if expression1; then
    statement...;
elif expression2; then
    statement...;
...
elif expressionN; then
    statement...;
else
    statement...;
fi

解释：

当expression的运行结果为0，表示真；

当expression的运行结果为非0，表示假。

if [ -f "/etc/alpine-release" ] ; then
    echo "your os is AlpineLinux"
elif [ -f "/etc/debian_version" ] ; then
    echo "your os is Debian"
elif [ -f "/etc/lsb-release" ] ; then
    echo "your os is Ubuntu"
elif [ -f "/etc/fedora-release" ] ; then
    echo "your os is Fedora"
elif [ -f "/etc/centos-release" ] ; then
    echo "your os is CentOS"
elif [ -f "/etc/redhat-release" ] ; then
    echo "your os is RHEL"
else
    echo "your os is not recognized!"
fi