1. ${variable:-value} 

    $variable如果为空或者未设置，则返回value的值；

[root@k8s-node1 ~]# vi koala.sh
#!/bin/bash

declare name
[ -n "$name" ] && echo $name || echo '$name is null'
echo "${name:-$HOSTNAME}"
[ -n "$name" ] && echo $name || echo '$name is null'
[root@k8s-node1 ~]# bash koala.sh
$name is null
k8s-node1
$name is null

2. ${variable:=value} 

    $variable如果为空或者未设置，则将value的值赋值给$variable，且返回value的值；

[root@k8s-node1 ~]# vi koala.sh
#!/bin/bash

declare name
[ -n "$name" ] && echo $name || echo '$name is null'
echo "${name:=$HOSTNAME}"
[ -n "$name" ] && echo $name || echo '$name is null'
[root@k8s-node1 ~]# bash koala.sh
$name is null
k8s-node1
k8s-node1

3. ${variable:?value} 

    $variable如果为空或者未设置，则进程退出且打印出value的内容，通常用调试阶段使用；

[root@k8s-node1 ~]# vi koala.sh
#!/bin/bash

declare name
[ -n "$name" ] && echo $name || echo '$name is null'
echo "${name:?$HOSTNAME}"
[ -n "$name" ] && echo $name || echo '$name is null'
[root@k8s-node1 ~]# bash koala.sh
$name is null
koala.sh: line 5: name: k8s-node1

1. ${variable:+value} 

    $variable的值非空时，返回value的值；否则返回null

[root@k8s-node1 ~]# vi koala.sh
#!/bin/bash

declare name
[ -n "$name" ] && echo $name || echo '$name is null'
echo "${name:+$HOSTNAME}"
[ -n "$name" ] && echo $name || echo '$name is null'
[root@k8s-node1 ~]# bash koala.sh
$name is null

$name is null