Print currently pressed keys to stdout and read them line by line

keyboardread

I want to know which keys are pressed on my keyboard and print the information to stdout.

A tool that can do this is showkey. However, if I want to pass the data of showkey to read:

while read line; do
  echo "$line" | otherprog
done <`showkey -a`

showkey -a | while read line; do
  echo "$line" | otherprog
done

Then showkey waits until a sum of 140 characters is typed in and then sends the buffered information to read.

showkey -a prints the pressed keys line by line, without any buffering.

Why does it buffer?
How do I avoid this buffering, so that I can read showkey's output truly line by line?
Is there an alternative to showkey?
Is there a file I can read the pressed keys directly from?
What is the correct way to pass data to read?

Solution:

I've used lornix's solution and included it into my simple keyboard keyboard :D!

stdbuf -o0 showkey -a | while read line; do
  perl -e 'print sprintf "%030s\n",shift' "$line" | aplay &> /dev/null &
done

Lasership version:

#!/bin/bash
MP3=(); for i in mp3/*.mp3; do MP3+=("$i"); done
NMP3=${#MP3[@]}
stdbuf -o0 showkey -a 2>/dev/null | while read line; do
    [ -z "$line" ] || ! [[ $line =~ ^[0-9] ]] && continue
    NUM="$(echo "$line" | awk '{print $2}')"
    mplayer "${MP3[$(($NUM % $NMP3))]}" &>/dev/null &
done

In the same folder, download some laser mp3 files into a folder called mp3.

Best Answer

Try setting showkey output to non-buffering with the stdbuf command:

stdbuf -o0 showkey -a | cat -

Will show the output as keys are pressed, rather than buffering a line.

stdbuf can adjust the buffering of stdin, stdout and stderr, setting them to none, line buffered, or block buffered, with a choosable block size. Very handy.

Related Solutions

Bash – How to Read Special Keys

What you are missing is that most terminal descriptions (linux is in the minority here, owing to the pervasive use of hard-coded strings in .inputrc) use application mode for special keys. That makes cursor-keys as shown by tput and infocmp differ from what your (uninitialized) terminal sends. curses applications always initialize the terminal, and the terminal data base is used for that purpose.

dialog has its uses, but does not directly address this question. On the other hand, it is cumbersome (technically doable, rarely done) to provide a bash-only solution. Generally we use other languages to do this.

The problem with reading special keys is that they often are multiple bytes, including awkward characters such as escape and ~. You can do this with bash, but then you have to solve the problem of portably determining what special key this was.

dialog both handles input of special keys and takes over (temporarily) your display. If you really want a simple command-line program, that isn't dialog.

Here is a simple program in C which reads a special key and prints it in printable (and portable) form:

#include <curses.h>

int
main(void)
{   
    int ch;
    const char *result;
    char buffer[80];

    filter();
    newterm(NULL, stderr, stdin);
    keypad(stdscr, TRUE);
    noecho();
    cbreak();
    ch = getch();
    if ((result = keyname(ch)) == 0) {
        /* ncurses does the whole thing, other implementations need this */
        if ((result = unctrl((chtype)ch)) == 0) {
            sprintf(buffer, "%#x", ch);
            result = buffer;
        }
    }
    endwin();
    printf("%s\n", result);
    return 0;
}

Supposing this were called tgetch, you would use it in your script like this:

case $(tgetch 2>/dev/null) in
KEY_UP)
   echo "got cursor-up"
   ;;
KEY_BACKSPACE|"^H")
   echo "got backspace"
   ;;
esac

Read file line by line and remember the last position in file

If you want to get the matches then you don't need to be using a loop at all. It would be much faster to just use a single grep command:

grep -Ff input_strings service.log > results.txt

That said, if you want to do literally what you stated in your question, then you could use a variable to keep track of the line on which the last match was found:

LINE_NUMBER=0
while read LINE; do

    # Search for the next match starting at the line number of the previous match
    MATCH="$(tail -n+${LINE_NUMBER} "service.log" | grep -n "${LINE}" | head -n1)";

    # Extract the line number from the match result
    LINE_NUMBER="${MATCH/:*/}";

    # Extract the matching string from the match result
    STRING="${x#*:}";

    # Output the matching string
    echo "${STRING}";

done < input_strings.txt > result.txt

Solution:

Best Answer

Related Solutions

Bash – How to Read Special Keys

Read file line by line and remember the last position in file

Related Question