01-1-Errorlevel.md

⚠️ Understanding ERRORLEVEL and IF ERRORLEVEL

• In Windows Batch scripting, ERRORLEVEL is used to check the exit code of the last executed command or program.

  It's a critical tool for error handling and control flow, especially in automation, scripting pipelines, or conditionally executing code.

  🧩 What is ERRORLEVEL?

  • ERRORLEVEL is an environment variable that stores the exit code of the last run program or script.

  • Most commands and utilities in Windows return an exit code:

    • 0 usually means success
    • Any other number (e.g., 1, 2, 9009, etc.) indicates some kind of failure

    🔍 Example

    @ECHO OFF
    DIR "C:\Some\Fake\Path"
    ECHO Last exit code was: %ERRORLEVEL%

    • If the path doesn't exist, %ERRORLEVEL% will be non-zero.

    ✅ Common Exit Code Meanings

    Code	Meaning
    0	Success
    1	General error
    2	Incorrect usage / syntax
    9009	Command not found

    🔎 IF ERRORLEVEL — How It Works

    The IF ERRORLEVEL syntax is a bit non-intuitive for beginners:

    IF ERRORLEVEL n command

    This checks if %ERRORLEVEL% is greater than or equal to n, not just equal to n.

    ⚠️ Important:

    IF ERRORLEVEL 1

    means:

    IF %ERRORLEVEL% GEQ 1
    

    So, if %ERRORLEVEL% is 5, the condition will still match!

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1. 🧪 Example 1: Check if command failed

   @ECHO OFF
   COPY file.txt D:\backup\
   IF ERRORLEVEL 1 (
       ECHO Copy failed!
   ) ELSE (
       ECHO Copy succeeded.
   )

2. 🧪 Example 2: Exact Match (Use Order Carefully!)

   If you want to check for a specific code, use descending order:

   @ECHO OFF
   SomeCommand
   
   IF ERRORLEVEL 3 (
       ECHO Exit code was 3 or more
   ) ELSE IF ERRORLEVEL 2 (
       ECHO Exit code was 2 or more
   ) ELSE IF ERRORLEVEL 1 (
       ECHO Exit code was 1 or more
   ) ELSE (
       ECHO Exit code was 0
   )

   ☑️ Always list the highest values first due to the GEQ behavior.

3. 🧪 Example 3: Setting Your Own ERRORLEVEL

   Use EXIT /B [number] to set your own error code inside a script or subroutine:

   @ECHO OFF
   CALL :Check 0
   CALL :Check 1
   GOTO :EOF
   
   :Check
   ECHO Checking with exit code %1
   EXIT /B %1

   Then use %ERRORLEVEL% or IF ERRORLEVEL to control behavior based on the result.

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📌 Best Practices

Tip	Why
Use IF ERRORLEVEL in descending order	Prevents multiple matches
Use EXIT /B n in subroutines	To simulate return codes
Reset %ERRORLEVEL% if needed	Some commands (like SET) don’t reset it
Combine with ` and&&` for simpler flow	Reduces nested blocks

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➕ Shortcut: && and ||

SomeCommand && ECHO Success || ECHO Failed

• && runs the next command only if the previous succeeded (ERRORLEVEL = 0)
• || runs the next command only if the previous failed (ERRORLEVEL > 0)

This behaves like a shorthand IF ERRORLEVEL check.

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✅ Summary Table

Concept	Usage Example	Notes
Check exit code ≥ n	IF ERRORLEVEL 2	GEQ behavior
Exact code check	Descending IF ERRORLEVEL blocks	Recommended
View current error	ECHO %ERRORLEVEL%	Shows last result
Set custom code	EXIT /B 3	Manual control
Short condition	`cmd && echo OK	echo Fail`

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