filepermissions.md

File Permissions:-

Overview

Unix-like operating systems, such as Linux, running on shared high-performance computers use settings called permissions to determine who can access and modify the files and directories stored in their file systems. Each file and directory in a file system is assigned "owner" and "group" attributes.

Most commonly, by default, the user who creates a file or directory is set as owner of that file or directory. When needed (for example, when a member of your research team leaves), the system's root administrator can change the user attribute for files and directories.

The group designation can be used to grant teammates and/or collaborators shared access to an owner's files and directories, and provides a convenient way to grant access to multiple users.

• Types of Files:

Symbol	Type of File
-	Normal file
d	Directory
l	Link file (shortcut)
b	Block file (Harddisk, Floppy disk)
c	Character file (Keyboard, Mouse

• Permissions are applied on three levels:-

  • Owner or User level
  • Group level
  • Others level

• Access modes are of three types:-

  • r read only
  • w write/edit/delete/append
  • x execute/run a command

• Access modes are different on file and directory:-

Permissions	Files	Directory
r	Open the file	'ls' the contents of dir
w	Write, edit, append, delete file	Add/Del/Rename contents of dir
x	To run a command/shell script	To enter into dir using 'cd'

To list of files and directories

$ ls -ld samplefile
-rw-rw-rw- 1 krishnaprasadkv krishnaprasadkv 0 May 27 14:45 samplefile

$ ls -ld sampledir/
drwxrwxrwx             1   krishnaprasadkv krishnaprasadkv         4096           May 27 14:45          sampledir/

Filetype+permission, links, owner,           group name of owner,  size in bytes, date of modification, file name

• Changing file/directory permissions with 'chmod' command

  • We can use the 'chmod' command which stands for 'change mode'. Using the command, we can set permissions (read, write, execute) on a file/directory for the owner, group and the world.
    Syntax: chmod permissions filename

• There are 2 ways to use the command -

  • Absolute mode
  • Symbolic mode

Absolute mode

• In this mode, file permissions are not represented as characters but a three-digit octal number.

• The table below gives numbers for all for permissions types.

Number	Permission Type	Symbol
0	No Permission	---
1	Execute	--x
2	Write	-w-
3	Execute + Write	-wx
4	Read	r--
5	Read + Execute	r-x
6	Read +Write	rw-
7	Read + Write +Execute	rwx

Lets see some examples by using absolute mode

• Change the permissions of the file 'samplefile to '764'.
  $ chmod 764 samplefile

$ ls -l samplefile
-rw-rw-rw- 1 krishnaprasadkv krishnaprasadkv    0 May 27 14:45 samplefile
$ chmod 764 samplefile
$ ls -l samplefile
-rwxrw-r-- 1 krishnaprasadkv krishnaprasadkv 0 May 27 14:45 samplefile

• '764' absolute code says the following:

  • Owner can read, write and execute (7 means rwx i.e. 4+2+1)
  • Usergroup can read and write (6 means rw i.e 4+2 )
  • others can only read (4 means r)

• Assigning full permission to the file i.e. rwx to all
  $ chmod 777 samplefile

$ ls -l samplefile
-rwxrw-r-- 1 krishnaprasadkv krishnaprasadkv 0 May 27 14:45 samplefile
$ chmod 777 samplefile
$ ls -l samplefile
-rwxrwxrwx 1 krishnaprasadkv krishnaprasadkv 0 May 27 14:45 samplefile

This is how you can change the permissions on file by assigning an absolute number.

Symbolic Mode

• In the Absolute mode, you change permissions for all 3 owners. In the symbolic mode, you can modify permissions of a specific owner. It makes use of mathematical symbols to modify the file permissions.

Operator	Description
+	Adds a permission to a file or directory
-	Removes the permission
=	Sets the permission and overrides the permissions set earlier.

• The various owners are represented as -

User	Denotations
u	user/owner
g	group
o	other
a	all

Lets see some examples of symbolic mode

$ chmod u=rwx,g=rw,o=r samplefile (user=rwx, group=rw and others=r)

$ ls -l samplefile
-rw-r----x 1 krishnaprasadkv krishnaprasadkv 0 May 27 14:45 samplefile
$ chmod u=rwx,g+wx,o-x samplefile
$ ls -l samplefile
-rwxrwx--- 1 krishnaprasadkv krishnaprasadkv 0 May 27 14:45 samplefile

• Assigning full permission to the file i.e. rwx to all.
  $ chmod ugo=rwx <file name>

$ ls -l samplefile
-rwxrwx--- 1 krishnaprasadkv krishnaprasadkv 0 May 27 14:45 samplefile
$ chmod ugo=rwx samplefile
$ ls -l samplefile
-rwxrwxrwx 1 krishnaprasadkv krishnaprasadkv 0 May 27 14:45 samplefile

• Adding execute permission to user only.
  $ chmod u+x samplefile
• Removing write and execute permissions from group and other.
  $ chmod go-wx samplefile
• Adding write and execute permissions from group and other.
  $ chmod go+wx samplefile
• Giving only read permission to group and other.
  $ chmod go=r samplefile