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lang-bash
-z? if not empty string.if [ ! -z "$VAR" ];-zis-nif [ -n "$VAR" ];$varon a command line will be split by its whitespace into a list of parameters, while"$var"will always be only one parameter. Quoting variables often is a good practice and stops you from tripping up on filenames containing whitespace (among other things). Example: after doinga="x --help", trycat $a- it will give you the help page forcat. Then trycat "$a"- it will (usually) saycat: x --help: No such file or directory. In short, quote early and quote often and you will almost never regret it.set -uthen the answer will break/fail at that point. There is a subtle (and usually ignored) difference betweenunsetandempty. Note that the distinction is only really important if it's important to you, or if the script has alreadyset -u.