${parameter:-word} #

ex1:
foo=
echo ${foo:-“hahaha”}
echo foo
输出的结果是 hahaha 空

ex2:
foo=“tengwang”
echo ${foo:-“hahaha”}
echo $foo
输出的结果是 tengwang tengwang

${parameter:=word} #

ex1:
foo=
echo ${foo:=hahaha}
echo $foo
输出的结果是 hahaha hahaha

ex2:
foo=“tengwang”
echo ${foo:=hahaha}
echo $foo
输出的结果是tengwang tengwang

${parameter:?word} #

ex1:
foo=
echo ${foo:?输入的变量为空，退出}
echo $foo
输出的结果是 -bash: foo: foo为空退出 空

ex2:
foo=“tengwang”
echo ${foo:?输入的变量为空，退出}
echo $foo
输出的结果是 tengwang tengwang

${parameter:+word} #

ex1:
foo=
echo ${foo:+hahaha}
echo $foo
输出的结果是 空 空

ex2:
foo=“tengwang”
echo ${foo:+hahaha}
echo $foo
输出的结果是 hahaha tengwang