Why Can’t I Write to /tmp/ with Root Permissions and Sticky Bit Set?

In the world of Linux, permissions and ownership are crucial concepts that every administrator should understand. One common issue that many users encounter is the inability to write to the /tmp/ directory, even with root permissions and the sticky bit set. This article aims to explain why this happens and how to resolve it.

Understanding Linux Permissions

In Linux, every file and directory has an associated set of permissions and an owner. The permissions determine who can read, write, or execute the file or directory. There are three types of permissions:

• Read (r): Allows the file to be read.
• Write (w): Allows the file to be modified.
• Execute (x): Allows the file to be executed as a program.

These permissions can be set for three types of users:

• User (u): The owner of the file.
• Group (g): The users who are members of the file’s group.
• Other (o): All other users.

The Role of the Sticky Bit

The sticky bit is a permission bit that protects the files within a directory. If the directory has the sticky bit set, then a file can be deleted only by the owner of the file, the owner of the directory, or by root. This is why it is often set on the /tmp directory – to prevent users from deleting each other’s temporary files.

The Issue with /tmp/ and Root Permissions

Even with root permissions and the sticky bit set, you may find that you are unable to write to the /tmp/ directory. This is usually because the permissions on the /tmp/ directory are not set correctly.

In a typical setup, the /tmp/ directory should have the following permissions: drwxrwxrwt. This means that it is a directory (d), and the owner (root) can read, write, and execute (rwx). The group and other users can also read, write, and execute (rwx), and the sticky bit is set (t).

If these permissions are not set correctly, you can set them using the chmod and chown commands.

Setting the Correct Permissions

To set the correct permissions on the /tmp/ directory, you can use the chmod command. The chmod command changes the permissions of a file or directory. The syntax is as follows:

chmod [who][operator][permissions] filename

• who can be user (u), group (g), other (o), or all (a).
• operator can be add (+), subtract (-), or set (=).
• permissions can be read (r), write (w), or execute (x).

To give everyone read, write, and execute permissions on the /tmp/ directory, and to set the sticky bit, you can run the following command:

sudo chmod 1777 /tmp

The 1 sets the sticky bit, and the 777 gives everyone read, write, and execute permissions.

You can also change the owner of the /tmp/ directory using the chown command. The chown command changes the owner of a file or directory. The syntax is as follows:

chown [owner][:[group]] filename

To change the owner to root, you can run the following command:

sudo chown root:root /tmp

Conclusion

In conclusion, the inability to write to the /tmp/ directory, even with root permissions and the sticky bit set, is usually due to incorrect permissions. By understanding Linux permissions and how to set them, you can easily resolve this issue. Remember to be cautious when modifying permissions on system directories like /tmp/, as it can have security implications.