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How to check if a variable is set in bash
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| +----------------------+------------+-----------------------+-----------------------+ | |
| | if VARIABLE is: | set | empty | 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" | "" | "" | | |
| +----------------------+------------+-----------------------+-----------------------| | |
| https://stackoverflow.com/questions/3601515/how-to-check-if-a-variable-is-set-in-bash |
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