How to compile multiple proto files in single command?

Here's an option using find

protoc --js_out=js \
        -Iproto/ \
        $(find proto/google -iname "*.proto")

The problem is that you are specifying --proto_path as an absolute path but your proto files as relative paths. You can either drop the --proto_path argument (it defaults to the current directory anyway), or you can do:

protoc --proto_path=C:\shekhar\proto_trial
       --java_out=C:\shekhar\proto_trial\java_op
       C:\shekhar\proto_trial\*.proto