chmod (File Permissions)

chmod (change mode) is a Unix and Linux command that sets file permissions. Each file has read, write, and execute permissions for three classes: the owner, the group, and others.

Why it matters

Correct permissions protect secrets and keep web servers working. A private key readable by everyone is a risk, while a script without execute permission will not run. Permissions can be written as letters or as a number, where read is 4, write is 2, and execute is 1, added together.

Web files typically need to be readable by the web server user, not writable.

Example

$ chmod 640 config.php
$ ls -l config.php
-rw-r----- 1 dev www-data 812 config.php

640 gives the owner read and write (6), the group read (4), and others nothing (0).

Common mistakes

• Using chmod 777 to fix an error, which lets any user modify the file.
• Setting execute on every file instead of only scripts and directories.
• Fixing permissions when the real problem is the file owner; check chown.

Related

• Root User (Administrator)
• File System
• Bash
• Linux

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