I want to know what exactly are Linux commands? & how to identify and locate them?In other words what are different kind/type/category of commands? How to identify them and locate (source) it.
What exactly are commands? How to identify?
command
Following is from from the book The Linux Command Line. The full PDF tutorial is also available here:A command can be one of four different things:An executable program like all those files we saw in /usr/bin. Within this category, programs can be compiled binaries such as  programs written in C and C++, or programs written in scripting  languages such as the shell, perl, python, ruby, etc.A command built into the shell itself. bash supports a number of commands internally called shell builtins. The cd command, for  example, is a shell builtin.A shell function. These are miniature shell scripts incorporated into the environ- ment. We will cover configuring the  environment and writing shell functions in later chapters, but for  now, just be aware that they exist.An alias. Commands that we can define ourselves, built from other commands.To identify the type of command, you can use type:NAME    type - Display information about command type.Example:$ type lsls is aliased to `ls --color=auto'$ type cdcd is a shell builtinTo display the location of command, you can use which:$ which info/usr/bin/info$ which init/sbin/initHope this is helpful for new users to know basic about commands.