In Linux and other Unix-like operating systems, a file or directory is considered "hidden" if its name begins with a period (.). This is a long-standing convention. Standard file-listing commands, such as ls, will not display these files by default. To view them, a specific flag must be used, typically -a (all) or -A (almost-all). This mechanism is commonly employed to conceal user-specific configuration files (e.g., .bashrc, .ssh/) from casual view, thereby preventing clutter in the user's home directory. An attacker might also use this technique to hide malicious tools or scripts on a compromised system.

A. Exclamation mark (!): This character is used in shell scripting for logical negation and history expansion, not for hiding files.
B. Underscore (): The underscore is a valid and common character in filenames but has no special property for hiding files.
C. Tilde (~): The tilde is a shell metacharacter that expands to the path of the current user's home directory; it is not used as a file-hiding prefix.

1. GNU Coreutils Documentation: The official documentation for the ls command explicitly states its behavior regarding files starting with a period.

Source: GNU Coreutils Manual

Section 10.1 ls: List directory contents.

Reference: "By default

ls does not list files whose names begin with .... The -a (--all) option disables this behavior." This is detailed in the description of the -a and -A options.

2. University Courseware (MIT): Reputable university computer science courses covering operating systems or shell usage confirm this convention.

Source: MIT's "The Missing Semester of Your CS Education" course materials.

Reference: Lecture 1: The Shell

Course Notes. "Files and directories that start with . are hidden by default. To list them

you have to pass the -a flag to ls." (https://missing.csail.mit.edu/2020/course-notes/)

3. Academic Publication (General OS Principles): Textbooks on operating system principles often cover file system conventions for major OS families like Unix/Linux.

Source: Bach

M. J. (1986). The Design of the UNIX Operating System. Prentice-Hall.

Reference: Chapter 4

"The File System

" describes the structure of directories and file naming conventions

where the special treatment of filenames beginning with a "." by shell programs like ls is a foundational concept.