参考: [ Unix & Linux ]
$echo ${JENKINS_VERSION:-2.7.4} 2.7.4 $JENKINS_VERSION=2.99 $echo ${JENKINS_VERSION:-2.7.4} 2.99
shell 给未定义的变量设定默认值 ${parameter:-word}
原文:https://www.cnblogs.com/tiantiandas/p/shell_set_default_value_for_unset_variable.html