File Permissions and Atrributes

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.

chmod

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

Note

When using numeric mode, all permissions are updated (i.e. you can’t just update user permisisons and not change group or other)

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