taoc

#!/usr/bin/env bash
# @Function
# tac lines colorfully. taoc means coat(*CO*lorful c*AT*) in reverse(last line first).
#
# @Usage
#   $ echo -e 'Hello\nWorld' | taoc
#   $ taoc /path/to/file1
#   $ taoc /path/to/file1 /path/to/file2
#
# @online-doc https://github.com/oldratlee/useful-scripts/blob/dev-3.x/docs/shell.md#-coat
# @author Jerry Lee (oldratlee at gmail dot com)
set -eEuo pipefail

readonly PROG=${0##*/}
readonly PROG_VERSION='3.x-dev'

################################################################################
# parse options
################################################################################

usage() {
  cat <<EOF
Usage: $PROG [OPTION]... [FILE]...
tac lines colorfully.

Support options:
  --help     display this help and exit
  --version  output version information and exit
All other options and arguments are delegated to command tac,
more info see the help/man of command tac(e.g. tac --help).
tac executable: $(type -P tac)
EOF

  exit
}

progVersion() {
  printf '%s version: %s\n' "$PROG" "$PROG_VERSION"
  printf 'tac executable: %s\n' "$(type -P tac)"
  exit
}

args=("$@")
# check arguments in reverse, so last option wins.
for ((idx = $# - 1; idx >= 0; --idx)); do
  [ "${args[idx]}" = --help ] && usage
  [ "${args[idx]}" = --version ] && progVersion
done
unset args idx

################################################################################
# biz logic
################################################################################

# if stdout is not a terminal, use `tac` directly.
#   '-t' check: is a terminal?
#   check isatty in bash https://stackoverflow.com/questions/10022323
[ -t 1 ] || exec tac "$@"

readonly -a ROTATE_COLORS=(33 35 36 31 32 37 34)
COLOR_INDEX=0
# CAUTION: print content WITHOUT new line
rotateColorPrint() {
  local content=$*
  # skip color for white space
  if [[ $content =~ ^[[:space:]]*$ ]]; then
    printf %s "$content"
  else
    local color=${ROTATE_COLORS[COLOR_INDEX++ % ${#ROTATE_COLORS[@]}]}
    printf '\e[1;%sm%s\e[0m' "$color" "$content"
  fi
}

rotateColorPrintln() {
  # NOTE: $'foo' is the escape sequence syntax of bash
  rotateColorPrint "$*"$'\n'
}

colorLines() {
  local line
  # Bash read line does not read leading spaces
  # https://stackoverflow.com/questions/29689172
  while IFS= read -r line; do
    rotateColorPrintln "$line"
  done
  # How to use `while read` (Bash) to read the last line in a file
  #   if there’s no newline at the end of the file?
  # https://stackoverflow.com/questions/4165135
  [ -z "$line" ] || rotateColorPrint "$line"
}

tac "$@" | colorLines