shell下变量比较最佳实践

https://stackoverflow.com/questions/13617843/unary-operator-expected

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If you know you‘re always going to use bash,it‘s much easier to always use the double bracket conditional compound command [[ ... ]],instead of the Posix-compatible? single bracket version [ ... ]. Inside a [[ ... ]] compound,word-splitting and pathname expansion are not applied to words,so you can rely on

if [[ $aug1 == "and" ]];

to compare the value of $aug1 with the string and.

If you use [ ... ],you always need to remember to double quote variables like this:

if [ "$aug1" = "and" ];

If you don‘t quote the variable expansion and the variable is undefined or empty,it vanishes from the scene of the crime,leaving only

if [ = "and" ];

which is not a valid syntax. (It would also fail with a different error message if $aug1 included white space or shell metacharacters.)

The modern [[ operator has lots of other nice features,including regular expression matching.