Sending some input into a process, then resuming input from command line

command lineinputterminal

I have an interactive terminal program, that accepts stdin (telnet for example).

I want to send it some input before interacting with it, like this:

echo "Hello" | telnet somewhere 123

But that only sends in Hello and kills telnet afterwards. How can I keep telnet alive and route input to it?

Best Answer

You can't change what STDIN of telnet is bound to after you start, but you can replace the simple echo with something that will perform more than one action - and let the second action be "copy user input to the target":

{ echo "hello"; cat; } | telnet somewhere 123

You can, naturally, replace cat with anything that will copy from the user and send to telnet.

Keep in mind that this will still be different to just typing into the process. You have attached a pipe to STDIN, rather than a TTY/PTY, so telnet will, for example, be unable to hide a password you type in.

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Terminal – Prevent Terminal Prompt from Being Overwritten by Program Output

This isn't as easy as it sounds to change. It's related to the terminal cooked vs. raw mode and whether echo is enabled or not.

When the terminal is in cooked mode (the default), the kernel reads everything that comes in as input and processes it using rudimentary line editing capabilities which include echoing normal text immediately, processing the erase and kill characters which erase a single character and the whole of the current line respectively, and a few other things. Lines of text only actually appear on the terminal's input when you press enter. During the whole time up until you press enter, everything happens entirely inside the kernel and no process running on the terminal receives a single byte, therefore the foreground application doesn't even know that the user is in the process of typing anything. A process running on the tty cannot suppress this echo even if it wants to just because the echo would come at an inopportune time (e.g. intermixed with output) because such a process is not even aware that the input is happening.

You can set the terminal to raw mode instead with no echo to suppress this (stty raw, or with termios), but then you lose the kernel's line editing capabilities completely — which means for example that you cannot correct a typo by pressing Ctrl-u and starting over. More importantly, you will have a lot of trouble using any program that depends on the kernel's cooked processing (basically anything that doesn't use readline or ncurses) because you will be typing completely blind at such programs!　Oh, and also: without the terminal cooked processing you lose the kernel's interception of job control shortcuts to interrupt and suspend (by default Ctrl-c and Ctrl-z respectively).

How to emulate sending keystrokes through a terminal program

That's typically what expect was written for:

expect -c 'spawn -noecho vi; send "iHello World!\r\33"; interact'

While expect was written for TCL in days prior to perl or python being popular, now similar modules for perl or python are also available.

Another option is to issue TIOCSTI ioctls to your tty device to insert characters (one byte at a time) in its input queue:

perl -le 'require "sys/ioctl.ph";
          ioctl(STDIN, &TIOCSTI, $_) for split "", join " ", @ARGV
         ' $'iHello World!\r\e'; vi

That has the benefit of avoiding an extra pseudo-terminal layer in between your terminal emulator and the application (here vi).

Best Answer

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Terminal – Prevent Terminal Prompt from Being Overwritten by Program Output

How to emulate sending keystrokes through a terminal program

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