What is the function of "fg %N" command?

Question

I have learned that fg %N means "go to task N"

I don't understand this command or how to use it. I have tried to see the manual entry of this command in the terminal, but that didn't work:

$ man fg
No manual entry for fg.

Answer 1

Second one first: fg is a bash shell built-in command and as such you need to refer to the man page for bash. In particular, the section JOB CONTROL says

   Simply naming a job can be used to bring it into the foreground: %1  is
   a  synonym  for  ``fg %1'', bringing job 1 from the background into the
   foreground.  Similarly, ``%1 &''  resumes  job  1  in  the  background,
   equivalent to ``bg %1''.

Alternatively, you can use the shell's interactive help system:

$ help fg
fg: fg [job_spec]
    Move job to the foreground.

    Place the job identified by JOB_SPEC in the foreground, making it the
    current job.  If JOB_SPEC is not present, the shell's notion of the
    current job is used.

    Exit Status:
    Status of command placed in foreground, or failure if an error occurs.

---

Now for the first part. The actual command you stated does not in fact redirect stdout to stderr: it redirects stdout to a file named 2 and then puts the whole command into the shell's background. Hence

$ man 1>2&
[1] 4662

runs man in the background (as job [1], with process ID 4662) - if you look in the current directory you will likely find a file called 2 with contents

 What manual page do you want?

The command you should have used is 1>&2

• &2 : file descriptor #2
• 2& : file named 2, command run in the background

For more information see the REDIRECTION section of man bash

Answer 2

1. fg is a bash builtin command:

   $ type fg
   fg is a shell builtin
   

   To get information on individual bash commands, use help:

   $ help fg
   fg: fg [job_spec]
       Move job to the foreground.
   
       Place the job identified by JOB_SPEC in the foreground, making it the
       current job.  If JOB_SPEC is not present, the shell's notion of the
       current job is used.
   
       Exit Status:
       Status of command placed in foreground, or failure if an error occurs.
   

2. As mentioned in the first version of the question, 1>&2 is an example of redirection. To read about redirection, run man bash and go to the section entitled REDIRECTION.