FOR

Conditionally perform a command several times.

 syntax-FOR-Files
       FOR %%parameter IN (set) DO command 
   
 syntax-FOR-Files-Rooted at Path   
       FOR /R [[drive:]path] %%parameter IN (set) DO command 
   
 syntax-FOR-Folders
       FOR /D %%parameter IN (folder_set) DO command 
   
 syntax-FOR-List of numbers   
       FOR /L %%parameter IN (start,step,end) DO command 
   
 syntax-FOR-File contents   
       FOR /F ["options"] %%parameter IN (filenameset) DO command 
   
       FOR /F ["options"] %%parameter IN ("Text string to process") DO command
   
 syntax-FOR-Command Results 
       FOR /F ["options"] %%parameter IN ('command to process') DO command

The operation of the FOR command can be summarised as...

• Take a set of data
• Make a FOR Parameter %%G equal to some part of that data
• Perform a command (optionally using the parameter as part of the command).
• Repeat for each item of data

If you are using the FOR command at the command line rather than in a batch program, specify %parameter instead of %%parameter.

FOR Parameters

The first parameter has to be defined using a single character, I tend to use the letter G.

e.g. FOR %%G IN ...

In each iteration of a FOR loop, the IN ( ....) clause is evaluated and %%G set to a different value

If this results in a single value then %%G is set equal to that value and the command is performed.

If this results in a multiple values then extra parameters are implicitly defined to hold each. These are automatically assigned in alphabetical order %%H %%I %%J ...(implicit parameter definition)

Also if the parameter refers to a file, you can use an enhanced variable reference to quickly extract the filename/path/date/size.

Example

FOR /F %%G IN ("This is a long sentence") DO @echo %%G %%H %%J

will result in the output

This is long

You can of course pick any letter of the alphabet other than %%G.

%%G is a good choice because it does not conflict with any of the pathname format letters (a, d, f, n, p, s, t, x) and provides the longest run of non-conflicting letters for use as implicit parameters.
G > H > I > J > K > L > M

Using variables correctly
Environment variables within a FOR loop are expanded at the beginning of the loop and won't change until AFTER the end of the DO section.
The following example counts the files in the current folder, but %count% always returns 1:

@echo off
SET count=1
 FOR /f "tokens=*" %%G IN ('dir /b') DO (
 echo %count%:%%G
 set /a count+=1)

To make this work correctly we must force the variable %count% to be evaluated during each iteration, using the CALL :subroutine mechanism:

@echo off
SET count=1
FOR /f "tokens=*" %%G IN ('dir /b') DO (call :s_do_sums "%%G")
GOTO :eof

:s_do_sums
 echo %count%:%1
 set /a count+=1
 GOTO :eof

Nested FOR commands

FOR commands can be nested FOR %%G... DO (for %%U... do ...)
when nesting commands choose a different letter for each part. you can then refer to both parameters in the final DO command.

If Command Extensions are disabled, the FOR command will only support the basic syntax with no enhanced variables:
FOR %%parameter IN (set) DO command [command-parameters]

"Those who cannot remember the past are condemned to repeat it" - George Santayana

Related:

FOR - Loop through a set of files in one folder
FOR /R - Loop through files (recurse subfolders)
FOR /D - Loop through several folders
FOR /L - Loop through a range of numbers
FOR /F - Loop through items in a text file
FOR /F - Loop through the output of a command
FORFILES - Batch process multiple files
GOTO - Direct a batch program to jump to a labelled line
IF - Conditionally perform a command
Powershell: ForEach-Object - Loop for each object in the pipeline
Equivalent bash command (Linux): for var in [list]; do - Expand list, and execute commands