Unbuffered shell-input

Posted 10-16-2021 at 01:46 AM by Michael Uplawski
Updated 11-13-2021 at 12:43 AM by Michael Uplawski (Wrong title, much misunderstanding and ill-chosen search criteria.)

There are solutions on the Web.

Having read some of these, I prefer a Ruby-Oneliner “readNB” which returns the 1 character that the user has had a chance to enter on the keyboard, before readNB returns.

---------------------- In the meantime -------------------------
GazL has – in the very first comment on this blog, below – presented a better solution with the shell built-in command read: read accepts some parameters and a call like

Code:

read -r -s -N1 [VARIABLE]

does the same as my ruby-statement. Details are found in the online-help to read (read --help)
-----------------------------------------------------------------------

In the script I call another routine “wait_for_user”, which I use in my ruby-scripts to provide non-blocking keyboard-input:

Code:

require 'io/wait'
require 'io/console'

# unblocking read from STDIN

def wait_for_user()
  	char = nil
        STDIN.raw do 
                STDIN.noecho do
                        until (STDIN.ready?)
                                sleep(0.1)
                        end

                        char = (STDIN.read_nonblock(1).ord rescue nil)
                end
        end
  return char
end

The oneliner:

Code:

puts wait_for_user.chr

One exemplary use of this in a shell script (bash):

Code:

#!/bin/bash

vim -R -M -p ~/.procmail/log

echo -e "\n\tempty log (N/y)?"
resp=`readNB`

if [ "$resp" == 'y' -o "$resp" == 'Y' ]
then
  echo -e "\tdeleting log..."
  cat /dev/null > ~/.procmail/log
else
 echo -e "\tokay, doing nothing."
fi

Posted in Wisdom caught up with me, though I am running fast.

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Comments

	I may be misunderstanding your requirements, do you really mean non-blocking, or do you mean non-line-buffered? If the latter, will read -r -s -N1 not serve your needs?
	Posted 10-18-2021 at 04:19 AM by GazL

Quote:

Originally Posted by GazL

I may be misunderstanding your requirements, do you really mean non-blocking, or do you mean non-line-buffered?

As far as I remember, my search for a solution produced examples with non-blocking input. So I considered this the correct terminology. I understand unbuffered just as well an cannot say why I would prefer one expression over the other. This is a language problem and I will correct my blog post, when I find better translations. Thank you for pointing it out.

Quote:

If the latter, will read -r -s -N1 not serve your needs?

It does. Believe it or not, I would have expected that the built-in read command were mentioned in the man-page to Bash, where it is not, and thus executed read --help for the very first time, this evening.

Thanks again,
Michael

Posted 10-18-2021 at 03:15 PM by Michael Uplawski Michael Uplawski is offline

	The non-blocking/unbuffered confusion is due to my reading English texts which only vaguely evoked the documentation that I had in German.
	Posted 10-19-2021 at 12:46 AM by Michael Uplawski

	No worries on the language, I just wanted to confirm what it was you were actually looking for. Within the context of the tty/terminal driver you commonly see it referred to a 'raw' vs 'cooked' input rather than the more verbose "non-line-buffered" I used.
	Posted 10-19-2021 at 05:27 AM by GazL