Is it possible in a batch file to read from a pipe line by line?

Question

I was wondering if it is possible to read from a pipe in a batch file. If I write:

echo Test

i get, unsurprising, Test. That's nice. But what if I want to pipe the output, and read it from another command?

echo Test | echo ???

How to obtain the same result as before, but through a pipe? Thanks!

EDIT: what I am after really after is this.

I have a list of files, and i need to filter this list with some words that i put, line by line, in a file named filter.txt. So I have to use findstr /g:filter.txt.

But then I need to do something to the list files that matches, and since findstr returns one row for each file, i have to read the matches line by line.

This is how i did it:

dir /b | findstr /g:filter.txt | for /F "delims=" %a in ('more') do del "%a"

SOLUTION:

It looks like that what I wanted to do was not reading from a pipe but just reading the output of another command in a batch file.

To do a single line read, you could use this:

echo Test | ( set /p line= & call echo %%line%%)

or you can use this, that works also with multi line input:

echo Test | for /F "delims=" %a in ('more') do @echo %a

(this trick of using more could be useful in some situations). But in my particular case, the solution is this:

for /F "delims=" %a in ('echo Test') do @echo %a

Thanks to everyone!

Answer 1

Based on this answer https://stackoverflow.com/a/6980605/1630171 it looks like that a way to answer my question is this:

echo Test | for /F "delims=" %a in ('more') do @echo %a

It's a bit weird but it works :)

It only looks a little strange to me that there's no native solution to this... but this does exactly what i want!

Answer 2

For reading a single line, you could also use set /p, but this only works with one line.

echo test | ( set /p line= & call echo %%line%%)

The problem is here, that a pipe creates two new cmd.exe contexts, for each side one.
They run in the same window as the parent cmd.exe, they can't change any variables of the parent cmd, as they are only childs.

That's the cause why this one fails

echo test | set /p line=
echo %line%

line will be set, but it will be destroyed when the pipe ends.

Answer 3

Excuse me, I think there is a confusion here...

You said you want to read from a pipe. A pipe is used to redirect the output of one command into the input of another command; the second command is called filter. For example, in

dir /b | findstr /g:filter.txt

there is a pipe between dir and findstr commands. A pipe is always established between two processes. There is no way to read the data that flow from dir command to findstr command (that is the only pipe that exist here). However, you can read from the output of findstr command.

If we insert an additional filter, the behavior is the same. For example, in

dir /b | findstr /g:filter.txt | more

there are two pipes, but there is no way to read from anyone of them. However, you can read from the output of the last command (more in this case). What is the native Batch solution to read the output of one command? It is the FOR /F command. For example, the native way to get echo command output in:

echo Test | for /F "delims=" %a in ('more') do @echo %a

is:

for /F "delims=" %a in ('echo Test') do @echo %a

Please note that in the first example the %a parameter does NOT get the information from the pipe that exist between echo and for commands, but from the output of more command.

In the same way, the natural method to achieve this task:

dir /b | findstr /g:filter.txt | for /F "delims=" %a in ('more') do del "%a"

is this way:

for /F "delims=" %a in ('dir /b ^| findstr /g:filter.txt') do del "%a"

that process the multi-line output of findstr command.

Second method is not just faster than the former, but it is also clearer because the inclusion of a more command that really do nothing may lead to undesired misconceptions or errors.

Antonio

Answer 4

Building on another answer I saw elsewhere, it is possible to capture the output of a command and store it in a variable without an intermediary file quite simply, so long as it is numeric.

Child processes, like those inside the pipe cannot share their environment variables, but can return a value which is picked up by %errorlevel%. %errorlevel% isn't an environment variable though, and is calculated by the shell every time it is invoked. It also cannot be set normally, and must be set using a child process. Example:

@echo off
echo %errorlevel%
cmd /c exit 56
echo %errorlevel%

Returns:

0
56

Interestingly, you can also do:

@echo off
echo %errorlevel%
cmd /c exit 56 & echo hi
echo %errorlevel%

Returns:

0
hi
56

Which I believe is because the echo hi is run by another child process in turn, which doesn't wait for the exit statement to finish before printing. This might be changed by a race condition though if the text printed is longer, I'm not to sure if the child process (running exit) which is parent to the one printing 'hi' will wait for it's child to exit (or any subsequent children either) before it completes the exit command. I tried to test this with a longer-running command like Tree, but I got a zero returned by a query to %errorlevel% which is probably due to Tree affecting the results by returning 0, possibly after the exit 56.

Anyway, to get back to what most will find useful:

@echo off
echo %errorlevel%
echo 456 | ( set /p line= & call exit %%line%% )
echo %errorlevel%
pause

Returns:

0
456

Here, the 456 printed by echo is captured and returned by subsequent queries to %errorlevel%. You can capture any command's output this way, although it's limited to one numeric value. This is still very useful, but unfortunately doesn't allow you to store textual output and I can't think of a way to make it work for multi line output either. (unexplored)

I think in theory you can chain as many commands as you want, and should be able to use && to force the order in which they run. I don't know how or if this can be used to capture multiple lines of input or allow the return of text, but should provide some additional wiggle room inside the pipe by providing nested child processes sharing their environment down and possibly return value up. (again untested, perhaps try multiple exit statements or something, and if I learn anything later I'll try to post it here)