T09

A .cmd / .bat file runs its comment text as commands

Symptom

Parts of rem comments come back as "is not recognized as an internal or external command", or lines are skipped. Changing the encoding does not fix it.

Check it in 30 seconds

Paste into Windows PowerShell 5.1. It only creates files under your %TEMP% folder. If the last line says REPRODUCED, it happens on your machine.

$d = Join-Path $env:TEMP ('cc-trap-' + (Get-Date -Format 'yyyyMMddHHmmss')); New-Item -ItemType Directory $d -Force | Out-Null
$k = [string][char]0x65E5 + [char]0x672C
[IO.File]::WriteAllText("$d\lf.cmd", "@echo off`nrem $k$k$k comment`nrem $k more`necho START`necho END`n", [Text.Encoding]::GetEncoding(932))
$o = cmd /c "`"$d\lf.cmd`" 2>&1" | Out-String; $e = $LASTEXITCODE
$extra = @($o -split "`r?`n" | Where-Object { $_.Trim() -ne '' -and $_.Trim() -ne 'START' -and $_.Trim() -ne 'END' })
$o; "console code page $([Console]::OutputEncoding.CodePage), exit=$e, unexpected lines=$($extra.Count)"
if ($e -ne 0 -or $o -notmatch 'START' -or $extra.Count -gt 0) { 'REPRODUCED' } else { 'NOT REPRODUCED' }

Why it happens

The real cause is the line endings. cmd.exe can misread line boundaries in an LF-only batch file. On the author's machine it broke under code page 932 (Japanese), not under 65001, 437 or 1252, so on English Windows this check may say NOT REPRODUCED.

Fix

Save .cmd / .bat files with CRLF line endings. In git, add *.cmd text eol=crlf to .gitattributes.

$enc = [Text.Encoding]::GetEncoding(28591)   # one byte per char: the file's bytes stay as they are
$t = [IO.File]::ReadAllText($path, $enc)
$t = ($t -replace "`r`n", "`n") -replace "`n", "`r`n"
[IO.File]::WriteAllText($path, $t, $enc)

On the author's machine

broke under cp932, ran fine under cp65001

Author's machine: Windows 11 / Windows PowerShell 5.1.26100 / Python 3.13.7 / Claude Code 2.1.304, Japanese Windows (code page 932). Checked 2026-10-20 to 22.

If you searched for

batch file rem line executed
bat file LF line endings
is not recognized as an internal or external command rem
cmd LF only line endings

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