Bash helper for displaying the last lines of an output continuously without cluttering the entire screen like `tail -f`.

ntail

#!/bin/bash
# Display last N lines of input like tail, but cleaning the screen before every update.
# Example: date; for i in $(seq 1 2000); do echo $i; sleep 0.03; done | ntail 10

function ntail {

    # default to 10 lines of tail output
    NUM_LINES=${1:-10}

    # gets the current time in milliseconds
    function mstime() {
        date +%s%3N
    }

    LAST_UPDATE=$(mstime)   # last time the screen was updated
    NEEDS_REFRESH=false     # whether to refresh the screen
    SCREEN_BUFFER_SIZE=0    # number of lines on the screen
    while IFS= read -r NEW_LINE; do

        # concatenate new the new line to the buffer
        TAIL_BUFFER="$TAIL_BUFFER$NEW_LINE"$'\n'

        # if last update is greater than 100ms, refresh screen
        if [ $(($(mstime) - LAST_UPDATE)) -gt 100 ]; then
            NEEDS_REFRESH=true
        fi

        # refresh screen if needed
        if [ "$NEEDS_REFRESH" = true ]; then

            # reduce buffer size to last NUM_LINES lines
            TAIL_BUFFER=$(echo "$TAIL_BUFFER" | tail -n "$NUM_LINES")$'\n'

            # clear the last SCREEN_BUFFER_SIZE lines, preserving the stdout above that
            for _ in $(seq 1 "$SCREEN_BUFFER_SIZE"); do
                printf "\033[1A\033[2K"
            done

            # print the new buffer
            printf "%s" "$TAIL_BUFFER"

            SCREEN_BUFFER_SIZE=$(echo "$TAIL_BUFFER" | wc -l)
            SCREEN_BUFFER_SIZE=$((SCREEN_BUFFER_SIZE - 1))
            LAST_UPDATE=$(mstime)
            NEEDS_REFRESH=false

        fi

    done < /dev/stdin
}

ntail "$@"