shell變量設置與顯示

[root@localhost ~]# echo ${MAIL}
/var/spool/mail/root
[root@localhost ~]# echo ${MAIL##/*/} #貪婪前匹配刪除
root
[root@localhost ~]# echo ${MAIL#/*/} #前匹配刪除
spool/mail/root
[root@localhost ~]# echo ${MAIL%/*} #後匹配刪除
/var/spool/mail
[root@localhost ~]# echo ${MAIL%%/*} #貪婪後匹配刪除

[root@localhost ~]# echo ${MAIL//oo/aa} #全局替換
/var/spaal/mail/raat
[root@localhost ~]# echo ${MAIL/oo/aa} #僅替換第一個
/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

變量設置方式	str沒有設置	str爲空字符	str已設置爲非空字符
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=${str=expr}	var=expr str=expr	str不變 var=	str不變 var=$str
var=${str:=expr}	var=expr str=expr	var=expr str=expr	str不變 var=$str
var=${str?expr}	expr輸出至stderr	var=	var=$str
var=${str:?expr}	expr輸出至stderr	expr輸出至stderr	var=$str