File Permissions and Atrributes

Working as part of a team in Linux environment require us to know how to share your files/directories with others.

Use ls -l (long format) to list files and their attributes.

-rw-r--r-- 1 user group 1002287 Oct 20 15:49 example.csv

Explanation of output:

• -rw-r--r--: Permissions of file

• 1: hard link count (out of scope)

• user: name of file owner

• group: name of group owner

• 1002287: file size in bytes

• Oct 20 15:49: last time file was modified

• example.csv: name of file

Permissions are grouped into 3 categories:

• user

• group

• other

Each category has 3 permissions:

• r (read)

• w (write)

• x (execute)

• A dash means no permission.

The command chmod (change mode) is used to edit file permissions. There are 2 ways to represent the permissions: numeric and symbolic.

Numeric mode

The rwx permissions are represented by octal numbers (0-7). The following table explains each value (credit: Wikipedia):

#	Sum	rwx	Permission
7	4(r) + 2(w) + 1(x)	rwx	read, write and execute
6	4(r) + 2(w)	rw-	read and write
5	4(r) + 1(x)	r-x	read and execute
4	4(r)	r–	read only
3	2(w) + 1(x)	-wx	write and execute
2	2(w)	-w-	write only
1	1(x)	–x	execute only
0	0	---	none

Common file permissions:

• 644 - owner can read and write file, all others can read

• 755 - owner can read, write, and execute, all others can read and execute

• 600 - owner can read and write, no access to anyone else

Symbolic mode

Use symbolic notation for finer grained control of permissions. Symbolic notation requires a reference, operator, and mode. From Wikipedia:

Reference	Class	Description
u	user	file owner
g	group	member’s of the files’s group
o	others	users who are neither the file’s owner nor members of the file’s group
a	all	all three of the above, same as ugo
(empty)	default	same as “all”, except that bits in the umask will be unchanged

Operator	Description
+	adds the specified modes to the specified classes
-	removes the specified modes from the specified classes
=	the modes specified are to be made the exact modes for the specified classes

Mode	Name	Description
r	read	read a file or list a directory’s contents
w	write	write to a file or directory
x	execute	execute a file or recurse a directory tree

Common commands:

Make file executable by all:

chmod +x file