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echo 与 sudo
众所周知，使用 echo 并配合命令重定向是实现向文件中写入信息的快捷方式。本文介绍如何将 echo 命令与 sudo 命令配合使用，实现向那些只有系统管理员才有权限操作的文件中写入信息。
比如要向 test.asc 文件中随便写入点内容，可以：
$ echo "信息" > test.asc
# 或者
$ echo "信息" >> test.asc
# 或者
$ echo "信息" >> test.asc
下面，如果将 test.asc 权限设置为只有 root 用户才有权限进行写操作：
$ sudo chown root.root test.asc
然后，我们使用 sudo 并配合 echo 命令再次向修改权限之后的 test.asc 文件中写入信息：
$ sudo echo "又一行信息" >> test.asc
-bash: test.asc: Permission denied
-bash: test.asc: Permission denied
这时，可以看到 bash 拒绝这么做，说是权限不够。这是因为重定向符号 “>” 和 ">>" 也是 bash 的命令。我们使用 sudo 只是让 echo 命令具有了 root 权限，但是没有让 “>” 和 ">>" 命令也具有 root 权限，所以 bash 会认为这两个命令都没有像 test.asc 文件写入信息的权限。
解决这一问题的途径有两种。第一种是利用 "sh -c" 命令，它可以让 bash 将一个字串作为完整的命令来执行，这样就可以将 sudo 的影响范围扩展到整条命令。具体用法如下：
$ sudo sh -c 'echo "又一行信息" >> test.asc'
另一种方法是利用管道和 tee 命令，该命令可以从标准输入中读入信息并将其写入标准输出或文件中，具体用法如下：
$ echo "第三条信息" | sudo tee -a test.asc
注意，tee 命令的 "-a" 选项的作用等同于 ">>" 命令，如果去除该选项，那么 tee 命令的作用就等同于 ">" 命令。
2009年4月23日 07:52
有道理!
我都是
sudo bash
command
exit
2009年4月23日 18:45
不错，这个介绍无论内容和风格都很好。
2009年4月25日 07:04
这个主题有些不太对吧.这个只能说明sudo的使用权限方式.
2009年4月25日 07:14
只是想通过一个具体的问题来揭示使用 sudo 时的注意事项。以后，有关 bash 的大部分文章，都可以通过具体的例子开展。
2009年4月26日 02:00
挺有用的！
2009年6月11日 18:47
最后的方法比较不错
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