Running Linux, Third Edition

File Ownership and Permissions

Ownership and permissions are central to security. It’s important to get them right, even when you’re the only user, because odd things can happen if you don’t. For the files that users create and use daily, these things usually work without much thought (although it’s still useful to know the concepts). For system administration, matters are not so easy. Assign the wrong ownership or permission, and you might get into a frustrating bind like not being able to read your mail. In general, the message:

Permission denied

means that someone has assigned an ownership or permission that restricts access more than you want.

What Permissions Mean

Permissions refer to the ways in which someone can use a file. There are three such permissions under Unix:

• Read permission means you can look at the file’s contents.

• Write permission means you can change or delete the file.

• Execute permission means you can run the file as a program.

When each file is created, the system assigns some default permissions that work most of the time. For instance, it gives you both read and write permission, but most of the world has only read permission. If you have a reason to be paranoid, you can set things up so that other people have no permissions at all.

Additionally, most utilities know how to assign permissions. For instance, when the compiler creates an executable program, it automatically assigns executable permission. When you check a file out of the revision control system ...