Comments in Batch Files

The REM command marks a line as a comment: cmd.exe reads it and simply ignores it, moving on to the next line. A second, widely used trick is starting a line with a double colon, ::. This works because a line beginning with a colon is treated as a label, and labels are never executed during normal top-to-bottom script flow -- they're only reached if a GOTO explicitly jumps to them -- so :: effectively behaves like a comment in most contexts.

The :: trick has a real pitfall: inside a parenthesized block, such as an IF (...) or a FOR (...) do (...) construct, cmd.exe's block parser can misinterpret :: as an invalid label and throw an error like 'was unexpected at this time.' REM does not have this problem -- it is always syntactically safe, in any position, including deep inside nested blocks. The only downsides of REM are that it's marginally slower to process and, unlike ::, its text is displayed if command echoing (ECHO ON) is active.

Good batch scripts use a consistent REM header block at the top: what the script does, who wrote it, what parameters it expects, and when it was last updated. This matters because batch has none of the tooling modern languages take for granted -- no docstrings, no IntelliSense hover text, no auto-generated documentation. Comments are the only place that context lives, so they should explain why a step exists or what edge case it handles, not just restate what the command obviously does.

In enterprise environments, batch scripts have a habit of outliving the person who wrote them by years, sometimes getting quietly copied across servers and modified by several different administrators over time. Without comments, a later maintainer has to reverse-engineer intent purely from the raw commands -- was that DEL command there to clean up temp files, or is it silently relying on a side effect something else depends on? A little documentation up front saves a lot of risky guesswork down the line.