Master Linux File Permissions: UGO, ACL, and chmod/chown Commands Explained

Basic UGO Permissions

In Linux, permissions are assigned to three categories of users: U (owner), G (group), and O (others). Each category can have three basic rights: r (read, value 4), w (write, value 2), and x (execute, value 1).

Permission symbols

r--

: read‑only

-w-

: write‑only

--x

: execute‑only

rw-

: read and write

r-x

: read and execute

-wx

: write and execute

rwx

: read, write, and execute

---

: no permissions

File vs. directory semantics

For files,

r

allows reading the content,

w

permits modifying the content, and

x

is generally irrelevant except for executable binaries. For directories,

r

lets you list entries,

w

lets you create or delete entries, and

x

allows you to enter the directory and access its contents.

Changing permissions with chmod

<code>chmod [options] &lt;mode&gt; &lt;file...&gt;</code>

Examples:

<code>chmod ugo+r a.conf
chmod u+rwx c.sh
chmod a+rw b.xml
chmod -R a+rw *
chmod 777 file   # equivalent to u=rwx,g=rwx,o=rwx
chmod 600 file   # owner read/write, others none</code>

Changing ownership with chown

<code>chown [options] user[:group] file...</code>

Example (requires root):

<code>chown tom:users d.key e.scrt
chown -R James:users *</code>

Changing group with chgrp

<code>chown user1 f1   # change group ownership</code>

Access Control Lists (ACL)

ACL extends the traditional UGO model, allowing fine‑grained permissions for individual users or groups.

Basic ACL commands

View ACL:

<code>getfacl /home/test.txt</code>

Set ACL entry:

<code>setfacl -m u:alice:rw /home/test.txt</code>

Mask permission

The mask defines the maximum effective permissions for ACL entries. It can be set with

-m

:

<code>setfacl -m m::rw- file</code>

Default permission

Default ACL entries are inherited by newly created files or sub‑directories within a directory. They are set with the

-d

option:

<code>setfacl -d d::rw- directory</code>

Important notes

Both mask and default entries use the

::

syntax.

Permissions can be expressed numerically or symbolically.

Values must be within the range

-

to

rwx

.

Practical uses of file permissions

Control user access to files.

Prevent execution of malicious programs.

Protect the integrity and confidentiality of data.