Question

I have an old shell script which needs to be moved to bash. This script prints progress of some activity and waits for user’s commands. If no action is taken by user for 15 seconds screen is redrawn with new progress and timer starts again. Here’s my problem:

I am trying to use read -t 15 myVar – this way after 15 seconds of waiting loop will be restarted. There is however a scenario which brings me a problem:

• screen redrawn and script waits for input (prints ‘Enter command:‘)
• user enters foo but doesn’t press enter
• after 15 seconds screen is again redrawn and script waits for input – note, that foo is not displayed anywhere on the screen (prints ‘Enter command:‘)
• user enters bar and presses enter

At this moment variable $myVar holds ‘foobar‘.

What do I need? I am looking for a way to find the first string typed by user, so I could redisplay it after refreshing status. This way user will see:
Enter command: foo

On Solaris I could use stty -pendin to save input into some sort of a buffer, and after refresh run stty pendin to get this input from buffer and print it on a screen.

Is there a Linux equivalent to stty pendin feature? Or maybe you know some bash solution to my problem?

Answer 1

OK, I think I have the solution. I took nhed’s proposition and worked a bit on it 🙂

The main code prints some status and waits for input:

while :
do
    # print the progress on the screen
    echo -n "Enter command: "
    tput sc # save the cursor position
    echo -n "$tmpBuffer" # this is buffer which holds typed text (no 'ENTER' key yet)
    waitForUserInput
    read arguments <<< $(echo $mainBuffer) # this buffer is set when user presses 'ENTER'
    mainBuffer="" # don't forget to clear it after reading
    # now do the action requested in $arguments
done

Function waitForUserInput waits 10 seconds for a keypress. If nothing typed – exits, but already entered keys are saved in a buffer. If key is pressed, it is parsed (added to buffer, or removed from buffer in case of backspace). On Enter buffer is saved to another buffer, from which it is read for further processing:

function waitForUserInput {
    saveIFS=$IFS # save current IFS
    IFS="" # change IFS to empty string, so 'ENTER' key can be read

    while :
    do
        read -t10 -n1 char
        if (( $? == 0 ))
        then
            # user pressed something, so parse it
            case $char in
                $'\b')
                    # remove last char from string with sed or perl
                    # move cursor to saved position with 'tput rc'
                    echo -n "$tmpBuffer"
                    ;;
                "")
                    # empty string is 'ENTER'
                    # so copy tmpBuffer to mainBuffer
                    # clear tmpBuffer and return to main loop
                    IFS=$saveIFS
                    return 0
                    ;;
                 *)
                    # any other char - add it to buffer
                    tmpBuffer=$tmpBuffer$char
                    ;;
            esac
        else
            # timeout occured, so return to main function
            IFS=$saveIFS
            return 1
        fi
    done
}

Thanks to all of you for your help!