Consistently use [[ … ]] in .bashrc

bashrc

@@ -3,7 +3,7 @@
 # Based on example .bashrc as provided by Debian
 
 # If not running interactively, don't do anything
-[ -z "$PS1" ] && return
+[[ -z "$PS1" ]] && return
 
 # Environment variables go here
 export LANG=en_US.UTF-8
@@ -31,7 +31,7 @@ shopt -s extglob
 shopt -s checkwinsize
 
 # make less more friendly for non-text input files, see lesspipe(1)
-[ -x /usr/bin/lesspipe ] && eval "$(SHELL=/bin/sh lesspipe)"
+[[ -x /usr/bin/lesspipe ]] && eval "$(SHELL=/bin/sh lesspipe)"
 
 # Simple prompt
 if [[ "$TERM" = "dumb" ]]; then
@@ -51,7 +51,7 @@ else
 fi
 
 # Enable color support of ls and also add handy aliases
-if [ -x /usr/bin/dircolors ]; then
+if [[ -x /usr/bin/dircolors ]]; then
   if [[ -r ~/.dircolors ]]; then
     eval "$(dircolors -b ~/.dircolors)"
   else
@@ -73,7 +73,7 @@ alias ..='cd ..'
 # enable programmable completion features (you don't need to enable
 # this, if it's already enabled in /etc/bash.bashrc and /etc/profile
 # sources /etc/bash.bashrc).
-if [ -f /etc/bash_completion ] && ! shopt -oq posix; then
+if [[ -f /etc/bash_completion ]] && ! shopt -oq posix; then
     . /etc/bash_completion
 fi