diff --git a/modules/dpkg/init.zsh b/modules/dpkg/init.zsh
index 1ba77470..4094c52a 100644
--- a/modules/dpkg/init.zsh
+++ b/modules/dpkg/init.zsh
@@ -7,6 +7,10 @@
 #   Sorin Ionescu <sorin.ionescu@gmail.com>
 #
 
+if (( ! $+commands[dpkg] )); then
+  return 1
+fi
+
 # Aliases
 alias as="aptitude -F \"* %p -> %d \n(%v/%V)\" --no-gui --disable-columns search" # Searches for a package.
 alias ad="sudo apt-get update"                                                    # Updates packages lists.
diff --git a/modules/git/init.zsh b/modules/git/init.zsh
index e3c3ac84..cf15851e 100644
--- a/modules/git/init.zsh
+++ b/modules/git/init.zsh
@@ -5,6 +5,10 @@
 #   Sorin Ionescu <sorin.ionescu@gmail.com>
 #
 
+if (( ! $+commands[git] )); then
+  return 1
+fi
+
 # Source module files.
 source "${0:h}/alias.zsh"
 source "${0:h}/hub.zsh"
diff --git a/modules/perl/init.zsh b/modules/perl/init.zsh
index c833bd8b..7a6329f4 100644
--- a/modules/perl/init.zsh
+++ b/modules/perl/init.zsh
@@ -5,6 +5,10 @@
 #   Sorin Ionescu <sorin.ionescu@gmail.com>
 #
 
+if (( ! $+commands[perl] )); then
+  return 1
+fi
+
 if [[ "$OSTYPE" == darwin* ]]; then
   # Perl is slow; cache its output.
   cache_file="${0:h}/cache.zsh"
diff --git a/modules/rsync/init.zsh b/modules/rsync/init.zsh
index e0f866a8..2117e20e 100644
--- a/modules/rsync/init.zsh
+++ b/modules/rsync/init.zsh
@@ -5,6 +5,10 @@
 #   Sorin Ionescu <sorin.ionescu@gmail.com>
 #
 
+if (( ! $+commands[rsync] )); then
+  return 1
+fi
+
 # Aliases
 _rsync_cmd='rsync --verbose --progress --human-readable --compress --archive --hard-links --one-file-system'
 
diff --git a/modules/screen/init.zsh b/modules/screen/init.zsh
index 8c0f1d57..f1275495 100644
--- a/modules/screen/init.zsh
+++ b/modules/screen/init.zsh
@@ -5,6 +5,10 @@
 #   Sorin Ionescu <sorin.ionescu@gmail.com>
 #
 
+if (( ! $+commands[screen] )); then
+  return 1
+fi
+
 # Aliases
 alias sl="screen -list"
 alias sn="screen -U -S"
diff --git a/modules/tmux/init.zsh b/modules/tmux/init.zsh
index c066ea98..7d3b266e 100644
--- a/modules/tmux/init.zsh
+++ b/modules/tmux/init.zsh
@@ -6,6 +6,10 @@
 #   Colin Hebert <hebert.colin@gmail.com>
 #
 
+if (( ! $+commands[tmux] )); then
+  return 1
+fi
+
 # Aliases
 alias ta="tmux attach-session"
 alias tl="tmux list-sessions"