for /f in cmd: read a file line by line or capture a command's output

for /f reads text line by line and splits each line into tokens. The source can be a file (in (file.txt)), a command (in ('command')) or a literal string (in ("text")). Options in quotes control parsing: tokens=1,2 picks fields, delims=, sets the separators (default: space and tab), skip=1 skips header lines. In a batch file the loop variable is written %%a; at the prompt it is %a.

for /f "skip=1 tokens=1,2 delims=," %%a in (data.csv) do echo %%a has %%b
for /f "delims=" %%l in ('git rev-parse --short HEAD') do set "COMMIT=%%l"

Examples

> data.csv echo name,qty
>> data.csv echo apple,3
>> data.csv echo pear,5
for /f "skip=1 tokens=1,2 delims=," %%a in (data.csv) do echo %%a has %%b
for /f "tokens=2" %%w in ("alpha beta gamma") do echo second word: %%w
for /f "tokens=1,*" %%a in ("key rest of the line") do echo key=[%%a] rest=[%%b]
for /f "delims=" %%v in ('cmd /c echo captured output') do set "RESULT=%%v"
echo RESULT=%RESULT%

apple has 3
pear has 5
second word: beta
key=[key] rest=[rest of the line]
RESULT=captured output

Note the redirection written first (> data.csv echo apple,3): echo apple,3>> data.csv would be read as "redirect stream 3", and the 3 would be lost.

Options

With tokens=1,2, the first token goes to %%a, the second to %%b (the next letters). Empty lines are always skipped.

Paths with spaces and commands with quotes

for /f "usebackq delims=" %%l in ("C:\My Data\list.txt") do echo %%l
for /f "usebackq delims=" %%l in (`where "node.exe"`) do echo found %%l

Without usebackq, ("C:\My Data\list.txt") is a literal string, not a file.