Some examples to add exit code to command line prompt in bash (put it into ~/.bashrc):

export PS1="\[\033[01;31m\]\$([ \$? == 0 ] || echo \"!\$? \" )\[\033[00m\]\[\033[01;32m\]\t \[\033[01;34m\]\w\[\033[00m\]\[\033[1;32m\]\n\$ \[\033[m\]"

Add to the default PS1 in CentOS:

export PS1="\[\033[01;31m\]\${?##0}\[\033[00m\][\u@\h \W]\\$"