In Shell, EOF is usually used in combination with < to indicate that the subsequent input is used as the input of subcommand or subshell, until EOF is encountered, and then returned to the calling Shell.
EOF can be replaced by something else, meaning that content is passed to the program as standard input.
Review the use of <. When the shell sees < < it knows that the next word is a delimiter. What follows the delimiter is treated as input until the shell sees the delimiter again (on a separate line). This delimiter can be anything you define character string.
[root@sys ~]# cat > /root/testeof.txt <<EOF
> WELCOME TO CHINA
[root@sys ~]# cat testeof.txt
WELCOME TO CHINA
Append content to / root/testeof.txt Until EOF
if [ -f /etc/redhat-release -a -n "`grep ' 7\.' /etc/redhat-release`" ];then #CentOS_REL=7 if [ ! -e /etc/yum.repos.d/epel.repo ];then cat > /etc/yum.repos.d/epel.repo << EOF [epel] name=Extra Packages for Enterprise Linux 7 - \$basearch #baseurl=http://download.fedoraproject.org/pub/epel/7/\$basearch mirrorlist=https://mirrors.fedoraproject.org/metalink?repo=epel-7&arch=\$basearch failovermethod=priority enabled=1 gpgcheck=0 EOF fi for Package in wget make openssl gcc-c++ ppp pptpd iptables iptables-services do yum -y install $Package done echo 'net.ipv4.ip_forward = 1' >> /etc/sysctl.conf elif [ -f /etc/redhat-release -a -n "`grep ' 6\.' /etc/redhat-release`" ];then #CentOS_REL=6 for Package in wget make openssl gcc-c++ iptables ppp do yum -y install $Package done sed -i 'email@example.com_forward.*@net.ipv4.ip_forward = 1@g' /etc/sysctl.conf rpm -Uvh http://poptop.sourceforge.net/yum/stable/rhel6/pptp-release-current.noarch.rpm yum -y install pptpd else echo -e "\033[31mDoes not support this OS, Please contact the author! \033[0m" exit 1 fi