File Permissions and Ownership with chmod and chown

Now that you can create users and groups, the next step is controlling what each user can do with files. Linux enforces this through file permissions and ownership.

ls -l shows a permission string like -rwxr-xr--. The first char is the file type; the next three triples are permissions for owner, group, and others.

• r (read) = 4
• w (write) = 2
• x (execute) = 1

Add them per triple to get an octal digit. rwx = 7, r-x = 5, r-- = 4.

chmod 754 file sets owner rwx, group r-x, others r--.

You can also add or remove specific bits: u=user, g=group, o=others, a=all.

A very common task: give a newly written script execute permission so you can run it directly.

chown user:group file reassigns who owns a file. This ties directly into the user/group management you already learned.

You can change just the group with chgrp or with the :group form of chown.

The -R flag applies changes to a directory and everything inside it. Use carefully.

New files get default permissions masked by umask. A umask of 022 removes write for group and others, producing 644 files and 755 directories.

When sharing a project directory with a team: set the group, grant group read/write, and keep others restricted.

What octal value represents rwxr-xr--?

You now control access to files:

• Permissions split into owner/group/others with r(4)/w(2)/x(1)
• chmod sets modes via octal or symbols
• chown/chgrp reassign ownership
• -R applies recursively; umask sets defaults