Linux/Unix.