Redirecting Output from within Batch file

I am creating a batch file with some simple commands to gather information from a system. The batch file contains commands to get the time, IP information, users, etc.

I assembled all the commands in a batch file, and it runs, but I would like the batch file, when run to output the results to a text file (log). Is there a command that I can add to the batch that would do so?

Keep in mind I do not want to run the batch from cmd, then redirect output ; I want to redirect the output from inside the batch, if that is possible.

The simple naive way that is slow because it opens and positions the file pointer to End-Of-File multiple times.

@echo off
command1 >output.txt
command2 >>output.txt
...
commandN >>output.txt

A better way - easier to write, and faster because the file is opened and positioned only once.

@echo off
>output.txt (
  command1
  command2
  ...
  commandN
)

Another good and fast way that only opens and positions the file once

@echo off
call :sub >output.txt
exit /b

:sub
command1
command2
...
commandN

Edit 2020-04-17

Every now and then you may want to repeatedly write to two or more files. You might also want different messages on the screen. It is still possible to to do this efficiently by redirecting to undefined handles outside a parenthesized block or subroutine, and then use the & notation to reference the already opened files.

call :sub 9>File1.txt 8>File2.txt
exit /b

:sub
echo Screen message 1
>&9 echo File 1 message 1
>&8 echo File 2 message 1
echo Screen message 2
>&9 echo File 1 message 2
>&8 echo File 2 message 2
exit /b

I chose to use handles 9 and 8 in reverse order because that way is more likely to avoid potential permanent redirection due to a Microsoft redirection implementation design flaw when performing multiple redirections on the same command. It is highly unlikely, but even that approach could expose the bug if you try hard enough. If you stage the redirection than you are guaranteed to avoid the problem.

3>File1.txt ( 4>File2.txt call :sub)
exit /b

:sub
etc.

if you want both out and err streams redirected

dir >> a.txt 2>&1

I know this is an older post, but someone will stumble across it in a Google search and it also looks like some questions the OP asked in comments weren't specifically addressed. Also, please go easy on me since this is my first answer posted on SO. :)

To redirect the output to a file using a dynamically generated file name, my go-to (read: quick & dirty) approach is the second solution offered by @dbenham. So for example, this:

@echo off
> filename_prefix-%DATE:~-4%-%DATE:~4,2%-%DATE:~7,2%_%time:~0,2%%time:~3,2%%time:~6,2%.log (
echo Your Name Here
echo Beginning Date/Time: %DATE:~-4%-%DATE:~4,2%-%DATE:~7,2%_%time:~0,2%%time:~3,2%%time:~6,2%.log
REM do some stuff here
echo Your Name Here
echo Ending Date/Time: %DATE:~-4%-%DATE:~4,2%-%DATE:~7,2%_%time:~0,2%%time:~3,2%%time:~6,2%.log
)

Will create a file like what you see in this screenshot of the file in the target directory

That will contain this output:

Your Name Here
Beginning Date/Time: 2016-09-16_141048.log
Your Name Here
Ending Date/Time: 2016-09-16_141048.log

Also keep in mind that this solution is locale-dependent, so be careful how/when you use it.