A shell script to reverse lines in a file or stdin. This is needed because some environments lack "tac" or "tail-r" command. I used "sed" as a last resort which may have a performance problem if you are processing enormous text data. ( Install "tac" then! )

reverser.sh

#!/bin/bash
#
# Reverse the order of lines in a file or stfin
#
# A tac-equivalent command for every environment.
# This script is needed because some environments
# like OS X and MingW lack tac command.
#
# Scriptd by Hiroshi Yamamoto ([email protected])
# Licensed under WTFPL version 2.0 (http://www.wtfpl.net/)

#######################################
# Sets a command that reverses the order of lines
# in a file or std input.
# Globals:
#   None
# Arguments:
#   Variable name that the command to be set
# Returns:
#   None
#######################################
function set_reverser() {
  if [ "$(uname)" == "Darwin" ]; then
    local FOUND="tail -r"
  else
    if hash tac 2>/dev/null; then
      local FOUND="tac"
    else
      # "sed" as a last fallback
      local FOUND="sed '1!G;h;$!d' ${@+\"$@\"}"
    fi
  fi
  local __resultvar=$1
  if [[ "$__resultvar" ]]; then
    eval $__resultvar="'$FOUND'"
  else
    echo "$FOUND"
  fi
}

# Call a reverser
set_reverser REVERSER
$REVERSER $1

exit