UPDATE 1-CBS joins Comcast On Demand Online trial

* CBS is the first broadcast network to join Comcast trial

* Trial to begin in coming weeks

* CBS shares up nearly 4 pct, Comcast shares up 1 pct

NEW YORK, July 14 (Reuters) - Comcast Corp (CMCSA.O) has signed up CBS Corp (CBS.N) as the first broadcast network to participate in the cable company's technical trial of a program to make more television shows available to Internet users.

CBS will offer both current and older shows in the On Demand Online program, which will begin a technical trial with about 5,000 U.S. customers in the coming weeks, CBS and Comcast said in a statement on Tuesday.

The statement comes after Comcast and Time Warner Inc (TWX.N) announced their "TV Everywhere" project, which would make more TV programs available online to people who subscribe to Comcast's cable TV service.

Broadcasters, cable operators and other programmers are trying to come up with ways to make TV shows and other content available online because more people are spending their time on the Web than ever before.

At the same time, they are trying to make sure that they do not gut their existing businesses by giving away content on the Internet or letting it get stolen and redistributed by digital pirates.

At stake are billions of dollars in advertising revenue as well as lucrative partnership agreements the various companies have with each other. They are trying to make the transition to the Internet to avoid similar pitfalls that the newspaper and music businesses have encountered there.

CBS shares were up 23 cents, or 3.85 percent, to $6.21 on the New York Stock Exchange. Comcast shares were up 16 cents, or 1.18 percent, on the Nasdaq stock market. (Reporting by Robert MacMillan; Editing by Derek Caney)