Shell – What allows a command to be typed while the previous command is still running in foreground

command lineshellterminal

Sometimes I have seen when one program (say A) is running in the foreground and printing its output to stdout, I can type another command (say B), and when I press Enter it is run, even though I had not been prompted to type B since A had not finished executing yet. I could do this in the tcsh shell and the end result was that B was executed after A.

What is this feature called? Is this shell specific? How does it work without me typing the command at the prompt?

Best Answer

This is called typeahead, and it's not shell specific. What you type ends up being buffered in the terminal, and the next time a program running in the terminal is ready for input it reads what is waiting in the buffer. In your example that program is the shell, so it executes the command you typed as if you'd waited for A to finish before typing it.

Some programs will explicitly clear the buffer before accepting input; for example most programs which ask for a password will clear the buffer, to make sure the user knows what was typed (since the input isn't echoed).

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A somewhat faster version of alex's Ctrl+A Ctrl+K (which moves to the front of the line and then cuts everything forward) is to just use Ctrl+U, which cuts backward on bash, and the entire line (regardless of your current position) on zsh. Then you use Ctrl+Y to paste it again

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It works the way it does because Unix is full-duplex. As Ritchie said in The UNIX Time-sharing System: A Retrospective:

One thing that seems trivial, yet makes a surprising difference once one is used to it, is full-duplex terminal I/O together with read-ahead. Even though programs generally communicate with the user in terms of lines, rather than single characters, full-duplex terminal I/O means that the user can type at any time, even if the system is typing back, without fear of losing or garbling characters. With read-ahead, one need not wait for a response to every line. A good typist entering a document becomes incredibly frustrated at having to pause before starting each new line; for anyone who knows what he wants to say any slowness in response becomes psychologically magnified if the information must be entered bit-by-bit instead of at full speed.

[end quote]

That being said, there are some modern programs that eat up or discard any typeahead; ssh and apt-get are two examples. If you type ahead while they're running, you may find that the first part of your input has disappeared. That could conceivably be a problem.

ssh remotehost do something that takes 20 seconds
mail bob
Dan has retired. Feel free to save any important files and then do
# ssh exits here, discarding the previous 2 lines
rm -fr *
.

Best Answer

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