watch + tail = peek: show command output in a limited scrolling region.

peek

#!/bin/bash

DESCRIPTION='watch + tail = peek: show command output in a limited scrolling region.'
VERSION=1.0.4
AUTHOR='@literallylara'
LICENSE='MIT'
WEBSITE='https://codeberg.org/literallylara/scripts/src/branch/main/utility/peek'

# Documentation:
# man console_codes
# man ascii
# man terminfo
# man bash

# ECMA-48 Select Graphic Rendition
SGR_RESET=$'\033[0m'
SGR_REVERSE_VIDEO_ON=$'\033[7m'
SGR_REVERSE_VIDEO_OFF=$'\033[27m'
SGR_COLOR_DEFAULT_FG=$'\033[39m'
SGR_COLOR_RED_FG=$'\033[31m'
SGR_COLOR_GREEN_FG=$'\033[32m'
SGR_COLOR_YELLOW_FG=$'\033[33m'

# Script settings
CHAR_RUNNING='⋯'
CHAR_SUCCESS='✓'
CHAR_ERROR='×'
OUTPUT_PREFIX="${SGR_COLOR_YELLOW_FG}${SGR_REVERSE_VIDEO_ON} ${SGR_REVERSE_VIDEO_OFF}${SGR_COLOR_DEFAULT_FG} "

#########################
### UTILITY FUNCTIONS ###
#########################

trunc() {
    n="$1"
    t="$2"
    if [ "${#t}" -le "$n" ]; then echo "$t"; else echo "${t:0:n-1}…"; fi
}

# Cursor position report (CPR)
# - Command  : ESC[6n
# - Response : ESC[y;xR
# man console_codes
term_get_cursor_pos() {
    # The `read -p` trick comes from Stack Overflow (see blow).
    # It ensures that `read` does not block when executed from within a script.
    # https://unix.stackexchange.com/questions/88296/get-vertical-cursor-position/183121#183121
    # -s  = silent
    # -dR = delimiter set to R
    # -p  = prompt
    # IFS = internal field separator(s)
    # _   = discarded variable (will be 'ESC')
    IFS='[;' read -r -s -dR -p $'\e[6n' _ row col

    echo "$row $col"
}

# Equivalent to: tput cup
# See: man console_codes
term_set_cursor_pos() {
    row=$1
    col=$2
    # ESC[y;xH, where y = row, x = column
    printf '\033[%d;%dH' "$((row + 1))" "$col" >"$(tty)"
    # tput cup $1 $2
}

# Equivalent to: tput csr
# See: man console_codes
term_set_scroll_region() {
    top=$1
    bottom=$2
    # ESC[t;br, where t = top, b = bottom
    printf '\033[%d;%dr' "$((top + 1))" "$((bottom + 1))" >"$(tty)"
    # tput csr $1 $2
}

term_del_from_cursor_to_end() {
    printf "\033[J" >"$(tty)"
    # tput ed
}

# Equivalent to: stty size
term_get_size() {
    if stty size 2>/dev/null; then
        : # already printed to stdout
    elif command -v tput >/dev/null; then
        echo "$(tput lines) $(tput cols)"
    else
        echo 'Missing `stty size` or `tput` command'
    fi
}

# Equivalent to tput cuu
# See: man console_codes
term_move_cursor_up() {
    # ESC[NA, where N = number of rows to move up
    printf '\033[%dA' "$((MAX_LINES + 1))" >"$(tty)"
}

# Dynamic globals
cols=$(term_get_size | cut -d' ' -f2)
MAX_TITLE_LENGTH=$((cols - 4))
MAX_LINE_LENGTH=$((cols - 2))
SCRIPT_NAME="$(basename "$0")"

print_help_and_exit() {
    echo "Usage: $SCRIPT_NAME [OPTIONS] -- COMMAND"
    echo ''
    echo 'Options:'
    echo '  -h, --help       print help and exit'
    echo '  -t, --title      header title to display. Defaults to COMMAND'
    echo '  -n, --max-lines  maxmimum number of COMMAND output lines to show at once'
    echo '  -o, --out        output file to write full COMMAND output to'
    echo "  -v, --version    print version and exit"
    echo ''
    echo 'Example:'
    echo "  $SCRIPT_NAME -t 'Counting to 100' -- sh -c 'for i in {1..100}; do echo \$i; sleep .01; done'"
    exit
}

# argument parsing
for arg in "$@"; do
    shift
    case "$arg" in
    '-h' | '--help') print_help_and_exit ;;
    '-t' | '--title') TITLE="$1" ;;
    '-n' | '--max-lines') MAX_LINES="$1" ;;
    '-o' | '--out') OUTFILE="$1" ;;
    '-v' | '--version')
        echo "$SCRIPT_NAME $VERSION - $DESCRIPTION"
        echo "License : $LICENSE"
        echo "Author  : $AUTHOR"
        echo "Website : $WEBSITE"
        exit
        ;;
    '--') break ;;
    esac
done

# argument defaults
if [ -z "$MAX_LINES" ] || [ "$MAX_LINES" = 0 ]; then MAX_LINES=5; fi
if [ -z "$TITLE" ]; then TITLE="$(trunc $MAX_TITLE_LENGTH "$*")"; fi
if [ -z "$OUTFILE" ]; then OUTFILE='/dev/null'; fi

print_header() {
    if [ -z "$1" ]; then
        color=$SGR_COLOR_YELLOW_FG
        status=$CHAR_RUNNING
    elif [ "$1" = 0 ]; then
        color=$SGR_COLOR_GREEN_FG
        status=$CHAR_SUCCESS
    else
        color=$SGR_COLOR_RED_FG
        status=$CHAR_ERROR
    fi

    printf '%s%s %s %s %s%s\n' "$color" "$SGR_REVERSE_VIDEO_ON" "$status" "$TITLE" "$SGR_REVERSE_VIDEO_OFF" "$SGR_COLOR_DEFAULT_FG"
}

print_header

# make sure we have $ARG_MAX_LINES below us
for _ in $(seq "$MAX_LINES"); do printf '\n'; done

# move cursor back up
term_move_cursor_up $((MAX_LINES + 1))

# get current row
row=$(term_get_cursor_pos | cut -d' ' -f1)

# set terminal scroll region from $row to $row + $ARG_MAX_LINES
term_set_scroll_region "$row" $((row + MAX_LINES))

# move cursor into scroll region
term_set_cursor_pos "$row" 0

# start command, truncate and prefix lines
(if [ "${#@}" -eq 1 ]; then
    # evaluate pipes, redirections, etc.
    eval "$@"
else
    # normal execution
    "$@"
fi) 2>&1 | tee "$OUTFILE" | sed -r "s/^(.{$MAX_LINE_LENGTH}).*/\1/; s/^/$OUTPUT_PREFIX/"

# get command output
EXIT_CODE="${PIPESTATUS[0]}"

# reset scroll region
term_set_scroll_region 0 "$(term_get_size | cut -d' ' -f1)"

# move cursor to header
term_set_cursor_pos $((row - 1)) 0

# update header
print_header "$EXIT_CODE"

# remove output
term_del_from_cursor_to_end

# forward exit code
exit "$EXIT_CODE"

$ peek --help
Usage: peek [OPTIONS] -- COMMAND

Options:
  -h, --help       print help and exit
  -t, --title      header title to display. Defaults to COMMAND
  -n, --max-lines  maxmimum number of COMMAND output lines to show at once
  -o, --out        output file to write full COMMAND output to
  -v, --version    print version and exit

Example:
  peek -t 'Counting to 100' -- sh -c 'for i in {1..100}; do echo $i; sleep .01; done'