What is the purpose of the SUID in Linux?

The purpose of SUID (Set User ID) in Linux is primarily to allow regular users to run applications with the privileges of the file owner, typically the root user. When a file has the SUID bit set, it temporarily gives the user executing the file the same permissions that the file owner has during the execution of that file. This is particularly useful for scenarios where a program requires elevated privileges to perform certain operations, such as changing system configurations or accessing restricted files, but where you still want to allow a regular user to run that program.

For instance, the passwd command, which is used to change user passwords, must be able to write to the /etc/shadow file, a file that is typically restricted to the root user. By setting the SUID bit on this command, users can run it to change their passwords without needing to log in as the root user.

This capability must be handled with caution because it can pose security risks if misused, allowing unauthorized access to sensitive system resources. Thus, its implementation should be carefully managed and audited.