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[translated] Understanding Linux cd command with Examples
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Understanding Linux cd Command with Examples
================================================================================
The cd command is the most basic command in *nix world. What linux cd command does is it changes your current working directory. This article details about this command, what this command can do and the internals about the command.
### cd command: An internal command ###
The BASH shell is the default shell in most Linux distributions. The shell has some internal commands of its own. cd is one od the internal commands. I will explain what internal commands are and why cd is an internal command. First, confirm that your current shell with SHELL environment variable:
![](http://linoxide.com/wp-content/uploads/2013/12/01.cd_shell.png)
Now check with which command what is the path of cd command binary(if any):
![](http://linoxide.com/wp-content/uploads/2013/12/02.cd_which.png)
It results in no output. Its because no binary is present on the system for cd command. Still you are able to execute the command. This is because cd is an BASH internal command. Internal commands are built into the shell. Another internal command ‘type’ will give you the information that cd is internal command.
![](http://linoxide.com/wp-content/uploads/2013/12/03.cd_type.png)
If you try to access the manual entry for any internal command, there is no separate manual page for them.
![](http://linoxide.com/wp-content/uploads/2013/12/04.cd_man.png)
For these internal commands, no separate process is created. So they are very fast.
To get full list of internal commands, you can give help command (which itself is an internal command):
![](http://linoxide.com/wp-content/uploads/2013/12/05.cd_help_1.png)
![](http://linoxide.com/wp-content/uploads/2013/12/06.cd_help_2.png)
### Why cd is internal command? ###
To keep things simple, I won’t go into much details, but it requires a little understanding of Unix processes to understand the answer to this question.
Whenever a processes is created by BASH, it is executed in a subshell of BASH (child process of current BASH process). The new processes makes changes and outputs (if reqquired) and no property of this subshell is returned to the parent when the process dies. Note that cd command changed the PWD of the shell. If cd were an external command, it would have changed the PWD of the subshell, returning nothing to the parent shell. And hence, the PWD of the shell will never change. All the commands that make changes to the environment of the shell, must be implemented as internal command. we could never achieve what we require by making cd an external command.
Now let us explore cd command usage.
### cd command usage: ###
If you issue cd command without any argument, it will take you to your home directory, no matter what your current directory is.
![](http://linoxide.com/wp-content/uploads/2013/12/07.cd_home.png)
The tilde sign (~) also represents the home directory. You can se this as well to go to your home directory.
![](http://linoxide.com/wp-content/uploads/2013/12/08.cd_home_tilde.png)
If you are root, you can go to any user’s home directory by using tilde sign followed by that user’s name. In some Linux distributions, unpriviledged users do not by default have permission to go to other users’ home directories.
![](http://linoxide.com/wp-content/uploads/2013/12/09.cd_home_user.png)
The . directory represents the current directory and the .. represents the parent directory. To go to parent directory, use ..
![](http://linoxide.com/wp-content/uploads/2013/12/10.cd_parent.png)
The . directory will not change your PWD in most of the cases. For example:
![](http://linoxide.com/wp-content/uploads/2013/12/11.cd_dot.png)
But if your current directory is renamed somehow, then your PWD will be changed with this command:
![](http://linoxide.com/wp-content/uploads/2013/12/12.cd_dot_renamed.png)
In BASH and most of other shells, you can provide two types of paths: Absolute path and Relative path. Absolute paths begin with / and are independent of your PWD. On the other hand, relative paths never begin with / and depend what your PWD is.
Changing PWD with absolute path:
![](http://linoxide.com/wp-content/uploads/2013/12/13.cd_abs_path.png)
Changing PWD with relative path:
![](http://linoxide.com/wp-content/uploads/2013/12/14cd_rel_path.png)
Toggling between two directories:
The “cd -” command will take you to the last working directory. You can toggle between two directories with this command.
![](http://linoxide.com/wp-content/uploads/2013/12/15.cd_toggle.png)
The last working directory is strored in OLDPWD variable. If you try to use previous command in a new terminal, it shows following error:
![](http://linoxide.com/wp-content/uploads/2013/12/16.cd_OLDPWD_not_set.png)
![](http://linoxide.com/wp-content/uploads/2013/12/16.cd_OLDPWD_not_set.png)
You can use some bash tricks as well with cd command.
With ? wild card:
![](http://linoxide.com/wp-content/uploads/2013/12/17.cd_question_mark_wild_card.png)
Using *
![](http://linoxide.com/wp-content/uploads/2013/12/18.cd_star_wild_card.png)
--------------------------------------------------------------------------------
via: http://linoxide.com/linux-command/linux-cd-command-examples/
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) 原创翻译，[Linux中国](http://linux.cn/) 荣誉推出
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通过例子来理解 Linux 的 cd 命令
================================================================================
在 *nix 的世界里，cd 命令是最为基础的命令，它所做的是改变当前的工作目录。 本文将深入讲解关于 cd 的技术细节。
### cd 命令：一个内在的命令
绝大多数的 Linux 发行版都将 Bash Shell 用作默认的 Shell。Bash 有一些自己内部的命令，cd 就是其中一个。我将会解释究竟什么是内部命令，以及为什么 cd 会是一个内部命令。那么首先，请检查确认你的当前 shell 以及环境变量：
![](http://linoxide.com/wp-content/uploads/2013/12/01.cd_shell.png)
现在继续使用 which 命令来检查 cd 所在的路径：
![](http://linoxide.com/wp-content/uploads/2013/12/02.cd_which.png)
我们发现没有输出。因为在我们的系统中 cd 命令其实并没有对应某个 binary，但是我们却又仍然可以执行这个命令，这就是因为 cd 其实是 BASH 的一个内部命令。内部命令都是被整合进入 shell 里面的。接下来我们可以继续使用 type 命令来查看 cd 这个内部命令的更多信息。
![](http://linoxide.com/wp-content/uploads/2013/12/03.cd_type.png)
如果你尝试去查看内部命令的手册，会发现并没有为它们单独设置手册。
![](http://linoxide.com/wp-content/uploads/2013/12/04.cd_man.png)
对于这些内部命令来说，并没有创建单独的进程，因此它们执行起来非常快。
我们可以使用 help 命令来查看所有的内部命令（help 命令本身也是一个内部命令）：
![](http://linoxide.com/wp-content/uploads/2013/12/05.cd_help_1.png)
![](http://linoxide.com/wp-content/uploads/2013/12/06.cd_help_2.png)
### 为什么 cd 会是一个内部命令：
让我们从简单的开始聊起吧。我不会说太多繁琐细节，但是想要弄明白这个问题，还是需要对 Unix 进程有一点了解。
不管 BASH 什么时候创建了一个进程，这个进程事实上都是在 BASH 的一个子 shell 中运行（也就是当前 BASH 进程的子进程）。这些新的进程会做一些改变，或是打印一些东西（如果需要的话），以及当这个进程死亡的时候，子 shell 并不会向当前 BASH 返回信息。请注意，cd 所做的是改变 shell 的 PWD。 因此倘若 cd 是一个外部命令，那么它改变的将会是子 shell 的 PWD，也不会向父 shell 返回任何东西。所以，当前 shell 的 PWD 就不会做任何改变。所有能对当前 shell
的环境作出改变的命令都必须是内部命令。 因此如果我们将 cd 做成外部命令，就无法像原来一样改变当前目录了。
好了，现在让我们再来聊聊 cd 命令的用法。
### cd 命令的用法：
如果你不加任何参数地来使用 cd 命令，不管你现在在哪，都会把你带到你的 home 目录下。
![](http://linoxide.com/wp-content/uploads/2013/12/07.cd_home.png)
其实波浪线符号（~）代表的也是 home 目录，我们同样可以用它回到 home。
![](http://linoxide.com/wp-content/uploads/2013/12/08.cd_home_tilde.png)
如果你是 root 用户，你可以通过使用波浪号加任何一个用户名来切换到该用户的 home 目录下。在一些 Linux 发行版中，没有特权的用户默认是没有权限访问其他用户的 home 目录的。
![](http://linoxide.com/wp-content/uploads/2013/12/09.cd_home_user.png)
再来看，*.* 目录代表的是当前目录，而 *..* 代表的是上级目录。所以我们可以使用 *..* 来回到上级目录。
![](http://linoxide.com/wp-content/uploads/2013/12/10.cd_parent.png)
大部分情况下，. 是不会改变你的 PWD。举例来看：
![](http://linoxide.com/wp-content/uploads/2013/12/11.cd_dot.png)
但是如果你的当前目录已经被重命名了，那么你的 PWD 就会发生改变：
![](http://linoxide.com/wp-content/uploads/2013/12/12.cd_dot_renamed.png)
在 BASH 和大部分其他的 shell 中，你可以提供两种不同的路径：绝对路径和相对路径。绝对路径以 / 开始，并且和你的 PWD 无关。而相对路径不会以 / 开始，并且和你的 PWD 相关。
Changing PWD with absolute path:
使用绝对路径来改变 PWD：
![](http://linoxide.com/wp-content/uploads/2013/12/13.cd_abs_path.png)
使用相对路径来改变 PWD：
![](http://linoxide.com/wp-content/uploads/2013/12/14cd_rel_path.png)
我们可以在两个目录切换：
*cd -* 命令可以返回上一次的工作目录，我们可以使用它在两个不同的目录中快速切换。
![](http://linoxide.com/wp-content/uploads/2013/12/15.cd_toggle.png)
我们上一次的工作目录会被储存在 OLDPWD 这个变量中。但是如果你在一个新开启的终端中来使用的话，会报出下面的错误：
![](http://linoxide.com/wp-content/uploads/2013/12/16.cd_OLDPWD_not_set.png)
![](http://linoxide.com/wp-content/uploads/2013/12/16.cd_OLDPWD_not_set.png)
你可以配合 cd 命令来使用一些 bash 小技巧。
比如，不妨使用 *?* 这个万能药：
![](http://linoxide.com/wp-content/uploads/2013/12/17.cd_question_mark_wild_card.png)
使用 ***：
![](http://linoxide.com/wp-content/uploads/2013/12/18.cd_star_wild_card.png)
--------------------------------------------------------------------------------
via: http://linoxide.com/linux-command/linux-cd-command-examples/
译者：[译者ID](https://github.com/ailurus1991) 校对：[校对者ID](https://github.com/校对者ID)
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创翻译，[Linux中国](http://linux.cn/) 荣誉推出
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