File Permissions & Ownership

Linux File Permissions

Every file in Linux has permissions, these define which actions can be undertaken by the user, group, and other.

As seen on the file below, permissions are set at the start of the line using 10 characters.

-r--r-xrw- 1 root root       27 May 26 10:56 test.txt

These 10 characters are the permission classes, and are used as follows:

(For file type: - is a file, d is a directory).

Permissions can also be represented in number format.

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chmod

The chmod command is used for changing file or directory permissions.

chmod [options] {mode} filename

chmod Modes

The chmod command supports 2 'modes'. These are methods in which the command can be used to implement permission alterations.

Symbolic Mode

Symbolic mode allows for changes to be made using 3 components;

Permission Contexts	Permission Operators	Permission Attributes
u/g/o/a (User/Group/Other/All)	+/-/= (Add/Remove/Exacy)	r/w/x (Read/Write/Execute)

To add write permission for the user;

chmod u+w test.txt

To exactly set permissions for u/g/o;

chmod u=rwx,g=rwx,o=r filename

Absolute Mode

Absolute mode allows for changes to be made using the octal numbering system, mentioned above.

chmod 641 filename

Flags

-R	Recursive.
-c	Report changes via CLI.
-f	Hide errors.
-v	Display diagnostic info.

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Linux File Ownership

Files in Linux are owned by a user and group.

-rw-r--r-- 1 root root       27 May 26 10:56 test.txt

Changing user/group

chown newuser:newgroup

For instances where you want to chown a directory, and all of the subdirectories & files within, we can use the -R (recursive) flag:

chown -R newuser:newgroup directoryname 

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FACL - File Access Control List

File Access Control Lists (FACLs) provide a robust mechanism for managing file permissions in Linux, offering greater flexibility and control than traditional Unix permissions. By using commands like setfacl and getfacl, administrators can easily set and view ACLs to fine-tune access to files and directories for multiple users and groups.

View file/directory ACL

getfacl filename

Grant an additional user permissions on a file

setfacl -m u:username:rwx filename

Remove a user's permissions on a file

setfacl -x u:username filename

Define default ownership/permissions for directories

setfacl -m d:u:username:rwx filename 

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Sticky bits

In Linux, the sticky bit is a special permission that can be set on directories to control user access to the files within those directories. When the sticky bit is set on a directory, it restricts the deletion or renaming of files within that directory. Specifically, only the file's owner, the directory's owner, or the root user can delete or rename files.

Enable sticky bits 

chmod o+t directoryname

Disable sticky bits

chmod o-t directoryname

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