Shell - set a default value to a variable when it has no value using ":-"

 

zzh@ZZHPC:~$ echo $a

zzh@ZZHPC:~$ a=111
zzh@ZZHPC:~$ a=${a:-aaa}
zzh@ZZHPC:~$ echo $a
111
zzh@ZZHPC:~$ echo $b

zzh@ZZHPC:~$ b=${b:-bbb}
zzh@ZZHPC:~$ echo $b
bbb