首先,请注意,当你这样说： @H_502_1@BAR=$(basename $FOO) # result is BAR="baz" BAZ=${BAR:0:1} # result is BAZ="b"

BAZ的构造中的第一个位是BAR,而不是要采用第一个字符的值.所以即使bash允许变量名包含任意字符,你的第二个表达式的结果也不会是你想要的.

但是,关于阻止这一点的规则,请允许我从bash手册页引用：

@H_502_1@DEFINITIONS The following definitions are used throughout the rest of this docu‐ ment. blank A space or tab. word A sequence of characters considered as a single unit by the shell. Also known as a token. name A word consisting only of alphanumeric characters and under‐ scores,and beginning with an alphabetic character or an under‐ score. Also referred to as an identifier.

然后稍后

@H_502_1@PARAMETERS A parameter is an entity that stores values. It can be a name,a num‐ ber,or one of the special characters listed below under Special Param‐ eters. A variable is a parameter denoted by a name. A variable has a value and zero or more attributes. Attributes are assigned using the declare builtin command (see declare below in SHELL BUILTIN COMMANDS).

之后当它定义你所要求的语法时：

@H_502_1@${parameter:offset:length} Substring Expansion. Expands to up to length characters of parameter starting at the character specified by offset.

因此,在联机帮助页中阐述的规则表示${foo：x：y}构造必须具有参数作为第一部分,并且参数只能是名称,数字或少数特殊参数字符之一. $(basename $FOO)不是参数允许的可能性之一.

对于在一个作业中执行此操作的方式,请使用管道到其他响应中提到的其他命令.