Is there a word for "shadow"?
The term &quot; to shadow &quot;(跟随/跟著) means 'follow (closely like a shadow)'.
Example:

&quot;Today is your first day on the job, you don't have to do anything, just shadowing John and watch how he works.&quot;
(今天是你工作的第一天，你不用做任何事情，只要跟著約翰, 看看他是如何工作便可以了.)

For a noun that describes 'someone who does the shadowing'
I suggest '見習生'
Example:

&quot;you would be John's shadow today&quot;
(你今天就當約翰的見習生吧)

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見習 (v) to watch and learn
見習生(n) a person on job probation (watch and learn)

見習生 is a general term, It can be a student training in actual work place or a new employee starting a job on probation. Different companies have different training methods. Shadowing is one kind of 見習, which trainee can learn by observing, and not hamper the trainer's regular work routine.