lesh/lesh

#!/bin/bash

# Lesh: a pure bash (I think) pager
# Lesh is moreish
# Intended to behave like less, more or less

# Currently lesh can:
#  read a file or pipe and display text
#  tell you if you didn't give it anything to read
#  scroll down/up with j/k or the down and up arrows
#  page up and page down with page up and page down
#  jump to the start/end of the file with g/G
#  quit with q
#
# It doesn't mess up your terminal when terminated
# It does when killed (printf '\e[;r' to fix)


# allow terminal size to be read, then run useless command to populate variables
shopt -s checkwinsize; (:;:)

# exit with error if file not provided
# and there's nothing being piped
if [[ ! -f "$1" ]] && [[ -t 0 ]]; then
    printf '%s\n' "nothing to read" >&2
    exit 1
fi

# if reading from a pipe, duplicate /dev/stdin
# then redirect the terminal back in order to read keypresses
[[ ! -t 0 ]] && exec 3<&0 && exec </dev/tty

# use the size of the terminal to determine the length of the scroll region
# and the width to fold text to
get_terminal_size() {
    scroll_size=$((LINES - 1))
    width=$COLUMNS
}

read_file() {
    mapfile -t file < "${file_name:-/proc/self/fd/3}"
}

# fold lines from file to $width, in a separate array (text)
fold_text() {
    # without setting IFS, "${file[*]}" is all one line
    IFS=$'\n'
    while read -r -n "$width" file_line; do
        text+=("$file_line")
    done <<< "${file[*]}"
    text_length="${#text[@]}"
    last_line=$((text_length-scroll_size))
}

setup_terminal() {
    # save cursor position, then switch to alternate screen
    printf '\e7\e[?1049h'
}

# move cursor to start/end of scroll region, or the prompt at the bottom
cursor() {
    case "$1" in
        start)
            printf '\e[H'
            ;;
        end)
            printf '\e[%s;H' "$scroll_size"
            ;;
        prompt)
            printf '\e[%s;H' "$LINES"
            ;;
    esac
}

# print a specified line of the folded text
# if that line is outside the contents of $text, print ~
print_line() {
    if [ "$1" -lt 0 ] || [ "$1" -gt "$text_length" ]; then
        printf '~'
    else
        printf '%s' "${text[$1]}"
    fi
}

# fill the scroll region with lines of $text
# starting at $line
draw_lines() {
    cursor prompt
    printf '\e[1J'
    cursor start
    print_line "$line"
    for ((i=line+1; i<line+scroll_size; i++)); do
        printf '\n'
        print_line "$i"
    done
}

# not used for anything yet, but looks like less
# show if end of file reached
draw_prompt() {
    cursor prompt
    # clear line
    printf '\e[2K'
    if [ "$line" -ge "$last_line" ]; then
        printf '\e[7m%s\e[0m' "(END)"
    else
        printf '%s' ":"
    fi
}

draw_screen() {
    # set scroll region, which moves cursor to start
    printf '\e[1;%sr' "$scroll_size"
    draw_lines
    draw_prompt
}

# print file name as given in argument
# intended to be erased when any key is pressed
print_message() {
    printf '\r\e[7m%s' "$file_name"
    [ "$line" -ge "$last_line" ] && printf " (END)"
    printf '\e[m'
}

# scroll the displayed text down, and print the next line up
scroll_up() {
    cursor start
    line=$((line-1))
    printf '\EM'
    print_line "$line"
}

# scroll the displayed text up, and print the next line down
scroll_down() {
    cursor end
    line=$((line+1))
    printf '\n'
    print_line "$((line-1+scroll_size))"
}

quit() {
    # reset scroll region, switch back to main screen, restore cursor
    printf '\e[;r\e[?1049l\e8'
    exit
}

# infinite loop waiting for single character input
# or escape sequences from (e.g) arrow keys
key_loop() {
    while read -rs -n 1; do
        # when escape recieved, read any characters sent immediately after
        # if there are none, then the escape key was pressed, but is ignored
        if [ "$REPLY" = $'\e' ]; then
            read -rs -t 0.0001
        fi
        case $REPLY in
            k|"[A")
                # if at start of the text, do nothing
                [ ! "$line" = 0 ] && scroll_up
                draw_prompt
                ;;
            j|"[B")
                # if at the end of text, do nothing
                [ ! "$line" -ge "$last_line" ] && scroll_down
                draw_prompt
                ;;
            "[5~")
                # page up
                if [ ! "$line" = 0 ]; then
                    line=$((line-scroll_size))
                    [ "$line" -lt 0 ] && line=0
                    draw_lines
                fi
                draw_prompt
                ;;
            "[6~")
                # page down
                if [ ! "$line" -ge "$last_line" ]; then
                    line=$((line+scroll_size))
                    [ "$line" -gt "$last_line" ] && line="$last_line"
                    draw_lines
                fi
                draw_prompt
                ;;
            g)
                if [ ! "$line" = 0 ]; then
                    line=0
                    draw_lines
                fi
                draw_prompt
                ;;
            G)
                if [ ! "$line" = "$last_line" ]; then
                    line="$last_line"
                    draw_lines
                fi
                draw_prompt
                ;;
            q)
                break
                ;;
            *)
                draw_prompt
                ;;
        esac
    done
}

main() {
    read_file
    fold_text

    setup_terminal
    draw_screen
    [ "$file_name" ] && print_message

    key_loop

    quit
}

trap quit EXIT INT HUP

# set or initialise variables
declare -a file text
file_name="$1"
line=0
get_terminal_size

main