i have my intrests in linux and networking. i am planning for rhce and ccna both. i have already joined training institutes for both.so is it beneficial for having a career as rhce+ccna from the job and salary point of view.? i am completely new to linux field so can anyone guide me how to gain knowledge in linux? what is the difference between red hat,ubuntu,fedora,centOS....do they have any similarty in commands?

Questions like this have been asked and answered many, MANY times here in the past. The answer is always the same: "Who knows?" We don't know you, your work habits, or anything meaningful about you. You could get a dozen certifications, and not make as much as someone who has NO certifications. There's no way for us to know how a company in the future will pay you, and why.

The best advice would be to actually LEARN the subjects...don't get 'certifications' based on practice tests or websites. Yes, you will have a 'certificate', but it won't have knowledge behind it. Think what happens after you get a job with your shiny new 'certification'...you'll be asked to do things that WERE NOT on that test....what will you do? It will be VERY clear, VERY quickly you can't perform the job you were hired for...and what do you think will happen then?
You learn it the same way you learned Windows or Mac that you use now; you LOAD IT and USE IT. Simple. Pick a version, load it onto your machine, and use it..first as a user, then start to configure more and more things. Read manuals; configure services, learn how to troubleshoot.
ALL of them are Linux...they have 98%+ IDENTICAL commands. There are differences in how you perform administration tasks ("yum" installs software for Fedora/Red Hat/Centos..."apt-get" for Ubuntu, for example), but most things are the same.

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