i want to write my own shell(micro),which has limited feautures.suppose the command copy old new to work,wat i need to do.i can write the program to perform the operation of copy,but how can i create executable for copy??

Copy (cp) is a separate program, and not part of the shell. You must learn the basics of programming before you can start replacing parts of the system.

Shell is a program, but most (or many) of the "commands" you run are executables of their own. You'll see it easily by issuing
in your shell, to see where commandname is - if it shows it's in some bin/ directory (or any other for that matter), it's a separate executable. Shells have some builtin "functions", like TAB-completion, if-else statements etc. but they depend on the shell. For example DOS doesn't have copy built-int; it's a separate executable in a system directory where DOS can find it.

If you want to write a shell, I suggest you start learning basics of programming, maybe best done (in this case) with C. Then you need a compiler to compile the code into an executable. Writing a very minimalist shell is not that difficult, but it's another matter to make one that is of use.

i thank you very much for your suggestions....

The shell is not nearly as magical as it seems at first. A rudimentary shell can be written in a few lines of code. In it's most basic form, it is a loop that reads its standard input, and invokes other programs using fork() + exec() + wait(). It may understand a set of it's own commands which it performs using internal code. Most of the functionality is already present in the standard C library. There are numerous examples of simple shells in textbooks and on the net. Depending on where in the spectrum of simplicity - complexity you want your shell to be, there is probably an open source version which will either work straight away, or can be easily modified to include the functions you want.
--- rod.

Another aspect of "what is a shell" is how it gets used. Some vendor supplied turnkey systems have user shells which are menus for that system -- for example, a library catalog system (before web interfaces took over). In this case, if you look at /etc/passwd, you would see that the shell assigned for user accounts is, maybe, something like /usr/local/bin/iiish. A user who logs into that system is locked in to the menu for the catalog.

Such shells can be relatively simple shell scripts themselves (e.g., #!/bin/ksh) which start out by trapping interrupts to prevent people from breaking out of them. The O'Reilly book on learning the korn shell has some examples. So, you don't have to get into C. You could really do it in almost anything, depending on what your requirements are.

Unix shell accounts on such a system would be rare and for sysadmins only.