Originally Posted by lipun4u
I am new in shell scripting and I have to cover bash, ksh and csh.
Then you should change to another course; this one is wasting your valuable time.
The Korn shell (ksh) was a great step forward from the original Bourne (sh) shell but was quickly replaced by the POSIX shell in a great drive to standardise divergent UNIXes. The POSIX shell is closely based on ksh. If portability is important, POSIX shell is arguably the best choice. Others can be argued for too but the case for ksh is weak.
Famously the csh should not be used for serious work for reasons explained in Csh Programming Considered Harmful
and Top Ten Reasons not to use the C shell
Bash is widely used so worth studying. It has become bloated though, so if performance is important other shells such as ash are used so are worth studying rather than ksh or csh.