Therefore "apropos whatis" will find only one matching man page and will report the same information as "whatis whatis" command.
Unix commands
There is no easy way - if there is a path name involved then it is external. You could use the 'whence' or 'whatis' commands to see if they are an alias or internal command, but that varies depending on which login shell you are using.
Commands you use in a Unix based computer OS to achieve certain things. Similar to MS/DOS commands in Windows. Mostly used in computers running the Linux OS. unix command
The lp and lpr commands are the traditional commands used to print jobs on UNIX.
It would take a very long time to learn all of the Unix commands, and frankly, that isn't necessary. Most Unix users have a subset of commands they use all the time, and that is how they learn them.
UNIX commands are designed to be simple in the first place; they basically do one task per command. To make a more powerful sequence, just put (or pipe) several commands together in a sequence. The ability to use many of the commands in different scenarios just by the command sequence is very powerful; the individual commands do not have to be that powerful or complex, but the result of using several of them in a row makes for a very powerful system.
Because Linux evolved from UNIX, but Windows evolved from DOS.
Unix files do not rely on extensions, therefore there is no command to find them.
Man (or manual) pages
There is none. For starters, you have it backwards, DOS actually copied most of its commands from Unix (The rest came from CP/M.), which Linux is inspired by. Commands like "cd" and "dir" were Unix commands long before DOS even existed.
Most of MS-DOS' commands were based on those of Unix and CP/M. 'cd', 'dir', 'clear', and 'echo' are usually found in both. MS-DOS added it's own commands, however, and made some different from those of existing versions of Unix, and no one saw any reason to change the names of existing ones in Unix.
William Holliker has written: 'UNIX Shell commands quick reference' -- subject(s): UNIX (Computer file), UNIX Shells