In UNIX and Linux, file permissions are defined by three sets of permissions for three types of users: the owner, the group, and others. These permissions are read (r), write (w), and execute (x). In addition to these basic permissions, there is a special type of file permission called setuid (short for “set user ID”).
When setuid is set on an executable file, the file will be executed with the privileges of the file’s owner instead of the privileges of the user who runs the file. In other words, when you run an executable file with setuid set on it, you temporarily become the owner of the file and gain the same permissions as the owner.
This is often used in situations where a non-privileged user needs to perform a task that requires elevated privileges. For example, the passwd command has setuid set on it so that regular users can change their passwords, even though this requires write access to the system’s password file, which is normally only accessible to the root user.
It’s important to note that setuid only applies to executable files, not to data files. Additionally, setuid can be a security risk if it’s not used carefully, since it can allow users to gain elevated privileges that they shouldn’t have.