File Ownership in Linux: How read, write and execute works for files and directories

published on Sun Mar 29 2020

As we’ve pointed out before,

Everything in Linux is a file.

Users

These files have permissions and users associated with them. There are three kinds of users in the Linux security model:

user - these are individual users who can own files and directories and has control over its access
group - individual users can belong to a group consisting of one or more users who are given access to files and directories by their owners
others - apart from a group, an owner may also grant some set of access rights to everybody, referred to as the world

Permissions

In Linux, access rights to files and directories are defined in terms of read access, write access and execution access.

The three kinds are:

Permission	File	Directory
`r`	Allows a file to be opened and read	Allows a directory’s contents to be listed if the execute attribute is also set
`w`	Allows a file to be written to or truncated; however, this attribute does not allow files to be renamed or deleted. The ability to delete or rename files is determined by directory attributes	Allows files within a directory to be created, deleted and renamed if the execute attribute is also set
`x`	Allows a file to be treated as a program and executed. Program files written in scripting languages must also be set as readable to executed	Allows a directory to be entered using `cd`

`ls` Output

If we look at an example output of the ls command we see,

-rwxrwxr-- 1 saikat saikat    0 Mar 29 19:20 dummy.sh

The first 10 characters of the listing are the file attributes. The first character is the file type. Common file types are:

- - a regular file
d - a directory
l - a symbolic link

Remaining characters of the file attributes are called the file mode. They represent the read, write and execute permissions for the file’s owner, the group owner and everybody else. In this case, they are:

rwx - file owner has read, write and execute permission
rwx - file group owner has read, write and execute permission
r-- - others only have read permission

Changing permissions

We use chmod to can change permissions of a particular file or directory in Linux. There are two ways of specifying mode changes:

octal number representation
symbolic representation

I use the symbolic method because it’s more visual and only changes the permissions that you wish to change and not the entire mode. You also don’t need to perform complicated binary to octal conversion to figure out the mode to pass.

Symbolic notation has 3 parts:

who the change will affect. Possible values are:
- u - short for user. Refers to the file or directory owner
- g - short for group owner
- o - short for others
- a - short for all
which operation will be performed. Possible values are:
- + - indicates that a permission needs to be added
- - - indicates that a permission needs to be taken away
- = - indicates that the specified permissions need to be set and all others are to be removed
what permission will be set. Possible values are:
- r - read permission
- w - write permission
- x - execute permission

Example:

chmod u=rwx,g=rx,o=r myfile

`chmod` modes examples

u+x - add execute permission for the owner
u-x - remove execute permission from the owner
a+x - add execute permission for the owner, group and others
o-rw - remove the read and write permissions from anyone besides the owner and group owner
go=rw - set the group owner and anyone besides the owner to have read and write permissions. If either had execute permission, that is removed
u+x,go=rx - add execute permission for the owner and set the permissions for the group and others to read and execute

Changing Ownership

Ownership of files and directories are changed with the chown command. Syntax is:

chown [owner][:[group]] file...

chown can change the file owner and/or the file group owner depending on the first argument of the command.

`chown` argument examples

saikat - changes the ownership of the file from its current owner to user saikat
saikat:users - changes the ownership of the file from its current owner to user saikat and group owner to group users
:admins - changes the group owner to the group admins. The file owner is unchanged
saikat: - changes the file owner from the current owner to user saikat and changes the group owner to the login group of user saikat