watch.sh

#!/usr/bin/env bash

# watch a file or command output and scroll within
#
# keys => arrow-up/down, page-up/down, pos1, end
#
# usages:
#           swatch.sh -n <timeout_watch> <file|command>
#           swatch.sh <file|command>
#
# version:          1.2 (macOS compatible)
# dependencies:     awk, tput, clear, read
# published:        https://unix.stackexchange.com/questions/3842/how-can-i-scroll-within-the-output-of-my-watch-command

# =============================================
# DEFAULTS
# =============================================
TMPFILE=$(mktemp)
line_show_begin="1"
line_show_begin_last="-1"
console_lines_correction="4"
timeout_watch="2"
timeout_read=".1"
# http://ascii-table.com/ansi-escape-sequences-vt-100.php
ESC_cursor_home='\033[H'

define(){ IFS=$'\n' read -r -d '' "${1}" || true; }
define helptext <<-EOF
  Usage: ${0##*/} [-n <timeout>] '[<command>]'

         timout : in seconds or fractions of seconds [default: ${timeout_watch}s]
         command: any command wished to be observed - best applied in quotes
EOF

__init() {
  trap "fn_console_size_change" SIGWINCH
  trap "fn_quit" INT HUP TERM QUIT EXIT
  fn_console_size_change
  tput civis
}

fn_help() {
  printf "%s\n" "${helptext}"

}

get_options() {
    [[ "$1" == "" ]] && fn_help 1
    while [ -n "$1" ]; do
        case "$1" in
            -h|--help)
                fn_help 0
            ;;
            -n)
                [[ "$2" == "" ]] && { echo "Error: option -n required <timeout>" ; exit 1 ; }
                timeout_watch="$2" && shift
            ;;
            -*) echo "Error: unknown option »$1«"; exit 1; ;;
            *)  command=$1;;
        esac
        shift
    done
    [[ "$command" == "" ]] && { echo "Error: command required" ; exit 1 ; }
}

fn_print_headline() {
    hdl_txt_right="${HOSTNAME}: $(date "+%Y-%m-%d %H:%M:%S")"
    hdl_txt_left="$command, [${timeout_watch}s, $line_show_begin]"
    hdl_txt_left_length=${#hdl_txt_left}
    printf '%s%*s\n\n' "$hdl_txt_left" "$((console_columns - hdl_txt_left_length))" "$hdl_txt_right"
}

fn_print_file() {
    # file length can change during watch execution
    eval "$command" > "$TMPFILE"
    lines_command=$(awk 'END {print NR}' "$TMPFILE")
    line_last=$((lines_command-console_lines))
    (( "$line_last" < "1" )) && { line_last=1; clear; }
    (( "$line_show_begin" > "$line_last" )) && { line_show_begin=$line_last; clear; }

    # print position changed
    if (( "$line_show_begin" != "$line_show_begin_last" )); then
        line_show_begin_last=$line_show_begin
        clear
    else
        printf "$ESC_cursor_home"
    fi

    # print file section
    fn_print_headline
    eval "$command" > "$TMPFILE"
    awk -v var1="$line_show_begin" -v var2="$console_lines" 'NR>=var1 {if (NR>var1+var2) {exit 0} else {printf "%s\n",$0 } }' "$TMPFILE"
}

fn_console_size_change() {
  console_columns=$(tput cols)
  console_lines=$(($(tput lines) - console_lines_correction))
  line_show_begin_last=-1
}

fn_quit() {
  tput cnorm
  tput clear
  exit 0
}

# =============================================
# MAIN
# =============================================
get_options "$@"
__init

while :; do
    fn_print_file
    read -rsn1 -t "$timeout_watch" k # char 1
    case "$k" in
        [[:graph:]]) # Normal input handling
        ;;
        $'\x09') # Routine for selecting current item
        ;;
        $'\x7f') # Routine for back-space
        ;;
        $'\x01') # Routine for ctrl+a
        ;;
        $'\x1b') # ESC
            read -rsn1 k # char 2
            [[ "$k" == ""  ]] && return # Esc-Key
            [[ "$k" == "[" ]] && read -rsn1 -t $timeout_read k # char 3
            [[ "$k" == "O" ]] && read -rsn1 -t $timeout_read k # char 3
            case "$k" in
                A)  (( "$line_show_begin" > "1" )) && ((line_show_begin--)) ;; # Arrow-Up-Key
                B)  (( "$line_show_begin" < "$line_last" )) && ((line_show_begin++)) ;; # Arrow-Down-Key
                H)  line_show_begin=1 ;; # Pos1-Key
                F)  line_show_begin=$line_last ;; # End-Key
                5)  # PgUp-Key
                    read -rsn1 -t $timeout_read k # char 4
                    if [[ "$k" == "~" ]] && (( "$line_show_begin" > "$((console_lines/2))" )) ; then
                        line_show_begin=$((line_show_begin-console_lines/2))
                    else
                        line_show_begin=1
                    fi
                ;;
                6)  # PgDown-Key
                    read -rsn1 -t $timeout_read k # char 4
                    if [[ "$k" == "~" ]] && (( "$line_show_begin" < "$((line_last-console_lines/2))" )) ; then
                        line_show_begin=$((line_show_begin+console_lines/2))
                    else
                        line_show_begin=$line_last
                    fi
                ;;
            esac
            read -rsn4 -t $timeout_read    # Try to flush out other sequences ...
        ;;
    esac
done