Quote:
Whenever we write a new command. Then we goto "/usr/bin" and then place this in this directory.And hence we can use it as a command anywhere from the terminal.
That is not a good idea, at all, for a variety of reasons. bash is already extensible - meaning you can write code to do special roll-your-own-commands, and then have bash include the command.
If you absolutely have to 'add commands' the way you are doing it, put the command scripts in /usr/local/bin or maybe /opt/local/bin/<some name>. The directory cannot be world write nor can any script be writable by anybody but root -- or you create giant security hole.
Now you can change the PATH variable at run time. If the PATH has ":/usr/local/bin" in it then you execute one of your special commands. If the PATH does not have it, then the special command will not be found. Simply change the PATH at runtime.