bash 判断centos 以及ubuntu

#!/bin/bash
a=`uname  -a`

public_string="Linux"

D="Darwin"
C="CentOS"
U="Ubuntu"


#centos os check
FILE_EXE=/etc/redhat-release

if [ -f "$FILE_EXE" ];then
	if [[ `cat /etc/redhat-release` =~ $C ]];then
		echo $C
		exit
	fi
fi


if [[ $a =~ $D ]];then
    echo "mac"
elif [[ $a =~ $C ]];then
    echo $C
elif [[ $a =~ $U ]];then
    echo $U
else
    echo $a
fi