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.