Bash: 检查变量是否设置

extra: http://devmanual.gentoo.org/tools-reference/bash/index.html

[ -n “${VAR+x}” ] ## Fails if VAR is unset
[ -n “${VAR:+x}” ] ## Fails if VAR is unset or empty
[ -n “${VAR-x}” ] ## Succeeds if VAR is unset
[ -n “${VAR:-x}” ] ## Succeeds if VAR is unset or empty