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Migrating to Linux: The Command Line
======
![](https://www.linux.com/sites/lcom/files/styles/rendered_file/public/migrate.jpg?itok=2PBkvV7s)
This is the fourth article in our series on migrating to Linux. If you missed the previous installments, we've covered [Linux for new users][1], [files and filesystems][2], and [graphical environments][3]. Linux is everywhere. It's used to run most Internet services like web servers, email servers, and others. It's also used in your cell phone, your car console, and a whole lot more. So, you might be curious to try out Linux and learn more about how it works.
Under Linux, the command line is very useful. On desktop Linux systems, although the command line is optional, you will often see people have a command line window open alongside other application windows. On Internet servers, and when Linux is running in a device, the command line is often the only way to interact directly with the system. So, it's good to know at least some command line basics.
In the command line (often called a shell in Linux), everything is done by entering commands. You can list files, move files, display the contents of files, edit files, and more, even display web pages, all from the command line.
If you are already familiar with using the command line in Windows (either CMD.EXE or PowerShell), you may want to jump down to the section titled Familiar with Windows Command Line? and read that first.
### Navigating
In the command line, there is the concept of the current working directory (Note: A folder and a directory are synonymous, and in Linux they're usually called directories). Many commands will look in this directory by default if no other directory path is specified. For example, typing ls to list files, will list files in this working directory. For example:
```
$ ls
Desktop Documents Downloads Music Pictures README.txt Videos
```
The command, ls Documents, will instead list files in the Documents directory:
```
$ ls Documents
report.txt todo.txt EmailHowTo.pdf
```
You can display the current working directory by typing pwd. For example:
```
$ pwd
/home/student
```
You can change the current directory by typing cd and then the directory you want to change to. For example:
```
$ pwd
/home/student
$ cd Downloads
$ pwd
/home/student/Downloads
```
A directory path is a list of directories separated by a / (slash) character. The directories in a path have an implied hierarchy, for example, where the path /home/student expects there to be a directory named home in the top directory, and a directory named student to be in that directory home.
Directory paths are either absolute or relative. Absolute directory paths start with the / character.
Relative paths start with either . (dot) or .. (dot dot). In a path, a . (dot) means the current directory, and .. (dot dot) means one directory up from the current one. For example, ls ../Documents means look in the directory up one from the current one and show the contents of the directory named Documents in there:
```
$ pwd
/home/student
$ ls
Desktop Documents Downloads Music Pictures README.txt Videos
$ cd Downloads
$ pwd
/home/student/Downloads
$ ls ../Documents
report.txt todo.txt EmailHowTo.pdf
```
When you first open a command line window on a Linux system, your current working directory is set to your home directory, usually: /home/<your login name here>. Your home directory is dedicated to your login where you can store your own files.
The environment variable $HOME expands to the directory path to your home directory. For example:
```
$ echo $HOME
/home/student
```
The following table shows a summary of some of the common commands used to navigate directories and manage simple text files.
### Searching
Sometimes I forget where a file resides, or I forget the name of the file I am looking for. There are a couple of commands in the Linux command line that you can use to help you find files and search the contents of files.
The first command is find. You can use find to search for files and directories by name or other attribute. For example, if I forgot where I kept my todo.txt file, I can run the following:
```
$ find $HOME -name todo.txt
/home/student/Documents/todo.txt
```
The find program has a lot of features and options. A simple form of the command is:
find <directory to search> -name <filename>
If there is more than one file named todo.txt from the example above, it will show me all the places where it found a file by that name. The find command has many options to search by type (file, directory, or other), by date, newer than date, by size, and more. You can type:
```
man find
```
to get help on how to use the find command.
You can also use a command called grep to search inside files for specific contents. For example:
```
grep "01/02/2018" todo.txt
```
will show me all the lines that have the January 2, 2018 date in them.
### Getting Help
There are a lot of commands in Linux, and it would be too much to describe all of them here. So the next best step to show how to get help on commands.
The command apropos helps you find commands that do certain things. Maybe you want to find out all the commands that operate on directories or get a list of open files, but you don't know what command to run. So, you can try:
```
apropos directory
```
which will give a list of commands and have the word "directory" in their help text. Or, you can do:
```
apropos "list open files"
```
which will show one command, lsof, that you can use to list open files.
If you know the command you need to use but aren't sure which options to use to get it to behave the way you want, you can use the command called man, which is short for manual. You would use man <command>, for example:
```
man ls
```
You can try man ls on your own. It will give several pages of information.
The man command explains all the options and parameters you can give to a command, and often will even give an example.
Many commands often also have a help option (e.g., ls --help), which will give information on how to use a command. The man pages are usually more detailed, while the --help option is useful for a quick lookup.
### Scripts
One of the best things about the Linux command line is that the commands that are typed in can be scripted, and run over and over again. Commands can be placed as separate lines in a file. You can put #!/bin/sh as the first line in the file, followed by the commands. Then, once the file is marked as executable, you can run the script as if it were its own command. For example,
```
--- contents of get_todays_todos.sh ---
#!/bin/sh
todays_date=`date +"%m/%d/%y"`
grep $todays_date $HOME/todos.txt
```
Scripts help automate certain tasks in a set of repeatable steps. Scripts can also get very sophisticated if needed, with loops, conditional statements, routines, and more. There's not space here to go into detail, but you can find more information about Linux bash scripting online.
Familiar with Windows Command Line?
If you are familiar with the Windows CMD or PowerShell program, typing commands at a command prompt should feel familiar. However, several things work differently in Linux and if you don't understand those differences, it may be confusing.
First, under Linux, the PATH environment variable works different than it does under Windows. In Windows, the current directory is assumed to be the first directory on the path, even though it's not listed in the list of directories in PATH. Under Linux, the current directory is not assumed to be on the path, and it is not explicitly put on the path either. Putting . in the PATH environment variable is considered to be a security risk under Linux. In Linux, to run a program in the current directory, you need to prefix it with ./ (which is the file's relative path from the current directory). This trips up a lot of CMD users. For example:
```
./my_program
```
rather than
```
my_program
```
In addition, in Windows paths are separated by a ; (semicolon) character in the PATH environment variable. On Linux, in PATH, directories are separated by a : (colon) character. Also in Linux, directories in a single path are separated by a / (slash) character while under Windows directories in a single path are separated by a \ (backslash) character. So a typical PATH environment variable in Windows might look like:
```
PATH="C:\Program Files;C:\Program Files\Firefox;"
while on Linux it might look like:
PATH="/usr/bin:/opt/mozilla/firefox"
```
Also note that environment variables are expanded with a $ on Linux, so $PATH expands to the contents of the PATH environment variable whereas in Windows you need to enclose the variable in percent symbols (e.g., %PATH%).
In Linux, options are commonly passed to programs using a - (dash) character in front of the option, while under Windows options are passed by preceding options with a / (slash) character. So, under Linux, you would do:
```
a_prog -h
```
rather than
```
a_prog /h
```
Under Linux, file extensions generally don't signify anything. For example, renaming myscript to myscript.bat doesn't make it executable. Instead to make a file executable, the file's executable permission flag needs to be set. File permissions are covered in more detail next time.
Under Linux when file and directory names start with a . (dot) character they are hidden. So, for example, if you're told to edit the file, .bashrc, and you don't see it in your home directory, it probably really is there. It's just hidden. In the command line, you can use option -a on the command ls to see hidden files. For example:
```
ls -a
```
Under Linux, common commands are also different from those in the Windows command line. The following table that shows a mapping from common items used under CMD and the alternative used under Linux.
![](https://www.linux.com/sites/lcom/files/styles/rendered_file/public/table-2_0.png?itok=NNc8TZFZ)
--------------------------------------------------------------------------------
via: https://www.linux.com/blog/learn/2018/1/migrating-linux-command-line
作者：[John Bonesio][a]
译者：[译者ID](https://github.com/译者ID)
校对：[校对者ID](https://github.com/校对者ID)
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
[a]:https://www.linux.com/users/johnbonesio
[1]:https://www.linux.com/blog/learn/intro-to-linux/2017/10/migrating-linux-introduction
[2]:https://www.linux.com/blog/learn/intro-to-linux/2017/11/migrating-linux-disks-files-and-filesystems
[3]:https://www.linux.com/blog/learn/2017/12/migrating-linux-graphical-environments

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迁徙到 Linux：命令行环境
======
![](https://www.linux.com/sites/lcom/files/styles/rendered_file/public/migrate.jpg?itok=2PBkvV7s)
这是关于迁徙到 Linux 系列的第四篇文章了。如果您错过了之前的内容，可以回顾我们之前谈到的内容 [新手之 Linux][1]、[文件和文件系统][2]、和 [图形环境][3]。Linux 无处不在，它可以运行在大部分的网络服务器，如 web、email 和其他服务器；它同样可以在您的手机、汽车控制台和其他很多设备上使用。现在，您可能会开始好奇 Linux 系统，并对学习 Linux 的工作原理萌发兴趣。
在 Linux 下，命令行非常实用。Linux 的桌面系统中，尽管命令行只是可选操作，但是您依旧能看见很多朋友开着一个命令行窗口和其他应用窗口并肩作战。在运行 Linux 系统的网络服务器中，命令行通常是唯一能直接与操作系统交互的工具。因此，命令行是有必要了解的，至少应当涉猎一些基础命令。
在命令行（通常称之为 Linux shell）中，所有操作都是通过键入命令完成。您可以执行查看文件列表、移动文件位置、显示文件内容、编辑文件内容等一系列操作，通过命令行，您甚至可以查看网页中的内容。
如果您在 Windows（CMD 或者 PowerShell） 上已经熟悉关于命令行的使用，您是否想跳转到了解 Windows 命令行的章节上去？先了阅读这些内容吧。
### 导语
在命令行中，这里有一个当前工作目录（文件夹和目录是同义词，在 Linux 中它们通常都被称为目录）的概念。如果没有特别指定目录，许多命令的执行会在当前目录下生效。比如，键入 ls 列出文件目录，当前工作目录的文件将会被列举出来。看一个例子：
```
$ ls
Desktop Documents Downloads Music Pictures README.txt Videos
```
`ls Documents` 这条命令将会列出 `Documents` 目录下的文件：
```
$ ls Documents
report.txt todo.txt EmailHowTo.pdf
```
通过 `pwd` 命令可以显示当前您的工作目录。比如：
```
$ pwd
/home/student
```
您可以通过 `cd` 命令改变当前目录并切换到您想要抵达的目录。比如：
```
$ pwd
/home/student
$ cd Downloads
$ pwd
/home/student/Downloads
```
路径中的目录由 `/`（左斜杠）字符分隔。路径中有一个隐含的层次关系，比如 `/home/student` 目录中，home 是顶层目录，而 student 是 home 的子目录。
路径要么是绝对路径，要么是相对路径。绝对路径由一个 `/` 字符打头。
相对路径由 `.` 或者 `..` 开始。在一个路径中，一个 `.` 意味着当前目录，`..` 意味着当前目录的上级目录。比如，`ls ../Documents` 意味着在此寻找当前目录的上级名为 `Documets` 的目录：
```
$ pwd
/home/student
$ ls
Desktop Documents Downloads Music Pictures README.txt Videos
$ cd Downloads
$ pwd
/home/student/Downloads
$ ls ../Documents
report.txt todo.txt EmailHowTo.pdf
```
当您第一次打开命令行窗口时，您当前的工作目录被设置为您的家目录，通常为 `/home/<您的登录名>`。家目录专用于登陆之后存储您的专属文件。
设置环境变量 `$HOME` 到您的家目录，比如：
```
$ echo $HOME
/home/student
```
下表显示了用于目录导航和管理简单的文本文件的一些命令摘要。
![table](https://www.linux.com/sites/lcom/files/styles/rendered_file/public/table-1_0.png?itok=j4Sgv6Vy)
### 搜索
有时我们会遗忘文件的位置，或者忘记了我要寻找的文件名。Linux 命令行有几个命令可以帮助您搜索到文件。
第一个命令是 `find`。您可以使用 `find` 命令通过文件名或其他属性搜索文件和目录。举个例子，当您遗忘了 todo.txt 文件的位置，我们可以执行下面的代码：
```
$ find $HOME -name todo.txt
/home/student/Documents/todo.txt
```
`find` 程序有很多功能和选项。一个简单的例子：
```
find <要寻找的目录> -name <文件名>
```
如果这里有 `todo.txt` 文件且不止一个，它将向我们列出拥有这个名字的所有文件的所有所在位置。`find` 命令有很多便于搜索的选项比如类型（文件或是目录等等）、时间、大小和其他一些选项。更多内容您可以同通过:`man find` 获取关于如何使用 `find` 命令的帮助。
您还可以使用 `grep` 命令搜索文件的特殊内容，比如：
```
grep "01/02/2018" todo.txt
```
这将为您展示 `todo` 文件中 `01/02/2018` 所在行。
### 获取帮助
Linux 有很多命令，这里,我们没有办法一一列举。授人以鱼不如授人以渔，所以下一步我们将向您介绍帮助命令。
`apropos` 命令可以帮助您查找需要使用的命令。也许您想要查找能够操作目录或是获得文件列表的所有命令，但是您并不希望让这些命令执行。您可以这样尝试：
```
apropos directory
```
要在帮助文档中，得到一个于 `directiory` 关键字的相关命令列表，您可以这样操作：
```
apropos "list open files"
```
这将提供一个 `lsof` 命令给您，帮助您打开文件列表。
当您明确您要使用的命令，但是不确定应该使用什么选项完成预期工作，您可以使用 man 命令，它是 manual 的缩写。您可以这样使用：
```
man ls
```
您可以在自己的设备上尝试这个命令。它会提供给您关于使用这个命令的完整信息。
通常，很多命令都会有能够给 `help` 选项（比如说，`ls --help`），列出命令使用的提示。`man` 页面的内容通常太繁琐，`--help` 选项可能更适合快速浏览。
### 脚本
Linux 命令行中最贴心的功能是能够运行脚本文件，并且能重复运行。Linux 命令可以存储在文本文件中，您可以在文件的开头写入 `#!/bin/sh`，之后追加命令。之后，一旦文件被存储为可执行文件，您就可以像执行命令一样运行脚本文件，比如，
```
--- contents of get_todays_todos.sh ---
#!/bin/sh
todays_date=`date +"%m/%d/%y"`
grep $todays_date $HOME/todos.txt
```
在一个确定的工作中脚本可以帮助自动化重复执行命令。如果需要的话，脚本也可以很复杂，能够使用循环、判断语句等。限于篇幅，这里不细述，但是您可以在网上查询到相关信息。
您是否已经熟悉了 Windows 命令行？
如果您对 Windows CMD 或者 PowerShell 程序很熟悉，在命令行输入命令应该是轻车熟路的。然而，它们之间有很多差异，如果您没有理解它们之间的差异可能会为之困扰。
首先，在 Linux 下的 PATH 环境于 Windows 不同。在 Windows 中，当前目录被认为是路径中的第一个文件夹，尽管该目录没有在环境变量中列出。而在 Linux 下，当前目录不会在路径中显示表示。Linux 下设置环境变量会被认为是风险操作。在 Linux 的当前目录执行程序，您需要使用 ./（代表当前目录的相对目录表示方式） 前缀。这可能会干扰很多 CMD 用户。比如：
```
./my_program
```
而不是
```
my_program
```
另外，在 Windows 环境变量的路径中是以 `;`（分号） 分割的。在 Linux 中，由 `:` 分割环境变量。同样，在 Linux 中路径由 `/` 字符分隔，而在 Windows 目录中路径由 `\` 字符分割。因此 Windows 中典型的环境变量会像这样：
```
PATH="C:\Program Files;C:\Program Files\Firefox;"
while on Linux it might look like:
PATH="/usr/bin:/opt/mozilla/firefox"
```
还要注意，在 Linux 中环境变量由 `$` 拓展，而在 Windows 中您需要使用百分号（就是这样: %PATH%）。
在 Linux 中，通过 `-` 使用命令选项，而在 Windows 中，使用选项要通过 `/` 字符。所以，在 Linux 中您应该：
```
a_prog -h
```
而不是
```
a_prog /h
```
在 Linux 下，文件拓展名并没有意义。例如，将 `myscript` 重命名为 `myscript.bat` 并不会因此而可执行，需要设置文件的执行权限。文件执行权限会在下次的内容中覆盖到。
在 Linux 中，如果文件或者目录名以 `.` 字符开头，意味着它们是隐藏文件。比如，如果您申请编辑 `.bashrc` 文件，您不能在 `home` 目录中找到它，但是它可能真的存在，只不过它是隐藏文件。在命令行中，您可以通过 `ls` 命令的 `-a` 选项查看隐藏文件，比如：
```
ls -a
```
在 Linux 中，普通的命令与 Windows 的命令不尽相同。下面的表格显示了常用命令中 CMD 命令和 Linux 命令行的差异。
![](https://www.linux.com/sites/lcom/files/styles/rendered_file/public/table-2_0.png?itok=NNc8TZFZ)
--------------------------------------------------------------------------------
via: https://www.linux.com/blog/learn/2018/1/migrating-linux-command-line
作者：[John Bonesio][a]
译者：[CYLeft](https://github.com/CYLeft)
校对：[校对者ID](https://github.com/校对者ID)
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
[a]:https://www.linux.com/users/johnbonesio
[1]:https://www.linux.com/blog/learn/intro-to-linux/2017/10/migrating-linux-introduction
[2]:https://www.linux.com/blog/learn/intro-to-linux/2017/11/migrating-linux-disks-files-and-filesystems
[3]:https://www.linux.com/blog/learn/2017/12/migrating-linux-graphical-environments