diff --git a/bin/dotfiles b/bin/dotfiles
index 00bc7338a8..98e9825eca 100755
--- a/bin/dotfiles
+++ b/bin/dotfiles
@@ -127,21 +127,22 @@ function _prompt_menu_add() {
   _prompt_menu_result=("${_prompt_menu_result[@]}" "${menu_options[i]}")
 }
 
-# Given a list of desired items and installed items, return a list
-# of uninstalled items. Arrays in bash are insane (not in a good way).
+# Given strings containing space-delimited words A and B, "setdiff A B" will
+# return all words in A that do not exist in B. Arrays in bash are insane
+# (and not in a good way).
 # From http://stackoverflow.com/a/1617303/142339
-function to_install() {
-  local debug oldifs desired installed remain a
+function setdiff() {
+  local debug oldifs A B C x
   if [[ "$1" == 1 ]]; then debug=1; shift; fi
   # Convert args to arrays.
-  desired=($1); installed=($2); remain=()
-  for a in "${desired[@]}"; do
-    [[ "${installed[*]}" =~ (^| )$a($| ) ]] || remain=("${remain[@]}" "$a")
+  A=($1); B=($2); C=()
+  for x in "${A[@]}"; do
+    [[ "${B[*]}" =~ (^| )$x($| ) ]] || C=("${C[@]}" "$x")
   done
-  [[ "$debug" ]] && for a in desired installed remain; do
-    echo "$a ($(eval echo "\${#$a[*]}")) $(eval echo "\${$a[*]}")" 1>&2
+  [[ "$debug" ]] && for x in A B C; do
+    echo "$x ($(eval echo "\${#$x[*]}")) $(eval echo "\${$x[*]}")" 1>&2
   done
-  echo "${remain[@]}"
+  echo "${C[@]}"
 }
 
 # If this file was being sourced, exit now.
diff --git a/init/20_osx_homebrew.sh b/init/20_osx_homebrew.sh
index 2d4bc8cc53..eb87862ccd 100644
--- a/init/20_osx_homebrew.sh
+++ b/init/20_osx_homebrew.sh
@@ -18,7 +18,7 @@ brew update
 
 # Tap Homebrew kegs.
 function brew_tap_kegs() {
-  kegs=($(to_install "${kegs[*]}" "$(brew tap)"))
+  kegs=($(setdiff "${kegs[*]}" "$(brew tap)"))
   if [[ ${#kegs[@]} != 0 ]]; then
     e_header "Tapping Homebrew kegs: ${kegs[@]}"
     for keg in "${kegs[@]}"; do
@@ -29,7 +29,7 @@ function brew_tap_kegs() {
 
 # Install Homebrew recipes.
 function brew_install_recipes() {
-  recipes=($(to_install "${recipes[*]}" "$(brew list)"))
+  recipes=($(setdiff "${recipes[*]}" "$(brew list)"))
   if [[ ${#recipes[@]} != 0 ]]; then
     e_header "Installing Homebrew recipes: ${recipes[@]}"
     for recipe in "${recipes[@]}"; do
diff --git a/init/30_osx_homebrew_casks.sh b/init/30_osx_homebrew_casks.sh
index 12eef577ee..6f9621c7c4 100644
--- a/init/30_osx_homebrew_casks.sh
+++ b/init/30_osx_homebrew_casks.sh
@@ -67,7 +67,7 @@ casks=(
 )
 
 # Install Homebrew casks.
-casks=($(to_install "${casks[*]}" "$(brew cask list 2>/dev/null)"))
+casks=($(setdiff "${casks[*]}" "$(brew cask list 2>/dev/null)"))
 if [[ ${#casks[@]} != 0 ]]; then
   e_header "Installing Homebrew casks: ${casks[@]}"
   for cask in "${casks[@]}"; do
diff --git a/init/50_node.sh b/init/50_node.sh
index 6b283146ff..dafb92cd20 100644
--- a/init/50_node.sh
+++ b/init/50_node.sh
@@ -24,7 +24,7 @@ if [[ "$(type -P npm)" ]]; then
   npm update -g npm
 
   { pushd "$(npm config get prefix)/lib/node_modules"; installed=(*); popd; } > /dev/null
-  list="$(to_install "${npm_globals[*]}" "${installed[*]}")"
+  list="$(setdiff "${npm_globals[*]}" "${installed[*]}")"
   if [[ "$list" ]]; then
     e_header "Installing Npm modules: $list"
     npm install -g $list
diff --git a/init/50_ruby.sh b/init/50_ruby.sh
index 70010d8cdb..fb7fcd0130 100644
--- a/init/50_ruby.sh
+++ b/init/50_ruby.sh
@@ -5,7 +5,7 @@ source $DOTFILES/source/50_ruby.sh
 if [[ "$(type -P rbenv)" ]]; then
   versions=(2.1.3) # 2.0.0-p576 1.9.3-p547)
 
-  list="$(to_install "${versions[*]}" "$(rbenv whence ruby)")"
+  list="$(setdiff "${versions[*]}" "$(rbenv whence ruby)")"
   if [[ "$list" ]]; then
     e_header "Installing Ruby versions: $list"
     for version in $list; do rbenv install "$version"; done
diff --git a/test/test_to_install.sh b/test/test_setdiff.sh
similarity index 95%
rename from test/test_to_install.sh
rename to test/test_setdiff.sh
index f386d2d402..0cb5653164 100755
--- a/test/test_to_install.sh
+++ b/test/test_setdiff.sh
@@ -4,7 +4,7 @@ source $DOTFILES/source/00_dotfiles.sh
 e_header "$(basename "$0" .sh)"
 
 function my_test() {
-  to_install "${desired[*]}" "${installed[*]}"
+  setdiff "${desired[*]}" "${installed[*]}"
 }
 
 desired=(a b c); installed=(); assert "a b c" my_test