Which comment style should I use in batch files?

Question

I've been writing some batch files, and I ran into this user guide, which has been quite informative. One thing it showed me was that lines can be commented not just with REM, but also with ::. It says:

Comments in batch code can be made by using a double-colon, this is better than using the REM command because labels are processed before redirection symbols. ::<remark> causes no problems but rem <remark> produces errors.

Why then, do most guides and examples I see use the REM command? Does :: work on all versions of Windows?

Answer 1

Another alternative is to express the comment as a variable expansion that always expands to nothing.

Variable names cannot contain =, except for undocumented dynamic variables like
%=ExitCode% and %=C:%. No variable name can ever contain an = after the 1st position. So I sometimes use the following to include comments within a parenthesized block:

::This comment hack is not always safe within parentheses.
(
  %= This comment hack is always safe, even within parentheses =%
)

It is also a good method for incorporating in-line comments

dir junk >nul 2>&1 && %= If found =% echo found || %= else =% echo not found

The leading = is not necessary, but I like if for the symmetry.

There are two restrictions:

1) the comment cannot contain %

2) the comment cannot contain :

Answer 2

good question... I've been looking for this functionality for long too...

after several tests and tricks it seem the better solution is the more obvious one...

--> best way I found to do it, preventing parser integrity fail, is reusing REM:

echo this will show until the next REM &REM this will not show

you can also use multiline with the "NULL LABEL" trick... (dont forget the ^ at the end of the line for continuity)

::(^
this is a multiline^
comment... inside a null label!^
dont forget the ^caret at the end-of-line^
to assure continuity of text^ 
)

Answer 3

tl;dr: REM is the documented and supported way to embed comments in batch files.

---

:: is essentially a blank label that can never be jumped to, whereas REM is an actual command that just does nothing. In neither case (at least on Windows 7) does the presence of redirection operators cause a problem.

However, :: is known to misbehave in blocks under certain circumstances, being parsed not as a label but as some sort of drive letter. I'm a little fuzzy on where exactly but that alone is enough to make me use REM exclusively. It's the documented and supported way to embed comments in batch files whereas :: is merely an artifact of a particular implementation.

---

Here is an example where :: produces a problem in a FOR loop.

This example will not work in a file called test.bat on your desktop:

@echo off
for /F "delims=" %%A in ('type C:\Users\%username%\Desktop\test.bat') do (
    ::echo hello>C:\Users\%username%\Desktop\text.txt
)
pause

While this example will work as a comment correctly:

@echo off
for /F "delims=" %%A in ('type C:\Users\%username%\Desktop\test.bat') do (
    REM echo hello>C:\Users\%username%\Desktop\text.txt
)
pause

The problem appears to be when trying to redirect output into a file. My best guess is that it is interpreting :: as an escaped label called :echo.

Answer 4

A very detailed and analytic discussion on the topic is available on THIS page

It has the example codes and the pros/cons of different options.

Answer 5

This page tell that using "::" will be faster under certain constraints Just a thing to consider when choosing

Answer 6

After I realized that I could use label :: to make comments and comment out code REM just looked plain ugly to me. As has been mentioned the double-colon can cause problems when used inside () blocked code, but I've discovered a work-around by alternating between the labels :: and :space

:: This, of course, does
:: not cause errors.

(
  :: But
   : neither
  :: does
   : this.
)

It's not ugly like REM, and actually adds a little style to your code.

So outside of code blocks I use :: and inside them I alternate between :: and :.

By the way, for large hunks of comments, like in the header of your batch file, you can avoid special commands and characters completely by simply gotoing over your comments. This let's you use any method or style of markup you want, despite that fact that if CMD ever actually tried to processes those lines it'd throw a hissy.

@echo off
goto :TopOfCode

=======================================================================
COOLCODE.BAT

Useage:
  COOLCODE [/?] | [ [/a][/c:[##][a][b][c]] INPUTFILE OUTPUTFILE ]

Switches:
       /?    - This menu
       /a    - Some option
       /c:## - Where ## is which line number to begin the processing at.
         :a  - Some optional method of processing
         :b  - A third option for processing
         :c  - A forth option
  INPUTFILE  - The file to process.
  OUTPUTFILE - Store results here.

 Notes:
   Bla bla bla.

:TopOfCode
CODE
.
.
.

Use what ever notation you wish *'s, @'s etc.