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      echo out > /tmp/out.txt
      BATSH

      # Perl 5.005_03 and later; pure-Perl, no external shell required.

DESCRIPTION
  Executive Summary
    BATsh is a bilingual shell interpreter written in pure Perl. It runs
    cmd.exe batch syntax and bash/sh syntax in the same script file,
    switching automatically between CMD mode and SH mode on a line-by-line
    basis. No external cmd.exe, bash, or sh is required -- everything runs
    inside Perl.

  Mixed-Mode Sample
    The following script demonstrates cmd.exe and bash sections coexisting
    and sharing variables through the common BATsh::Env variable store.

      :: -- CMD section: sets a variable and calls a SH function via bridge --
      @ECHO OFF
      SET LANG=BATsh
      SET COUNT=3

      # -- SH section: reads CMD variables, uses functions and pipeline --
      greet() {
          echo "Hello from $1 (bash/sh mode)"
      }
      greet $LANG
      for i in 1 2 3; do echo "  item $i of $COUNT"; done
      result=$(echo "$LANG" | perl -ne "print uc")
      echo "Uppercase: $result"
      echo "log line" >> /tmp/batsh_demo.txt

      :: -- CMD section again: reads variable set by SH side --
      ECHO Back in CMD mode
      ECHO Uppercase result: %result%

    BATsh features (both modes): pipelines (|), I/O redirection (> >> <
    2>&1), variable expansion (${var%pat} ${var^^} ${#var}), functions,
    shift, local.

FULL DESCRIPTION
    BATsh is a bilingual shell interpreter written in pure Perl. It
    implements both the cmd.exe command set and the sh/bash command set
    entirely in Perl -- no external cmd.exe, bash, or sh is required.

    Scripts are divided into CMD sections (uppercase first token) and SH
    sections (lowercase first token). Both sections share a common variable
    store via BATsh::Env, so variables set in a CMD section are immediately
    visible in the next SH section and vice versa.

CMD MODE
    Any line whose first token is all uppercase (A-Z, 0-9, path chars) is a
    CMD line. CMD sections are executed by BATsh::CMD, which implements:

      ECHO, @ECHO OFF/ON
      SET VAR=value, SET /A expr (arithmetic)
      SET /P VAR=Prompt  (interactive prompt input from STDIN)
      IF "A"=="B" ... ELSE ..., IF /I (case-insensitive), IF NOT
      IF EXIST "path with spaces", IF DEFINED var, IF ERRORLEVEL n
      FOR %%V IN (list) DO ..., FOR /L %%V IN (s,step,e) DO ...
      FOR /F "tokens= delims= skip= eol= usebackq" %%V IN (src) DO ...
      GOTO :label, :label, GOTO :EOF
      CALL :label [args], CALL file.batsh
      SHIFT, SHIFT /N
      SETLOCAL [ENABLEDELAYEDEXPANSION|DISABLEDELAYEDEXPANSION], ENDLOCAL
      CD, DIR, COPY, DEL, MOVE, MKDIR, RMDIR, REN, TYPE
      PAUSE, EXIT [/B] [code], CLS, TITLE, VER, PUSHD, POPD
      cmd1 | cmd2  (pipeline via temporary file)
      &, &&, ||  (sequential, conditional-and, conditional-or)

  Variable Expansion
    "%VAR%" references are expanded before each line is dispatched. Variable
    names are case-insensitive ("SET foo=x" is visible as "%FOO%").

    Inside parenthesised IF and FOR blocks, "%VAR%" is expanded at parse
    time (before any commands in the block run), matching cmd.exe behaviour.
    To see a value updated inside a block, use delayed expansion:

      SETLOCAL ENABLEDELAYEDEXPANSION
      SET X=old
      IF 1==1 (
          SET X=new
          ECHO !X!       &:: prints "new" (delayed)
          ECHO %X%       &:: prints "old" (parse-time)
      )
      ENDLOCAL

  Batch Parameters
    %0 is the script path (absolute); %1..%9 are positional arguments; "%*"
    is all arguments joined by space.

    "CALL :label arg1 arg2 ..." invokes a subroutine as a true call frame:
    the subroutine receives its own %0 (the ":label" token), %1..%9 (the
    call arguments) and "%*" (their join), and the caller's parameters are
    saved before the call and restored on return. Arguments are "%"-expanded
    before the call and split with double-quote awareness, so "CALL :sub "a
    b" %FILE%" passes "a b" as one argument and the expanded value of
    "%FILE%" as the next. Nested calls each get an independent frame. The
    same arguments are also visible as $1..$9 / $@ when the subroutine body
    is written in SH mode.

    "SHIFT" moves %2 into %1, %3 into %2, and so on, clears %9, and rebuilds
    "%*"; "SHIFT /N" begins the shift at %N (%1.."%(N-1)" are left
    unchanged).

    Batch-parameter tilde modifiers expand %0..%9 components:

      %~0    dequote (strip surrounding "...")
      %~f1   full absolute path of %1
      %~d1   drive letter only   (e.g. C:)
      %~p1   directory path only (with trailing /)
      %~n1   filename without extension
      %~x1   extension only       (e.g. .bat)
      %~dp0  drive + directory    (most common usage)
      %~nx1  filename + extension

  Redirection and Compound Commands
      ECHO text > file      stdout overwrite
      ECHO text >> file     stdout append
      prog 2> err.txt       stderr redirect
      & cmd                 sequential execution