02-IF-Else.md

Conditional Statements in Windows Batch Scripts (IF, IF EXIST)

Windows batch provides several forms of the IF statement:

1. IF <condition> <command>

2. IF EXIST <filename> <command>

3. IF DEFINED <variable> <command>

4. IF ERRORLEVEL <number> <command>

5. IF <string1>==<string2> <command>

   ✅ Basic IF Syntax

   IF <condition> (commands) ELSE (commands)

   Or, without parentheses:

   IF <condition> command1 ELSE command2

   However, parentheses are preferred for multi-line blocks and to avoid ambiguity.

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1. IF with String Comparisons

• 🧷 Syntax

  IF "string1"=="string2" (echo match) ELSE (echo no match)

  Always quote your strings to avoid parsing issues, especially when dealing with spaces or empty variables.

• 🛑 Common Pitfall: Missing Quotes

  :: Problematic when variable is empty!
  IF %USERNAME%==admin echo Welcome
  
  :: Better:
  IF "%USERNAME%"=="admin" echo Welcome

  If %USERNAME% is empty, the first line becomes:

  IF ==admin echo Welcome

  ...which causes a syntax error.

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  🔎 Case-Insensitive Matching

  Batch string comparison using == is case-insensitive by default.

  IF "admin"=="ADMIN" echo This will match

  To do case-sensitive comparisons, use CALL with string manipulation or external tools (not natively supported).

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  🧠 Variables and Delayed Expansion

  Consider this:

  SET VAR=hello
  IF "%VAR%"=="hello" ECHO match

  Now try:

  SET VAR=hello & SET VAR=world
  IF "%VAR%"=="world" ECHO match

  This won’t work reliably in complex loops. You’ll need delayed variable expansion:

  SETLOCAL ENABLEDELAYEDEXPANSION
  SET VAR=hello
  IF "!VAR!"=="hello" ECHO match
  ENDLOCAL

  Use !VAR! instead of %VAR% when using delayed expansion, especially inside loops or IF blocks.

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2. IF EXIST: File/Folder Existence Check

• ✅ Syntax

  IF EXIST "filename.txt" (
      ECHO File found!
  ) ELSE (
      ECHO File not found!
  )

• 📁 Checking Directories

  IF EXIST "C:\MyFolder\" ECHO Folder exists

  Yes, folders are treated like files with trailing slashes.

  ❗ Gotcha: Quoted Paths and Wildcards

  Avoid combining wildcards and quotes:

  :: WRONG – This won't work
  IF EXIST "*.txt" echo Match found
  
  :: Correct
  IF EXIST *.txt echo Match found

  Quotes are fine for exact names, not wildcard patterns.

  🔁 Nested IFs

  IF EXIST "file.txt" (
      IF "%MODE%"=="debug" (
          ECHO Found file in debug mode
      ) ELSE (
          ECHO Found file in normal mode
      )
  ) ELSE (
      ECHO File not found
  )

  Just like other languages, you can nest conditionals, but be mindful of readability.

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🆚 Comparing Numbers vs Strings

• All IF string comparisons using == are string-based, not numeric.

  SET A=10
  SET B=2
  IF %A% GTR %B% echo A is greater

• 🔢 Numeric Comparison Operators

  Use these:

  • EQU – Equal
  • NEQ – Not equal
  • LSS – Less than
  • LEQ – Less than or equal
  • GTR – Greater than
  • GEQ – Greater than or equal

  Example:

  SET /A A=5
  IF %A% GEQ 3 echo A is 3 or more

  🔍 %A% GEQ 3 works only if both sides are numeric.

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3. IF ERRORLEVEL

• Checks the exit code from the previous command:

  ping -n 1 127.0.0.1
  IF ERRORLEVEL 1 (
      ECHO Ping failed
  ) ELSE (
      ECHO Ping succeeded
  )

  Important:

  IF ERRORLEVEL 2 …  :: Matches 2, 3, 4, … (NOT just 2)

• To check for exact error level, check from highest to lowest:

  IF ERRORLEVEL 3 (
      ECHO 3 or more
  ) ELSE IF ERRORLEVEL 2 (
      ECHO 2
  ) ELSE IF ERRORLEVEL 1 (
      ECHO 1
  )

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4. IF DEFINED

• Checks if a variable exists:

  IF DEFINED PATH (
      ECHO PATH is set
  ) ELSE (
      ECHO PATH is not set
  )

Great for checking if user or system variables are available.

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🛑 Common Mistakes

Mistake	Why it’s a problem
Unquoted strings	Breaks when variables are empty or contain spaces
No delayed expansion in loops	%VAR% gets stale values
Wrong file path in IF EXIST	Missing quotes, wrong slashes, case sensitivity
Assuming ERRORLEVEL means exact	It's ≥ comparison, not equality

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🧪 Real-world Example: Backup Script

@ECHO OFF
SET FILE=backup.zip

IF EXIST "%FILE%" (
    ECHO Found existing backup. Deleting...
    DEL "%FILE%"
)

ECHO Creating new backup...
REM simulate backup with dummy file
COPY NUL "%FILE%"

IF EXIST "%FILE%" (
    ECHO Backup created successfully.
) ELSE (
    ECHO Backup failed!
)

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🧠 Pro Tip: Combine Conditions

IF EXIST "input.txt" IF "%MODE%"=="prod" (
    ECHO Processing file in prod mode
)

Or use nested form for clarity.

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📘 Summary

Type	Example	Description
String Compare	IF "%A%"=="B"	String equality
File Check	IF EXIST "file.txt"	File or folder presence
Variable Check	IF DEFINED MYVAR	Whether a variable is defined
Numeric Compare	IF %X% GTR 3	Numerical comparisons
Exit Code	IF ERRORLEVEL 1	Previous command's exit status

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