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Looping

In the Windows CMD shell, looping is performed with the FOR statement.

Loops are controlled by an iterator variable, which has special rules:

  • The iterator variable name must be a single letter
  • The variable name is case sensitive
  • The variable name is written with a single preceeding percent sign [%] at all times when used from the command line, or double preceeding percent signs [%%] inside a script

Delayed Expansion​

When the body of a FOR loop is executed, the variables are expanded (replaced by their values) before the loop begins. That means that any variables that are contained in the loop have their values locked-in and they can't be changed while the loop is executing.​

To allow updated variable values to be accessed within a loop:​

1. Set the EnableDelayedExpansion option:​

SETLOCAL EnableDelayedExpansion​

​2. Change any variables which will be updated during the execution of the loop by replacing the percent-signs ( % ) with exclaimation-marks ( ! ):​

%ERRORLEVEL%    ->    !ERRORLEVEL!​

Loop throush a List of Files or Parameters​

To loop through a list of values such as filenames, use:

FOR %variable IN (files) DO list​

The variable will be sequentially set to each of the given files values, and the loop body (list) will be executed once for each value.

The files value may be:

  • A list of filenames: FOR %F IN (file1.txt file2.pdf file3.c) DO ECHO %F
  • A filename pattern: FOR %F IN (*.pdf) DO ECHO %F
  • Fixed strings: FOR %C IN (Red Green Blue) DO ECHO %C
  • Or a list of all of the parameters: FOR %P IN (%*) DO ECHO %P

The /D option can be used along with a filename pattern to match only directories:

FOR /D %variable IN (files) DO list

Example​

@echo off​
SETLOCAL EnableDelayedExpansion​
FOR %%F IN (\*) DO (​

rem The CHOICE command presents a Y/N choice and sets %ERRORLEVEL%
rem to 1 if the user selected Y and 2 if the user selected N

CHOICE /M "DELETE %%F"​
IF !ERRORLEVEL!==1 (​
  ECHO ...Deleting %%F​
  DEL %%F​
) ELSE (​
  ECHO ...Skipping %%F​
)​

Loop through a Range of Integers​

FOR /L (start, step, end) DO list

This type of loop counts forward or backwards from start to end by a given step.

Example:​ ​

@echo off​
rem Count from 0 to 5 in increments of 1​
FOR /L %%I IN (0,  1, 5) DO ECHO ... %%I ...​
 
rem Count from 4 to 0 in increments of -1​
FOR /L %%I IN (4, -1, 0) DO ECHO ... %%I ...

Output:

0
1
2
3
4
5   <- This is the end of output from the first loop, having gone from 0 to 5
4   <- The second loop starts output here, going from 4 to 0
3
2
1
0