ps
ProcessesPackage: procps
The list of processes at this instant, with PID, owner and command. ps aux shows everything; ps -o pid,ppid,cmd picks the columns, which makes the output readable by a script.
What its options do in the lessons
From the same glossary the lessons render under their commands, so the two cannot disagree.
ps -e- Every process on the machine, not only those of your terminal.
ps -f- Full format: adds the PPID (the parent process) and the whole command line.
-efis what makespsuseful. ps -o- Chooses the columns, in the order you want:
ps -o pid,ppid,stat,cmd. Useful for keeping only what you actually read. ps -p- Restricts the output to the given PIDs, for when you already hold one and want to know what it is.
ps --forest- Draws the parentage as a tree rather than a flat list. An orphaned process shows immediately: it hangs off PID 1.