In UNIX or Linux, what does the ps command typically display by default about running processes?

Introduction / Context:
The ps command is one of the most basic tools for examining processes on UNIX and Linux systems. Administrators and users rely on ps to see which programs are running, what resources they use, and what states they are in. However, the default behavior of ps varies slightly by implementation and often does not show every process on the system without additional options. Understanding what ps shows by default is useful for interpreting its output and for answering exam questions about process monitoring.

Given Data / Assumptions:

• We are considering typical UNIX or Linux implementations of ps.
• Most versions of ps have a default mode when invoked with no options.
• The system may have many processes, including those owned by other users and kernel threads.
• The question asks what ps normally displays by default.

Concept / Approach:
By default, ps commonly shows processes associated with the current terminal or the current user session. For example, on many systems ps without arguments lists processes that share the controlling terminal with the shell from which ps was run. To see all processes, users typically need to supply options such as -e, -A, aux, or similar. Therefore, describing ps as showing only some of the running processes by default, usually those attached to the current terminal or user, is accurate.

Step-by-Step Solution:
Step 1: Recall that UNIX systems can run many processes belonging to different users and different terminals.Step 2: Check typical defaults: running ps with no options often shows processes associated with the current terminal session, such as the shell and programs started from it.Step 3: To display all processes system wide, additional options are required, such as ps -e or ps aux.Step 4: Therefore, the default output of ps does not include every process on the system.Step 5: Conclude that ps by default displays only a subset of running processes, usually tied to the current terminal or user session.

Verification / Alternative check:
If you run ps with no arguments on a typical Linux machine, you may see only a small list including the shell and a few foreground jobs. Using ps -e or ps aux causes the list to expand dramatically, showing many more processes. Documentation for ps notes that default behavior is implementation specific but usually limited, and that additional flags are needed for a complete system wide view. This confirms that ps prints the status of only some processes by default.

Why Other Options Are Wrong:
Option B is incorrect because seeing all processes on the system normally requires special options or privileges. Option C is wrong because ps is not limited to kernel threads; by default it mainly shows user processes, and kernel threads may be hidden or shown depending on options. Option D is incorrect because ps is specifically a process status command; file system usage is handled by other commands such as df or du.

Common Pitfalls:
A common misunderstanding is to assume that running ps always shows a comprehensive list of all processes, which can lead to confusion when expected processes are not displayed. Another pitfall is to confuse ps with other monitoring tools such as top or htop, which have interactive interfaces and broader default views. Remembering that ps without options generally shows only the processes for the current terminal or session helps interpret its output correctly.

Final Answer:
By default, the ps command generally displays the status and details of processes associated with the current terminal or user session, not every process on the entire system.