In Windows batch files, use REM to add a comment. Everything after REM on the same line is ignored by the command interpreter. The alternative is :: (double colon), which is technically a label that goes nowhere but is widely used as a comment because it is shorter and visually cleaner. Both approaches are single-line only. Batch has no native block comment syntax.

REM: The Official Comment Command

REM (short for "remark") is the documented, supported way to add comments in batch files. The command interpreter skips everything after REM on the same line.

1@echo off
2REM This script backs up the database before deployment
3REM Author: deploy-team
4REM Last updated: 2026-06-01
5
6echo Starting backup...

REM works everywhere in a batch file: at the top level, inside IF blocks, inside FOR loops, and inside parenthesized code blocks. This universality is its primary advantage over ::.

Inline comments with REM

You can place REM after a command using &:

set DB_HOST=localhost & REM Database host for local development
set DB_PORT=5432 & REM Default PostgreSQL port

The & chains two commands on one line. The second command is REM, which does nothing. This is the closest batch scripting gets to an inline comment.

:: (Double Colon): The Practical Alternative

:: is technically a label with an invalid name. Because the interpreter cannot jump to it, it effectively does nothing. Developers adopted it as a comment syntax because it is shorter than REM and visually stands out more.

1@echo off
2:: =============================================
3:: Deploy script for production environment
4:: =============================================
5
6echo Deploying to production...

The visual clarity is real. In scripts with heavy commenting, :: creates a cleaner look than REM because it uses fewer characters and creates a more consistent left margin.

REM vs :: Comparison

The performance difference is negligible in practice. The real decision point is whether your comment appears inside a code block.

The :: Problem Inside Code Blocks

This is the most important thing to know about :: in batch scripts. Inside parenthesized blocks (which includes IF/ELSE, FOR, and multi-line grouped commands), :: can cause syntax errors or unpredictable behavior.

This works:

1@echo off
2IF "%1"=="deploy" (
3    REM Run the deployment
4    echo Deploying...
5    echo Done.
6)

This can break:

1@echo off
2IF "%1"=="deploy" (
3    :: Run the deployment
4    echo Deploying...
5    echo Done.
6)

The interpreter sometimes treats :: inside parenthesized blocks as a malformed label definition, which disrupts the parsing of the block. The behavior varies across Windows versions and depends on surrounding code. The safe rule: always use REM inside parenthesized blocks.

Commenting Out Code for Debugging

When you need to temporarily disable lines during troubleshooting, prefix them with REM:

1@echo off
2echo Step 1: Preparing files...
3REM echo Step 2: Uploading to server...
4REM echo Step 3: Running migrations...
5echo Step 4: Sending notification...

Only Step 1 and Step 4 execute. Steps 2 and 3 are commented out but preserved in the script for easy re-enabling.

For multiple consecutive lines, REM is the only reliable approach:

1@echo off
2REM ---- Temporarily disabled for debugging ----
3REM xcopy /s /y "C:\app\build" "\\server\deploy\"
4REM net stop MyService
5REM net start MyService
6REM ---- End disabled section ----
7echo Debug mode: skipping deployment steps.

Simulating Block Comments With GOTO

Batch has no multi-line comment syntax, but you can simulate it with a GOTO that skips a section:

1@echo off
2echo Before the comment block.
3
4goto :skip_comment
5This entire block is skipped.
6You can write anything here.
7No REM prefix needed.
8Even special characters like | > < work.
9:skip_comment
10
11echo After the comment block.

The GOTO jumps to the :skip_comment label, bypassing everything in between. This is useful for temporarily disabling large sections of a script without adding REM to every line.

A cleaner version uses a descriptive label:

1goto :end_old_logic
2REM Old backup logic - replaced 2026-06-01
3xcopy /s /y "%SOURCE%" "%DEST%"
4if errorlevel 1 echo Backup failed
5:end_old_logic

Using % Comments in Special Cases

Some developers use a variable expansion trick for end-of-line comments:

set count=5 %= This sets the retry count =%

The %= and =% delimiters create an undefined variable name containing a space, which expands to nothing. This is a hack, not official syntax, and it fails if EnableDelayedExpansion interacts badly with the content. It is mentioned here for completeness, but & REM is more readable and reliable.

Best Practices for Batch File Comments

Script headers

Start every non-trivial batch file with a header block:

1@echo off
2REM ================================================================
3REM Script:  deploy.bat
4REM Purpose: Deploy application to staging environment
5REM Usage:   deploy.bat [environment] [version]
6REM Example: deploy.bat staging 2.1.0
7REM ================================================================

Section markers

Use comment blocks to separate logical sections:

1REM ---- Configuration ----
2set APP_NAME=myapp
3set DEPLOY_DIR=C:\deploy\%APP_NAME%
4
5REM ---- Validation ----
6if "%1"=="" (
7    echo Error: Environment argument required.
8    exit /b 1
9)
10
11REM ---- Deployment ----
12echo Deploying %APP_NAME% to %1...
13xcopy /s /y "build\*" "%DEPLOY_DIR%\"

Explain the "why," not the "what"

1REM Bad: Set the timeout to 30
2set TIMEOUT=30
3
4REM Good: 30 seconds allows the database connection pool to drain fully
5set TIMEOUT=30

Common Pitfalls

Using :: inside IF, FOR, or parenthesized blocks. This is the most common batch commenting mistake. The interpreter can misparse :: as a label inside these constructs, producing cryptic errors. Always use REM inside code blocks.

Assuming batch supports block comments. Developers coming from C, Java, or Python expect /* */ or """ syntax. Batch has no equivalent. Use repeated REM lines or the GOTO skip pattern.

Forgetting & before inline REM. Writing set X=5 REM comment does not create a comment. It sets X to 5 REM comment. You need set X=5 & REM comment with the & command separator.

Over-commenting obvious code. Adding REM increment counter above set /a counter+=1 adds noise without value. Comment on the purpose and context, not the mechanics.

Leaving debug REM lines in production scripts. Commented-out code that stays in a script indefinitely creates confusion about whether it should be re-enabled or deleted. Clean up disabled code once the debugging session is complete.

Summary

• REM is the standard, universally safe comment command in batch files. Use it by default, especially inside code blocks.
• :: is a popular shorthand that is visually cleaner but breaks inside parenthesized blocks (IF, FOR, grouped commands).
• For inline comments after a command, use & REM comment text.
• Simulate block comments with GOTO :label to skip large sections without prefixing every line.
• Always use REM inside IF/ELSE and FOR loops to avoid parsing errors.
• Comment on purpose and context rather than restating what the code literally does.