Users, Groups & Permissions
Control who can read, write, and execute files using Linux's user/group/other model and chmod.
Every Linux file has an owner and a group, and three sets of permissions: for the owner (user), the group, and everyone else (other). Each set grants read (r), write (w), and execute (x).
When you run ls -l you see something like -rwxr-xr--. The first character is
the file type (- file, d directory, l symlink). The next nine are the three
permission triples. Each permission has a numeric value: r=4, w=2, x=1. Add
them per triple to get the familiar three-digit modes like 755 or 644.
chmodchanges permissionschownchanges the owner/group- On directories,
xmeans “can enter/traverse”, not “execute”
ls -l script.sh # -rw-r--r-- 1 alex devs 0 Jun 22 script.sh
chmod +x script.sh # add execute for everyone
chmod 750 script.sh # owner rwx, group r-x, others nothing
chmod u+w,o-r notes.txt # symbolic: add owner write, remove other read
chown alex:devs file # set owner to alex, group to devs
sudo chmod -R 755 /var/www # recurse into a directory tree- Create a file and view its default permissions with
ls -l. - Make it executable for the owner only using symbolic mode.
- Set a file to
640and explain who can read it. - Use
chown(with sudo) to change a file’s group, then verify withls -l.
Cheat Sheet▾
| Mode | Meaning |
|---|---|
r = 4, w = 2, x = 1 | Permission values |
chmod 644 f | Owner rw, others read |
chmod 755 d | Common for dirs/scripts |
chmod +x f | Make executable |
chmod -R | Apply recursively |
chown user:group f | Change ownership |
umask | Default-permission mask |
| SUID / SGID / sticky | Special bits (4/2/1 prefix) |
Common Interview Questions▾
What does chmod 755 mean?
Owner gets read+write+execute (7), group and others get read+execute (5). Common for directories and executable scripts.
What does the execute bit do on a directory?
It allows traversing into the directory (accessing files inside by name). Without it, you cannot cd into or access paths under the directory even if you can list it.
What is the sticky bit used for?
On a shared directory like /tmp, it restricts file deletion so only the file’s owner (or root) can remove it, even though everyone can write there.