creating a new binary
 

I need to change the functions of some linux commands. We can't edit the binary files provided in /bin, is there any other method other than alias.
For ex. - I need to change the function chmod so that it takes only three consecutive integers as input (chmod 777 filename) and nothing else ?
Do I have to write by own code for it, or is there any other alternate method.

I think it would be better you describe what you want to achieve. I don't think that writing your own commands is a good Idea.

Markus

What's so bad with the original command? You could define a function and hide it from the user. Inside the function you can check for the proper arguments and call the real binary:

Be aware, that either the user could still call the original command, or even install his own version.