1 分钟搞懂什么是 XY 问题

<n00b> How can I echo the last three characters in a filename?<feline> If they're in a variable: echo ${foo: -3}<feline> Why 3 characters? What do you REALLY want?<feline> Do you want the extension?<n00b> Yes.<feline> There's no guarantee that every filename will have a three-letter extension,<feline> so blindly grabbing three characters does not solve the problem.<feline> echo ${foo##*.}

Angela: 'nmap -O -A 127.0.0.1' returns some lines starting with 'OS:'. How to change it?Obama: Look in the sourcecode for nmap, find how it figures out the Linux part, then rewrite your TCP/IP stack to not operate in a way nmap can detect.Angela: Yeah, but I don't know about linux system api at all.Obama: Well, nmap's fingerprint is based on the way the TCP/IP stack works, there's no real way except to rewrite the appropriate parts of said stack.Angela: I really need to avoid these messages. Can iptables do this work?Obama: Well, don't use OS detection or version scanningAngela: I want to prevent others from knowing the type of my OS