Shell script that normalizes a path without requiring the path to actually exist on the filesystem. Works in sh, bash, and zsh.

normalize_path.sh

#!/usr/bin/env sh

#
# Removes all '.', '..', and '//' segments, creating a canonical
# form of the path string. The path does not have to actually exist 
# in the filesystem.
#
normalize_path() {
  if [[ "$1" == "" ]] ; then
    echo "$1"
    return
  fi
  parts=()
  if [ -n "${ZSH_VERSION:-}" ] ; then
    IFS="/" read -r -A parts <<< "$1"
  else
    IFS="/" read -r -a parts <<< "$1"
    parts=("" "${parts[@]}")
  fi
  length=${#parts[@]}
  skip=0
  out=""
  abs=""
  for ((i=length; i>0; i--)); do
    s=${parts[i]:-}
    if [[ "$s" == "." ]] ; then
  		# if (( i == 0 )) ; then out=".${out:+/$out}"; fi
	  	continue
	  fi
	  if [[ "$s" == "" ]] ; then
  		if (( i == 1 )) ; then abs="/" ; fi
	  	continue;
	  fi
	  if [[ "$s" == ".." ]] ; then
		  skip=$((skip + 1))
		  continue
    fi
	  if (( skip > 0 )) ; then
		  skip=$((skip - 1))
		  continue
	  fi
	  out="${parts[i]}${out:+"/$out"}"
  done

  if [[ ! ($abs || "$out" == "." || $out == ./*) ]] ; then
  	# add leading '..' segments only if it makes sense to do so
	  for ((i=skip; i>0; i--)); do
      out="..${out:+"/$out"}"
	  done
  fi
  if [[ $abs ]] ; then
	  # output is absolute if input was absolute
	  out="/${out}"
  fi
  if [[ "$1" == */ ]] ; then
	  # use a trailing slash if the input had one
	  out="${out%/}/"
  fi
  echo "$out"
}

normalize_path "$@"