shell如何判断变量是否定义并做相应转换：
+----------------------+------------+-----------------------+-----------------------------+
| if VARIABLE is: | set | empty | unset |
| | Set and Not Null | Set But Null | Unset |
+----------------------+------------------+-----------------------+-----------------------+
- | ${VARIABLE-default} | $VARIABLE | "" | "default" |
= | ${VARIABLE=default} | $VARIABLE | "" | $(VARIABLE="default") |
? | ${VARIABLE?default} | $VARIABLE | "" | exit 127 |
+ | ${VARIABLE+default} | "default" | "default" | "" |
+----------------------+------------------+-----------------------+-----------------------+
:- | ${VARIABLE:-default} | $VARIABLE | "default" | "default" |
:= | ${VARIABLE:=default} | $VARIABLE | $(VARIABLE="default") | $(VARIABLE="default") |
:? | ${VARIABLE:?default} | $VARIABLE | exit 127 | exit 127 |
:+ | ${VARIABLE:+default} | "default" | "" | "" |
+----------------------+------------------+-----------------------+-----------------------+
参考文档 http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_02
