8. Redirection & Pipes

What & why

Redirection sends command output to a file instead of the screen; pipes send it to another command. Batch scripts use these constantly — to build files, suppress noise, capture results, and log. The stream numbers (1 for stdout, 2 for stderr) are where it gets fiddly.

The three streams

Number	Name	Default
0	stdin	keyboard
1	stdout	screen
2	stderr	screen

Writing to a file

echo Starting > run.log            &rem > overwrites (truncates first)
echo Step 1 >> run.log             &rem >> appends
dir /b *.txt >> run.log

> always starts fresh; >> adds to the end (and creates the file if missing).

The null device

nul is a magic name that discards everything written to it:

ping -n 1 example.com >nul                &rem hide normal output
copy a.txt b.txt >nul                     &rem hide "1 file(s) copied."
where git >nul 2>nul && echo git present  &rem hide both streams, act on success

Redirecting stderr

somecommand 2> errors.log                 &rem stderr to a file, stdout still on screen
somecommand 2>nul                          &rem discard errors only
somecommand > out.log 2> err.log           &rem split the streams
somecommand > out.log 2>&1                  &rem BOTH streams into out.log
somecommand 1>&2                            &rem send stdout to stderr

2>&1 means "make stream 2 go wherever stream 1 currently goes." Order matters: > out.log 2>&1 works (redirect 1 to the file, then point 2 at the same place). 2>&1 > out.log does not combine them (2 gets pointed at the screen, then 1 moves to the file).

Writing errors from your own script

echo ERROR: config not found 1>&2
exit /b 1

Sending your error messages to stderr lets callers separate them from real output.

Reading from a file: <

rem Feed a file to a command's stdin:
sort < unsorted.txt > sorted.txt

rem Answer an interactive prompt:
del *.tmp < nul

< nul feeds an immediate end-of-input, which makes some prompting commands take their default or bail — occasionally useful, but prefer a real /q switch.

Pipes

dir /b /s *.log | find /c /v ""          &rem count lines (= file count)
tasklist | findstr /i "chrome"           &rem filter process list
type big.txt | more                       &rem page through output
echo %PATH% | find /i "python"            &rem is python on PATH?

The left command's stdout becomes the right command's stdin. stderr is not piped unless you add 2>&1 on the left.

Each side of a pipe runs in its own cmd.exe

something | something spawns two subshells. Consequences:

• Variables set inside a piped command do not survive: echo x | set /p "v=" — v is set in the subshell and lost.
• Delayed-expansion state and setlocal don't carry across.
• It's slower than it looks (two process launches).

Workarounds are in Intermediate Lesson 3 (for /F to capture output) and Advanced Lesson 6.

Capturing command output into a variable

You can't do set v=$(cmd) like bash. Use for /F:

for /F "delims=" %%v in ('hostname') do set "host=%%v"
echo Running on %host%

for /F "tokens=2 delims=:" %%v in ('ipconfig ^| findstr /i "IPv4"') do set "ip=%%v"
echo IP:%ip%

Note the ^| — inside the for /F ('...') quotes, pipes and redirects must be caret-escaped.

Combining and grouping

(echo Line 1 & echo Line 2 & echo Line 3) > file.txt

(
    echo [config]
    echo debug=1
    echo path=%CD%
) > app.ini

command1 & command2                       &rem run both, unconditionally
command1 && command2                      &rem run command2 only if command1 succeeded
command1 || command2                      &rem run command2 only if command1 failed

Special characters and escaping

To write a literal >, <, |, & to a file, escape with ^:

echo Redirect with ^> and pipe with ^| >> notes.txt

Inside for /F '...' command blocks, the same applies to |, >, <.

Common mistakes

• 2>&1 before the > — doesn't merge the streams. Put 2>&1 last.
• Space in > >file or between 2 and > (2 > file) — the 2 gets treated as an argument. Write 2>file or 2> file (space after > is fine, not before).
• Expecting set inside a pipe to stick — it runs in a subshell.
• Piping when you meant to capture — use for /F to get output into a variable.
• > a file that's open/locked — "The process cannot access the file." Close it, or write elsewhere.
• Trailing space before >>: echo hi >> log writes hi (with the space). Use echo hi>>log or accept the space.
• >nul not hiding errors — that's stdout only; add 2>nul.

Exercises

1. Run a command that fails (e.g. dir Z:\nope) and send only its error text to errors.log, keeping stdout on screen. Then send both to one file.
2. Build report.txt in one grouped ( ... ) > report.txt block: a title line, a blank line, and the output of date /t and time /t.
3. Count how many running processes contain "svchost" using tasklist | findstr /i piped into find /c /v "".
4. Capture the output of hostname into a variable with for /F and print This is <name>.
5. Capture your IPv4 address into a variable (ipconfig ^| findstr /i IPv4, split on :), trimming the leading space.
6. Append a timestamped line to run.log every time the script runs: echo %date% %time% run>>run.log.

Recap

• > overwrites, >> appends, nul discards.
• Streams: 1=stdout, 2=stderr. 2>&1 merges — and must come after the >.
• < feeds stdin from a file. | pipes stdout to the next command's stdin.
• Each pipe side is a separate cmd.exe — set inside a pipe is lost.
• Capture output into a variable with for /F "..." %%v in ('command') do set "x=%%v" (escape |/> as ^|/^> inside).
• Group with ( ... ); chain with &, &&, ||.

Next: Subroutines with call & goto →