如果您是Linux新手，那么许多命令及其变体可能看起来有点混乱。以“echo”命令为例。为什么人们在安装软件时使用它？今天的超级用户问答帖子回答了一位Linux新用户的问题。

今天的问答环节是由SuperUser提供的，SuperUser是Stack Exchange的一个分支，是一个由社区驱动的问答网站分组。

问题

超级用户读者PallavBakshi想知道为什么人们在Linux中安装软件时使用“echo”命令：

I am new to the computing world. While installing ROS Indigo, the first step said that I should use the following code:

• sudo sh -c ‘echo “deb http://packages.ros.org/ros/ubuntu $(l**_release -sc) main” > /etc/apt/sources.list.d/ros-latest.list’

Why do people use the “echo” command along with “sh -c” in this context? I have seen the “echo” command used in other installation processes as well.

Links I Looked Through

What Exactly is the “sh” Command?

Ubuntu Install of ROS Indigo

为什么人们在Linux中安装软件时使用“echo”命令？

答案

超级用户舰队司令部给了我们答案：

Ordinarily, the function of the “echo” command is to display a string (piece of text) on the c***ole. But this time, a (greater than) > character is added after the echo command, redirecting its output to a text file located in /etc/apt/sources.list.d/ros-latest.list.

Basically, this whole command writes a piece of text to a text file. Now, here comes the tricky part:

The string written to the file may be different for each computer. The part, $(l**_release -sc), is resolved (changed into something else) when the “echo” command runs.

You can open /etc/apt/sources.list.d/ros-latest.list in a text editor before and after the command to see the changes for yourself. Keep in mind that the file might not exist before using this command.

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