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  Bash variable expansion modifier
  Add Date : 2018-11-21      
  Bash variable expansion modifier

1, is not provided on a temporary replacement (:-)

Colon: used to test whether a variable is set before, if there is no colon, then that is set before, do not replace
$ Fruit = peach
$ Echo $ {fruit: -plum}

$ Fruit =
$ Echo $ {fruit: -plum}

$ Echo $ fruit

2, is not set on the permanent replacement (: =)

$ Name =
$ Echo $ {name: = Peter}

$ Echo $ name

3, set up on a temporary replacement (: +)

$ Foo = grapes
$ Echo $ {foo: + pears}
$ Echo $ foo

4, is not set to print messages (:?)

$ Echo $ {namex:? "Namex is undefined"}
namex: namex is undefined

$ Echo name = $ {1:? "Requires an argument"} // apply the script parameter detection position
1: requires an argument

echo $ {y?}
y: parameter null or not set // print a default message

5, substring (: m: n)

$ Var = notebook

$ Echo $ {var: 0}

$ Echo $ {var: 0: 4}

$ Echo $ {var: 4: 4}
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