diff --git a/init.sh b/init.sh
index 353dd3a..541a2d9 100644
--- a/init.sh
+++ b/init.sh
@@ -10,7 +10,7 @@ export PATH="$TMUXIFIER/bin:$PATH"
 
 # If `tmuxifier` is available, and `$TMUXIFIER_NO_COMPLETE` is not set, then
 # load tmuxifier shell completion.
-if [ ! -z $(command -v "tmuxifier") ] && [ -z "$TMUXIFIER_NO_COMPLETE" ]; then
+if [ -n "$(command -v "tmuxifier")" ] && [ -z "$TMUXIFIER_NO_COMPLETE" ]; then
   if [ -n "$BASH_VERSION" ]; then
     source "$TMUXIFIER/completion/tmuxifier.bash"
   elif [ -n "$ZSH_VERSION" ]; then
diff --git a/lib/layout-helpers.sh b/lib/layout-helpers.sh
index 91d823c..0dafd63 100644
--- a/lib/layout-helpers.sh
+++ b/lib/layout-helpers.sh
@@ -13,14 +13,14 @@
 #   - $2: (optional) Shell command to execute when window is created.
 #
 new_window() {
-  if [ ! -z "$1" ]; then
+  if [ -n "$1" ]; then
     window="$1"
   fi
   local command=()
-  if [ ! -z "$2" ]; then
+  if [ -n "$2" ]; then
     command+=("$2")
   fi
-  if [ ! -z "$window" ]; then
+  if [ -n "$window" ]; then
     local winarg=(-n "$window")
   fi
   tmux new-window -t "$session:" "${winarg[@]}" "${command[@]}"
@@ -114,7 +114,7 @@ window_root() {
 #   fi
 #
 initialize_session() {
-  if [ ! -z "$1" ]; then
+  if [ -n "$1" ]; then
     session="$1"
   fi
 
@@ -127,7 +127,7 @@ initialize_session() {
     env TMUX= tmux new-session -d -s "$session"
 
     # Set default-path for session
-    if [ ! -z "$session_root" ] && [ -d "$session_root" ]; then
+    if [ -n "$session_root" ] && [ -d "$session_root" ]; then
       cd "$session_root"
       tmux set-option -t "$session" default-path "$session_root" 1>/dev/null
     fi
@@ -180,7 +180,7 @@ __expand_path() {
 }
 
 __go_to_session() {
-  if [ -z $TMUX ]; then
+  if [ -z "$TMUX" ]; then
     tmux -u attach-session -t "$session"
   else
     tmux -u switch-client -t "$session"
diff --git a/libexec/tmuxifier-completions b/libexec/tmuxifier-completions
index ec24335..57598ad 100755
--- a/libexec/tmuxifier-completions
+++ b/libexec/tmuxifier-completions
@@ -20,7 +20,7 @@ fi
 
 ! command_path="$(tmuxifier-resolve-command-path "$command")"
 
-if [ ! -z "$command_path" ] && has-completions "$command_path"; then
+if [ -n "$command_path" ] && has-completions "$command_path"; then
   shift
   exec "$command_path" --complete "$@"
 fi
diff --git a/libexec/tmuxifier-current-session b/libexec/tmuxifier-current-session
index c9cb465..280ab59 100755
--- a/libexec/tmuxifier-current-session
+++ b/libexec/tmuxifier-current-session
@@ -2,7 +2,7 @@
 set -e
 [ -n "$TMUXIFIER_DEBUG" ] && set -x
 
-if [ ! -z $TMUX ]; then
+if [ -n "$TMUX" ]; then
   for item in $(tmux list-sessions -F "#{?session_attached,1,0}:#S"); do
     if [[ "$item" == "1:"* ]]; then
       echo ${item/1:/}