刚才输入命令 echo "This is file content, over!" > filename 竟然会出错，提示：
bash: !": event not found
原来字符串中的！会被解析。如果不出错的话呢，转义！就可以了，如 \! ，
或者把双引号改成单引号也行。
Google了下错误信息，找到如下内容：
Message bash: !__: event not found
Notes When you type a word preceeded by an "!", bash thinks you want to recall a previous command or "event". The message indicates no matching event was found in your recent command history. You can suppress the special meaning of ! by quoting it, for example: \!
Notes When you type a word preceeded by an "!", bash thinks you want to recall a previous command or "event". The message indicates no matching event was found in your recent command history. You can suppress the special meaning of ! by quoting it, for example: \!
这段英文的大概意思就是说 ！的作用就是调用之前的命令或事件。
比如 我之前输入了一条命令 touch aaa bbb ，然后又删了 rm aaa bbb，如果我又想重新创建文件aaa bbb 我直接输入 ！touch 就可以了，该命令就会重复执行上一次touch命令，即touch aaa bbb
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