shell变量设置与显示

[root@localhost ~]# echo ${MAIL}
/var/spool/mail/root
[root@localhost ~]# echo ${MAIL##/*/} #贪婪前匹配删除
root
[root@localhost ~]# echo ${MAIL#/*/} #前匹配删除
spool/mail/root
[root@localhost ~]# echo ${MAIL%/*} #后匹配删除
/var/spool/mail
[root@localhost ~]# echo ${MAIL%%/*} #贪婪后匹配删除

[root@localhost ~]# echo ${MAIL//oo/aa} #全局替换
/var/spaal/mail/raat
[root@localhost ~]# echo ${MAIL/oo/aa} #仅替换第一个
/var/spaal/mail/root

[root@localhost ~]# unset a;var=${a-b};echo $a,$var # var = (a == null) ? a : b
,b
[root@localhost ~]# a='foo';var=${a-b};echo $a,$var
foo,foo
[root@localhost ~]# unset a;var=${a=b};echo $a,$var # var = a= (a == null)? a : b
b,b
[root@localhost ~]# a='foo';var=${a=b};echo $a,$var
foo,foo

变量设置方式	str没有设置	str为空字符	str已设置为非空字符
var=${str-expr}	var=expr	var=	var=$str
var=${str:-expr}	var=expr	var=expr	var=$str
var=${str+expr}	var=expr	var=expr	var=expr
var=${str:+expr}	var=expr	var=	var=expr
var=${str=expr}	var=expr str=expr	str不变 var=	str不变 var=$str
var=${str:=expr}	var=expr str=expr	var=expr str=expr	str不变 var=$str
var=${str?expr}	expr输出至stderr	var=	var=$str
var=${str:?expr}	expr输出至stderr	expr输出至stderr	var=$str

来源：https://www.cnblogs.com/goodspeed/archive/2012/12/13/2815798.html