An internal command is one that is built in to the shell interpreter and is likely to be used quite a bit.
An external command is a file in a directory that can be searched, loaded, and executed.
The reason for having internal commands is for performance; it takes a lot less time to have the shell just execute the given internal command than it would take to find it on the system, load it into memory, and then execute it.
A list of internal/builtin commands is available for each shell environment by looking at the 'man' entry. Anything not listed there is either an alias or an external command.
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.
Internal commands are executed by the shell and do not exist as a separate binary program. You can find out which of these there are by looking at the 'man' entry for the shell you are using. External commands can be found in various directories, such as /bin, /usr/bin, etc.
Unix commands
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.
INTERNAL COMMANDSThese are those commands which are contained in command.com files of MS-DOS.These are those functions that are built into the command interpreter.There is no need of any external file in computer to read internal MS-DOS command.These commands can be used as long as DOS is running on the system.Internal commands do not vary from system to system.These are ver, time, del, md, cd, copy con, cls, date, vol, ren, copy etc.EXTERNAL COMMANDSThese are those commands which are not in-built in MS-DOS.External commands are those which are not included in the interpreter.There is a need of an internal file in the computer to read external MS-DOS command.External command may vary from system to system. This means any two computers with same version of MS-DOS may have same internal commands, but may have different external commands.These are tree, xcopy, diskcopy, more, print etc.
You really can't. There is nothing in a prompt that would give that information.
There is no answer to this question. Under UNIX and Linux, commands are either shell native or external programs found on the path (which usually includes /bin /sbin /usr/bin and ~/bin) Most UNIX and distros allow the admin to choose a shell program of choice and modify the contents of other directories and many admins allow individual users to change their personal shell and add additional executable files adding more commands. That being the case, theoretically any command can be valid under almost any UNIX or Linux. YMMV.
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.