shell编程如何判断一个字符串是否只是由字母数字下划线组成?

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代码:

[color=blue]-(guest@mac:tty1)-(root)-
[5970 0] %[/color] name=a_b
[color=blue]-(guest@mac:tty1)-(root)-
[5970 0] %[/color] if echo "$name" | grep -q '^[a-zA-Z0-9_]\+$'; then echo ok; else echo invalid; fi
ok
[color=blue]-(guest@mac:tty1)-(root)-
[5970 0] %[/color] name=a-b
[color=blue]-(guest@mac:tty1)-(root)-
[5970 0] %[/color] if echo "$name" | grep -q '^[a-zA-Z0-9_]\+$'; then echo ok; else echo invalid; fi
invalid
[color=blue]-(guest@mac:tty1)-(root)-
[5970 0] %[/color]