prepend_path and append_path functions for zsh

edit_paths.zsh

# The following functions append/prepend the specified directory to the path variable specified by name
# If the directory is already in the path, it is first removed and then appended/prepended.
# Examples:
#
# Prepend `~/bin` to `$PATH`:
#   prepend_path PATH ~/bin
#
# Append `~bin` to `$PATH` instead:
#  append_path PATH ~/bin
# (this will remove `~/bin` from the beginning of `$PATH` first)
#
# Append `~/lib/python-2.7/site-packages` to `$PYTHONPATH`:
#  append_path PYTHONPATH ~/lib/python-2.7/site-packages
#
# and so on ...


# Remove $2 from path with name $1, e.g.
#   remove_from_path PATH ~/bin
#   remove_from_path PYTHONPATH ~/lib/python2.7/site-packages
function remove_from_path() {
  local a
  local p
  local s
  local r
  eval "p=\$$1"  # get value of specified path
  a=( ${(s/:/)p} )  # turn it into an array
  # return if $2 isn't in path
  if [[ ${a[(i)${2}]} -gt ${#a} ]] && return
  # rebuild path from elements not matching $2
  for s in $a; do
    if [[ ! $s == $2 ]]; then
      [[ -z "$r" ]] && r=$s || r="$r:$s"
    fi
  done
  eval $1="$r"
}

# Add path to start of named path, removing any occurences
# already in it, e.g.
#   prepend_path PATH ~/bin
#   prepend_path PYTHONPATH ~/my-py-stuff
function prepend_path() {
  # Exit if directory doesn't exit
  [[ ! -d "$2" ]] && return
  local p
  remove_from_path "$1" "$2"
  eval "p=\$$1"
  eval export $1="$2:$p"
}

# As above, but add to end of path
function append_path() {
  # Exit if directory doesn't exit
  [[ ! -d "$2" ]] && return
  local p
  remove_from_path "$1" "$2"
  eval "p=\$$1"
  eval export $1="$p:$2"
}