Bash: read stdin from file and write stdout to file

Question

I'm trying to run an app (let's say top) so it will read from a file for stdin and write to another file from stdout.

Currently I have

mkfifo stdin.pipe
(tail -f stdin.pipe) | top

which works as expected, as I can then echo something to that file and top will receive it. But I'm unable to redirect the output of top. How can I achieve this?

EDIT:

Ok, let's scratch top. I'm testing with this:

cat test.sh

echo Say something
read something
echo you said $something

Are you sure top reads from stdin? Mine doesn't. You can redirect output from top using the -b (batch) option: top -n 1 -b > gash — cdarke
– cdarke, Commented Mar 28, 2013 at 11:35
@cdarke I'm sorry, you're right. I was making tests with another app. top doesn't work the same way. Is there a way I can map stdin and stdout to files and use them to control a cli app? — Alexander Nestorov
– Alexander Nestorov, Commented Mar 28, 2013 at 11:38

cdarke · Accepted Answer · 2013-03-28 11:44:31Z

54

Let's forget about top, that appears to be a red herring.

To map stdin or stdout to files, you can use redirection:

some_program < input_file          # Redirects stdin

another_program > output_file      # Redirects stdout

or even:

yet_another  < input_file > output_file

answered Mar 28, 2013 at 11:44

cdarke

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6 Comments

Alexander Nestorov Over a year ago

Is this async or sync? I just tested it with a 2 lines bash that will say "Hi", wait for input and then say "Bye". I was able to see the output in the output_file after I redirected something to the input_file (aka, when the app finished running)

cdarke Over a year ago

If you are commuicating between programs then use an anonymous pipe, it will be more efficient than using a temporary file. Depends what you mean by sync/async in this context, but all shell IO is buffered. The buffer is flushed and the file is closed after each redirection, but you can get issues if a program is reading while one is writing (to a file, not a pipe).

Alexander Nestorov Over a year ago

The problem is that I'm trying to control a CLI app from PHP so I can't save the handle to the pipe, that's why I need to be able to read/write to/from files. How can I force output to be flushed instead of going to buffer?

cdarke Over a year ago

Use a named pipe instead of a file, you can redirect to a named pipe just as if it was a file (um, actually, it is a file). You can also read a named pipe from PHP just as if it was a text file.

Alexander Nestorov Over a year ago

are you sure that this will work with the example script I posted in my question? (the edit part). I just created mkfifo in and out and I ran my script. I expected to get Say something after cat out but I didn't get anything.

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Joni · Accepted Answer · 2013-03-28 12:49:50Z

8

Is there a way I can map stdin and stdout to files and use them to control a cli app?

It sounds like you are looking for coprocesses, added to Bash in 4.0.

coproc cat                    # Start cat in background
echo Hello >&${COPROC[1]}     # Say "Hello" to cat
read LINE <&${COPROC[0]}      # Read response
echo $LINE                    # cat replied "Hello"!

Before 4.0 you had to use two named pipes to achieve this.

answered Mar 28, 2013 at 12:49

Joni

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1 Comment

hesham_EE Over a year ago

Interesting addition!

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