8. The Sticky Bit

Beyond the standard read, write, and execute permissions, Linux offers special permissions for advanced access control. The last of these special permissions we will cover is the sticky bit.

What is the Sticky Bit?

The sticky bit is a permission setting that can be applied to a directory. When a directory has the sticky bit set, files within that directory can only be deleted or renamed by the file's owner, the directory's owner, or the root user. This is particularly useful for shared directories where multiple users need to create and manage their own files without interfering with others. This concept is a key part of Unix file permissions sticky bit management.

A Practical Example: The /tmp Directory

A common use case for the sticky bit in Linux is the /tmp directory, which is a world-writable location for temporary files. Let's examine its permissions:

$ ls -ld /tmp
drwxrwxrwt 17 root root 4096 Dec 15 11:45 /tmp

Notice the t at the end of the permission string (rwxrwxrwt). This t indicates that the sticky bit is set. Because of this, while any user can create files in /tmp, they cannot delete or move files created by other users. This prevents one user from disrupting another's work in this shared space.

How to Set the Sticky Bit

You can set the sticky bit using the chmod command in two ways: symbolic mode or octal (numeric) mode.

To add the sticky bit using symbolic mode:

chmod +t my_shared_dir

To set permissions using octal mode, you prepend a 1 to the standard three-digit permission code. The numerical representation for the sticky bit is 1.

# This sets permissions to rwxr-xr-x with the sticky bit
chmod 1755 my_shared_dir

Understanding the sticky bit is essential for managing multi-user environments and securing shared directories effectively.