GNU bash exit codes with special meanings

$printenv > /dev/null 
$echo $?
0
$printenv /dev/null 
$echo $?
1
$/dev/null
bash: /dev/null: Permission denied
$echo $?
126
$nocommand
bash: nocommand: command not found...

$
$echo $?
127
$

http://www.tldp.org/LDP/abs/html/exitcodes.html