It all depends on which linux version you are using. There are some with boot screens which require you to use your password,and some that don't have one and that automatically boot up without one.
What Are Security And Protection In Linux Operating System?
MEMORY MANAGEMENT
Linux uses virtual memory. Virtual memory makes the system appear to have more memory than it actually has by sharing it between competing processes as they need it. Virtual memory does more than just make your computer's memory go further. The memory management subsystem provides Large Address Spaces, Protection, Memory, Mapping, Fair Physical Memory Allocation and Shared Virtual Memory.
FILE SECURITY
On a Linux system, every file is owned by a user and a group user. For each category of users, read, write and execute permissions can be granted or denied. Linux separates access control on files and directories according to three characteristics: Owner, group, and other. There is always exactly one owner, any number of members of the group, and everyone else. When the set user ID access mode is set in the owner permissions, and the file is executable, processes which run it are granted access to system resources based on the owner of the file.
Linux uses virtual memory. Virtual memory makes the system appear to have more memory than it actually has by sharing it between competing processes as they need it. Virtual memory does more than just make your computer's memory go further. The memory management subsystem provides Large Address Spaces, Protection, Memory, Mapping, Fair Physical Memory Allocation and Shared Virtual Memory.
FILE SECURITY
On a Linux system, every file is owned by a user and a group user. For each category of users, read, write and execute permissions can be granted or denied. Linux separates access control on files and directories according to three characteristics: Owner, group, and other. There is always exactly one owner, any number of members of the group, and everyone else. When the set user ID access mode is set in the owner permissions, and the file is executable, processes which run it are granted access to system resources based on the owner of the file.