checking the existence of some commands

.xinitrc

@@ -1,13 +1,14 @@
 [ -f /etc/xprofile ] && . /etc/xprofile
 [ -f ~/.xprofile ] && . ~/.xprofile
 [ -f ~/.Xresources ] && xrdb -merge ~/.Xresources
-/usr/bin/xcompmgr &
-/usr/bin/xscreensaver -no-splash &
-eval $(/usr/bin/gpg-agent --daemon --enable-ssh-support --write-env-file "${HOME}/.gpg-agent-info")
+
+hash xcompmgr 2>/dev/null && xcompmgr &
+hash xscreensaver 2>/dev/null && xscreensaver -no-splash &
+hash gpg-agent 2>/dev/null && eval $(gpg-agent --daemon --enable-ssh-support --write-env-file "${HOME}/.gpg-agent-info")
 
 DEFAULT_SESSION=awesome
 case "$1" in
-    default|awesome|"")
+    awesome|default|"")
 	exec awesome
 	;;
     qtile)

.zshrc

@@ -51,6 +51,6 @@ alias grep='grep -n --color=auto'
 alias j='jobs'
 alias ne='emacs'
 alias df='df -h'
-alias top='TERM=screen-256color htop'
 alias weechat='TERM=screen-256color weechat'
 alias lock='xscreensaver-command --lock'
+hash htop 2>/dev/null && alias top='TERM=screen-256color htop'