Sending data to stdin of another process through linux terminal

问题

I've been trying to send data to stdin of a running process. Here is what I do:

1. In a terminal I've started a c++ program that simply reads a string and prints it. Code excerpt:

   while (true) {
       cin >> s;
       cout << "I've just read " << s << endl;
   }
   

2. I get the PID of the running program

3. I go to /proc/PID/fd/
4. I execute echo text > 0

Result: text appears in the terminal where the program is run. Note, not I've just read text, but simply text. What am I doing wrong and what should I do to get this thing to print 'I've just read text'?

回答1:

When you're starting your C++ program you need to make sure its input comes from a pipe but not from a terminal. You may use cat | myapp to do that. Once it's running you may use PID of your application for echo text > /proc/PID/fd/0

回答2:

It could be a matter of stdout not being properly flushed -- see "Unix Buffering". Or you could be in a different shell as some commentators have suggested.

Generally, it's more reliable to handle basic interprocess communication via FIFOs or NODs -- named pipes. (Or alternatively redirect stdout and/or stderr to a file and read from that with your c++ program.)

Here's some good resources on how to use those in both the terminal and c++.

"FIFO – Named pipes: mkfifo, mknod"
"Using Pipes in Linux Processes"
"Programming with FIFO: mkfifo(), mknod()"

回答3:

FD 0 is the terminal the program is running from. When you write to FD 0, you are writing to the terminal the program is running from. FD 0 is not required to be opened in read-only mode; in practice it seems to be read/write mode, so you can write to it. (I suspect this is because FDs 0, 1 and 2 all refer to the same file description)

So echo text > /proc/PID/fd/0 just echoes text to the terminal.

To pipe input to the program, you would need to write to the other end of the pipe (actually a PTY, which mostly behaves like a pair of pipes). Most likely, whatever terminal emulator you're using (xterm, konsole, gnome-terminal) will have the other end open, so you could try writing to that.

来源：https://stackoverflow.com/questions/37424497/sending-data-to-stdin-of-another-process-through-linux-terminal