IO Redirection – Difference Between Cat and ‘>’ to Zero Out a File

The cat file | command syntax is considered a Useless Use of Cat. Of all your options, it takes a performance hit because it has to spawn another process in the kernel. However insignificant this may turn out to be in the big picture, it's overhead the other forms don't have. This has been covered on questions such as: Should I care about unnecessary cats?

Between the other two forms there are virtually no performance differences. STDIN is a special file node that the process has to open and read just like any other. Passing a file name instead of STDIN just makes it open a different file.

The difference would be in what features / flexibility you are looking for.

• Passing the file name to the program would mean the input file was seekable. This may or may not matter to the program but some operations can be sped up if the stream is seekable.
• Knowing the actual input file allows your program to potentially write to it. For example sed -i for in-place editing. (Note: since this has to create a new file behind the scenes it's not a performance gain over other redirects but it is a convenience step.)
• Using shell redirects gives you the ability to concatenate multiple files or even use process redirection. sed [exp] < file1 file2 or even sed [exp] < <(grep command). Details of this use case can be found on this question: Process substitution and pipe

It's the fd to the master side of the pseudo-terminal in the terminal emulator that you want to monitor if you want to see what's displayed on it. That master fd is what simulates the wire that goes to a real terminal. What xterm writes on it is the characters generated from the key you press. What it reads from it is what it displays.

For instance, on Linux:

$ lsof -ac xterm /dev/ptmx
COMMAND   PID     USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
xterm   15173 chazelas    4u   CHR    5,2      0t0 2131 /dev/ptmx

And then run for instance:

stty -echo -opost
strace -e read -e read=4 -p15173 2>&1 | stdbuf -o0 sh -c '
  grep "^ |" | cut -b11-60 | tr -d " " | xxd -r -p'

Of course, it works better if you run that in a terminal of same type and size as the one you're trying to monitor. You can get the size with:

stty size < /dev/pts/that-terminal

That dumps what is read by xterm from the master side of the terminal, so what is displayed there, including the local echo of what is being typed.

The -e read=4 above is for strace to output a hexdump of what xterm reads on its fd 4. The rest of the command is to convert that to the actual characters. I tried peekfd -n -8 15173 4 but for some reason that only gave what was being written.

We're using -opost to disable any post-processing in our monitoring terminal, so that everything xxd writes to the slave side makes it unchanged to our master side, so that our monitoring xterm gets the same thing as the monitored one. -echo is so that if the application in the monitored terminal sends an escape sequence that requests an answer from the terminal (such as those that request the cursor position or the terminal type or window title), that will make its way to our monitoring xterm and our xterm will reply as well. We don't want a local echo of that.

You could also monitor what is being typed by tracing the write system calls to that same fd (replace read with write above). Note that upon pressing Enter, the terminal emulator sends a CR character, not LF. Also, since we're tracing on the master side, if the user types a<Backspace>b, we'll see all 3 keystrokes even if the terminal device is in canonical mode.

As to why yours doesn't work:

tee /dev/pts/user_pts </dev/pts/user_pts

Reading from the terminal device is reading the user input, and writing to it is to display it to the user.

You're telling tee to read from the terminal device. So what it reads (the user input) won't be read by the application(s) running in the terminal (and vis versa, tee and that application will fight for the terminal input). Writing to the terminal device, is for display there, it is not for putting it back there as input. When you do

echo test

(with echo's stdout being the terminal), it is not the same thing as if you had typed test.

There is an ioctl (TIOCSTI) to put characters back as input, but even that would not really work because you could put it back after the application as already read some more, so it would change the order the application is reading input, and any way, that would mean you would read it over and over again.