Conditionally perform a command.
File syntax
IF [NOT] EXIST filename command IF [NOT] EXIST filename (command) ELSE (command) String syntax
IF [/I] [NOT] item1==item2 command IF [/I] item1 compare-op item2 command IF [/I] item1 compare-op item2 (command) ELSE (command) Error Check Syntax IF [NOT] DEFINED variable command IF [NOT] ERRORLEVEL number command IF CMDEXTVERSION number command key item May be a text string or an environment variable a variable may be modified using either
Substring syntax or Search syntax command The command to perform NOT perform the command if the condition is false. == perform the command if the two strings are equal. /I Do a case Insensitive string comparison. compare-op May be one of EQU : Equal NEQ : Not equal LSS : Less than < LEQ : Less than or Equal <= GTR : Greater than > GEQ : Greater than or equal >= This 3 digit syntax is necessary because the > and < symbols are recognised as redirection operators
IF ERRORLEVEL n statements should be read as IF Errorlevel >= number
i.e.
IF ERRORLEVEL 0 will return TRUE when the errorlevel is 64
An alternative and often better method of checking Errorlevels is to use the string syntax along with the %ERRORLEVEL% variable:
IF %ERRORLEVEL% GTR 0 Echo An error was found
IF %ERRORLEVEL% LSS 0 Echo An error was found
IF %ERRORLEVEL% EQU 0 Echo No error found
IF %ERRORLEVEL% EQU 0 (Echo No error found) ELSE (Echo An error was found)
IF %ERRORLEVEL% EQU 0 Echo No error found || Echo An error was found
Note some errors are negative numbers.
When working with errorlevels in a batch file it's a good idea to also use SETLOCAL so that the %ERRORLEVEL% variable is reset each time the batch file runs.
IF EXIST filename will return true if the file exists (this is not case sensitive).
Examples:
IF EXIST C:\install.log (echo complete) ELSE (echo failed) IF DEFINED _department ECHO Got the department variable IF DEFINED _commission SET /A _salary=%_salary% + %_commission% IF CMDEXTVERSION 1 GOTO start_process IF %ERRORLEVEL% EQU 2 goto sub_problem2
Does %1 exist?
To test for the existence of a command line parameter - use empty brackets like this
IF [%1]==[] ECHO Value Missing
or
IF [%1] EQU [] ECHO Value Missing
In the case of a variable that may be NULL - a null variable will remove the variable definition altogether, so testing for NULLs becomes easy:
IF NOT DEFINED _example ECHO Value Missing
IF DEFINED will return true if the variable contains any value (even if the value is just a space)
Test the existence of files and folders
IF EXIST name - will detect the existence of a file or a folder - the script empty.cmd will show if the folder is empty or not.
Brackets
You can improve the readability of a batch script by writing a complex IF...ELSE command over several lines using brackets
e.g.
IF EXIST filename (
del filename
) ELSE (
echo The file was not found.
)
The IF statement does not use any great intelligence when evaluating Brackets, so for example the command below will fail:
IF EXIST MyFile.txt (ECHO Some(more)Potatoes)
This version will work:
IF EXIST MyFile.txt (ECHO Some[more]Potatoes)
If the string being compared by an IF command includes delimiters such as [Space] or [Comma], then either the delimiters must be escaped with a caret ^ or the whole string must be "quoted".
This is so that the IF statement will treat the string as a single item and not as several separate strings.
Testing Numeric values
Do not use brackets or quotes when comparing numeric values
e.g.
IF (2) GEQ (15) echo "bigger"
or
IF "2" GEQ "15" echo "bigger"
These will perform a character comparison and will always echo "bigger"
however the command
IF 2 GEQ 15 echo "bigger"
Will perform a numeric comparison and works as expected - notice that this behaviour is exactly opposite to the SET /a command where quotes are required.
The examples here all use GEQ, but this applies equally to all the compare-op operators: EQU, NEQ, LSS, LEQ, GTR, GEQ
when comparing numbers as a string "026" > "26"
Wildcards
Wildcards are not supported by IF, so %COMPUTERNAME%==SS6* will not match SS64
A workaround is to retrieve the substring and compare just those characters:
SET _prefix=%COMPUTERNAME:~0,3%
IF %_prefix%==SS6 GOTO they_matched
Pipes
When piping commands, the expression is evaluated from left to right, so
IF... | ... is equivalent to (IF ... ) | ...
you can also use the explicit syntax IF (... | ...)
ERRORLEVEL
To deliberately raise an ERRORLEVEL in a batch script use the EXIT /B command.
It is possible (though not a good idea) to create a string variable called %ERRORLEVEL% (user variable)
if present such a variable will prevent the real ERRORLEVEL (a system variable) from being used by commands such as ECHO and IF.
To test for the existence of a user variable use SET errorlevel, or IF DEFINED ERRORLEVEL
If Command Extensions are disabled IF will only support direct comparisons: IF ==, IF EXIST, IF ERRORLEVEL
also the system variable CMDEXTVERSION will be disabled.
You see things; and you say 'Why?' But I dream things that never were; and I say 'why not?' - George Bernard Shaw
Related:
Conditional execution syntax (AND / OR)
SET - Display or Edit environment variables
ECHO - Display message on screen
EXIT - Set a specific ERRORLEVEL
IFMEMBER - NT Workgroup member (Resource kit)
SC - Is a Service running (Resource kit)
Powershell: if - Conditionally perform a command
Equivalent bash command (Linux):
if - Conditionally perform a command
