Bash: non blocking read inside a loop

bashreadshell-script

I have a following small bash script

var=a

while true :
do
    echo $var
    sleep 0.5
    read -n 1 -s var
done

It just prints the character entered by the user and waits for the next input. What I want to do is actually not block on the read, i.e. every 0.5 second print the last character entered by the user. When user presses some key then it should continue to print the new key infinitely until the next key press and so on.

Any suggestions?

Best Answer

From help read:

  -t timeout    time out and return failure if a complete line of input is
        not read within TIMEOUT seconds.  The value of the TMOUT
        variable is the default timeout.  TIMEOUT may be a
        fractional number.  If TIMEOUT is 0, read returns immediately,
        without trying to read any data, returning success only if
        input is available on the specified file descriptor.  The
        exit status is greater than 128 if the timeout is exceeded

So try:

while true
do
    echo "$var"
    IFS= read -r -t 0.5 -n 1 -s holder && var="$holder"
done

The holder variable is used since a variable loses its contents when used with read unless it is readonly (in which case it's not useful anyway), even if read timed out:

$ declare -r a    
$ read -t 0.5 a
bash: a: readonly variable
code 1

I couldn't find any way to prevent this.

Related Solutions

BASH function to read user input OR interrupt on timeout

read has a parameter for timeout, you can use:

read -t 3 answer

If you want read to wait for a single character (whole line + Enter is default), you can limit input to 1 char:

read -t 3 -n 1 answer

After proper input, return value will be 0, so you can check for it like this:

if [ $? == 0 ]; then
    echo "Your answer is: $answer"
else
    echo "Can't wait anymore!"
fi

I guess there is no need to implement background jobs in your situation, but if you want to, here is an example:

#!/bin/bash

function ProcessA() {
    sleep 1  # do some thing
    echo 'A is done'
}

function ProcessB() {
    sleep 2  # do some other thing
    echo 'B is done'
}

echo "Starting background jobs..."

ProcessA &  # spawn process "A"
pid_a=$!    # get its PID

ProcessB &  # spawn process "B"
pid_b=$!    # get its PID too

echo "Waiting... ($pid_a, $pid_b)"
wait  # wait for all children to finish

echo 'All done.'

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

Best Answer

Related Solutions

BASH function to read user input OR interrupt on timeout

Bash – How to Read Special Keys

Related Question