Introduction to Data Communications
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51b. Access and Permissions

User/Group/Other

With respect to file and directory access, the user community is divided into 3 categories: user (or owner), group and other.

	user (u)	The owner of the files or directories



	group (g)	Group members. Groups are users who agree to share certain files and

			directories. Groups are usually formed along project or business

			organizational lines.



	other (o)	All other users of the system.

Each file has a set of values stored in its inode that specifies its permissions. An inode is an entry in the table of inodes that describes the file or directory. The Table of Inodes is Unix's version of DOS's File Allocation Table.

Read/Write/Execute

The permissions indicate, for each category of user, the kind of access allowed. Permission is also called the file's protection mode or simple mode.

Type		File Action					Directory Access



read (r)	Allows file to be viewed		Allows directory to be listed

		copied and printed



write (w)	Allows file to be moved,		Allows files to be created in

		removed and modified			directory



execute (x)	Allows file to be run as a		Allows directory to be searched

		command

Unix displays a file's permissions in the following order:

	rwx	where "r" is read, "w" is write and "x" is execute

It is also expressed in Octal Code:

	Mode	Octal	Binary	Descriptions

	---	0	000	No permissions allowed

	--x	1	001	Execute only

	-w-	2	010	Write only

	-wx	3	011	Write and execute only

	r--	4	100	Read only

	r-x	5	101	Read and execute only

	rw-	6	110	Read and write only

	rwx	7	111	Read, Write and Execute

It is important to know the octal code or how to figure out the octal code if you need to change permissions.

When listing a directory using the "ls -l" long directory listing, the files permissions will appear:

		ls -l



		total 8

		drwxr-xr-x		2	rocky		other	96 	Dec 26 	23:16	.

		drwxrwxr-x	7	root		sys	96	Dec 24	07:40 	..

		-rw-r--r--		1	rocky		other	613	Nov 2	12:30		readme.txt

		drwxr-xr-x		2	rocky		other	234	Feb 28	03:40		x-files

The long directory lists all of the rights associated with the file or directory. The mode bits are organized as follows:

The default permissions when you create a file are 777 which is 111 111 111 in binary or (rwxrwxrwx). When a directory is created, the default permissions are 666 which is 110 110 110 or (rw-rw-rw-).

Changing Permissions

When a file is first created, it is created with the default permissions 777 (rwxrwxrwx). This means that anyone can read, write or execute the new file. Unix provides a command to modify the default permissions: umask. "umask" works by deselecting the permissions that you do not want from the default permissions. "umask" by itself reports what the current mask is.

Ex	umask		will report current mask



	000		000 indicates no mask and default permissions exist



Ex	umask 027	will set the mask to 0278 or 000 010 1112



					000  010  111

	777 (default permission)	rwx  rwx  rwx

	027 corresponds to 		---  -w-  rwx	permissions deselected



	Resulting permissions:		rwx  r-x  ---



					user has rwx	(all permissions) 000

					group has r-x	(read and execute only) 010

					other has ---	(no permissions) 111

The umask command is used during your shell startup script or login script. You set it once during logging into the system and normally won't have to use it again.

chmod

If you need to change a resource (file or program rights) , use chmod to alter the permissions. chmod is the more common method of changing permissions. You can alter the permissions 2 ways: use the read/write/execute switches or use octal coding.

Ex.	chmod +r index.txt	changes the permission for user, group &

				other to read.



	chmod u +r index.txt	changes the permission for only the user



	chmod 755 index.txt	changes the permission to rwxr-xr-x

Notice that chmod works the opposite of unmask. You set the permissions that you want.

Changing Ownership & Group

You can change the owner of a file by using the chown (change owner) command. You must be the current owner of the file to change its owner.

Ex. chown bullwinkle help.rocky.txt	This changes the owner of the file help.rocky.txt to

					"bullwinkle"

Similarly, you can change the group identification of the file by running chgrp (change group). You must be the owner of the files or a user who has group write privileges to change the group ownership of files.

Ex. chgrp brains mr.peabody.doc		This changes the group of the file

					mr.peabody.doc to the group "brains".

Note: You can remove a file that you don't own if it is in a directory in which you have write permission.

Introduction to Data Communications
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