Bash variable extension modifier

1. Temporary replacement if not set (:-)

Colon: Used to verify that the variable has been set, if there is no colon, it is assumed to have been set, not replaced
$fruit =peach
$echo ${fruit:-plum}
Peach

$fruit =
$echo ${fruit:-plum}
Plum

$echo $fruit
$

2, not set on permanent replacement (: =)

$name =
$echo ${name:=peter}
Peter

$echo $name
Peter

3, set the temporary replacement (: +)

$foo =grapes
$echo ${foo:+pears}
Pears
$echo $foo
Grapes

4. Print the message without setting it (:?)

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

$echo name=${1:? " Requires an argument "}//For detecting positional parameters in scripts
1:requires an argument

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

5. Substring (: m:n)

$var =notebook

$echo ${var:0}
Notebook

$echo ${var:0:4}
Note

$echo ${var:4:4}
Book

Bash variable extension modifier