Variable testing of shell programming

a test expression

Complex format, simple logic

Variable tests are used when variables are optimized

 

 

Two actual combat

[root@localhost ~]# unset y

[root@localhost ~]# x=${y-2}

[root@localhost ~]# echo $x

2

[root@localhost ~]# y=""

[root@localhost ~]# x=${y-2}

[root@localhost ~]# echo $x

 

[root@localhost ~]# y=1

[root@localhost ~]# x=${y-2}

[root@localhost ~]# echo $x

1