File Permissions

On Linux and macOS, every file has an owner, a group, and permissions that decide who can read, write, or run it.

Reading permissions

# illustrative
ls -l

-rw-r--r-- 1 ada staff  1024 Apr 22 09:30 notes.txt
-rwxr-xr-x 1 ada staff   512 Apr 22 09:31 deploy.sh
drwxr-xr-x 2 ada staff  4096 Apr 22 09:32 reports

The first column: the first character is the type (- file, d directory), then three groups of three:

-   rwx   r-x   r-x
│   │     │     └── others (everyone else)
│   │     └──────── group
│   └────────────── owner (user)
└────────────────── type

r = read, w = write, x = execute (for a directory, x means you can enter it). - means the permission is missing.

Changing permissions: chmod

# illustrative
chmod +x deploy.sh       # add execute for everyone
chmod u+x,go-w notes.txt # user +execute; group and others -write
chmod 644 notes.txt      # rw-r--r--
chmod 755 deploy.sh      # rwxr-xr-x
chmod 600 secrets.env    # rw------- : only you can read it

The numeric form adds up r=4, w=2, x=1 for owner, group, and others: 7 = rwx, 6 = rw-, 5 = r-x, 4 = r--.

Owners and sudo

chown user:group file changes the owner. System files belong to the root user; sudo command runs a single command as root, after asking for your password.

Common mistakes

• chmod 777 to "fix" a permission error — it lets anyone change or run the file. Give only the permissions actually needed.
• Running everything with sudo. Use it only for commands that genuinely need administrator rights, and read the command carefully first.