Master Linux File Permissions: chown, chgrp, and chmod Explained

Introduction

Linux is a multi‑user, multitasking operating system that defines three identities for each file or directory—owner, group, and others—each with three possible permissions: readable, writable, and executable.

File Attributes

Running ls -al --full-time (or its alias ll) displays seven columns for each entry:

File type and permission bits (e.g., -r-xr-x--- means a regular file where the owner can read and execute, the group can read and write, and others have no permissions).

Link count.

Owner name.

Group name.

File size in bytes.

Last modification timestamp (not creation time).

File name (names starting with a dot are hidden).

Changing Owner (chown)

Location

etc/passwd

Syntax

chown [-R] [owner] [file|directory]

Use -R for recursive changes. You can also change the group at the same time with chown [-R] owner:group file, though chgrp is preferred for group changes.

Examples

chown daemon test          # change owner of "test" to daemon

chown daemon:root test     # change owner to daemon and group to root

chown root.users test     # change owner to root and group to users

chown .root test           # change only the group to root

Note: The specified owner must exist in /etc/passwd .

Changing Group (chgrp)

Location

etc/group

Syntax

chgrp [-options] group file|directory

Use man chgrp or chgrp --help for detailed options.

Example

chgrp -R users test   # recursively change group of "test" and its contents to users

Invalid group names will cause an error.

Changing Permissions (chmod)

Linux permissions consist of read (r), write (w), and execute (x) for each of the three identities, yielding nine possible permission bits.

Symbolic Method

Use u, g, o (or a for all) to select identities, r, w, x for permissions, and +, -, = to add, remove, or set permissions.

Set Permissions (=)

chmod u=rwx,g=rwx,o=rwx test   # give everyone read, write, execute

chmod ugo=rwx test

chmod a=rwx test

Remove Permissions (-)

chmod u-x,g-x,o-x test   # remove execute permission from everyone

chmod ugo-x test

chmod a-x test

Removing execute (x) on a directory prevents users from entering it with cd .

Add Permissions (+)

chmod u+x,g+x,o+x test   # add execute permission for everyone

chmod ugo+x test

chmod a+x test

Typical use: chmod a+x test.sh makes a shell script executable.

Numeric Method

Permissions are represented by numbers: read=4, write=2, execute=1. Sum them for each identity.

chmod 777 test   # rwx for owner, group, others

chmod 666 test   # rw- for all

chmod 755 test   # rwx for owner, r-x for group and others

The numeric method is often simpler.

File vs. Directory Permission Differences

Files

readable : can read file contents.

writable : can modify file contents.

executable : can be executed as a program.

Write permission on a file does not allow deletion; deletion is controlled by the directory's permissions.

Directories

readable : can list the directory contents (e.g., ls).

writable : can create, rename, move, or delete entries within the directory.

executable : can enter the directory (e.g., cd).

To allow browsing, a directory needs at least r or x . To read a file inside, the directory must have x and the file must have r .

Conclusion

Linux files can have separate rwx permissions for owner, group, and others. Use chown to change owners, chgrp to change groups, and chmod to modify permissions, thereby protecting data security.