Help Topic: Command Line Survival Basics

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Command Line Quick Survival Basics

The Terminal, i.e., the command line environment, offers an incredibly powerful set of tools out of the box. These tools are readily extendible through the addition of countless open source additions, as well as through additions made by you, in this course and hopefully well afterwards. To get started the below list represents the bare bones basics for someone just getting start. This list represents my own perspective, but you can find similar material by just doing a web search on "command-line basics".

Here we use the terms Terminal and command line interchangeably. This reflects a Mac user's perspective since the Terminal application is the most common way to interact with the command line. If you're a GNU/Linux user, you're more likely to refer to the Console instead of the terminal.

Let's get started.

Viewing the Contents of Your Directory with ls Command    [top]

The command line offers just another way of navigating through the same file system you might otherwise navigate with GUIs such as Finder. Perhaps the most commonly used command is ls which just displays the contents of present directory: 

  $ ls
  mail  project_fango   public_html   report_a.pdf   report_b.pdf   test.txt

The ls command comes with a bunch of command line options. These are extra things you can type on the command line as arguments to the command being invoked. For example, in the above, six items are reported. It turns out that mail, project_fango, and public_html are directories, i.e., folders, containing other files or directories. This is hard to discern from the above output. But if you use a simple command line option, "-F", you'll be able to see this right away: 

  $ ls -F
  mail/  project_fango/   public_html/   report_a.pdf   report_b.pdf   test.txt

Note all directories have the trailing "/" character. I also use -G to make the directories show up in color. This can be done by typing ls -F -G, or more simply, ls -FG. There are lots of useful options worth exploring when you get a chance. Just type man ls to see some options.

Note also that arguments provided on the command line are separated by spaces. This is why most command line users never name files with spaces. Note the names report_a.pdf above uses the underscore character instead.

Knowing what directory (folder) you are in with pwd    [top]

Using the command line, you are always "in" a particular directory. Remember, command line users tend to refer to a directory where other users might refer to a folder. To know what the "present working directory" is, simply use the "pwd" command with no arguments. 

  $ pwd
  /Users/mary/lab_one

This is such a useful piece of information to have that many users customize their command line prompt (the dollar-sign at the left) to show the present working directory. As an exercise, google "customize command line prompt" and see if you can set this up.

Changing Directories with the cd Command    [top]

Now that you know how to tell what directory you're, you might be wondering how to change directories, moving around the overall file structure. To do this we use the cd command. The most common argument is the full path name of the directory you want to go to, as in: 

  $ cd /Users/mary/lab_one
  $ pwd
  /Users/mary/lab_one
  $ cd /Users/bob/projects/lab_three
  $ pwd
  /Users/bob/projects/lab_three

There are a few handy shortcuts that should be in your toolbox right away. The first is for changing to the directory right above your current directory. For example, suppose you are presently in the directory /Users/bob/projects/lab_three, and you wish to move up to the directory /Users/bob/projects. You could do it the hard way: 

  $ pwd
  /Users/bob/projects/lab_three
  $ cd /Users/bob/projects
  $ pwd
  /Users/bob/projects

Or you could do it the easy way, replacing the third line above with the simpler version below: 

  $ pwd
  /Users/bob/projects/lab_three
  $ cd ..
  $ pwd
  /Users/bob/projects

The ../ notation is an actual "directory" in every directory in your file system. This special shorthand name has been around forever. Another shorthand name is ./ which stands for the current directory. Because both directories begin with a dot, they are also known as hidden directories, and don't show up when you use the ls command. They can be seen if you use the ls -a option for showing "all" files and directories, including the hidden ones. Go ahead and try it now, you will see both ./ and ../ regardless of which directory you are in. Even the "root" directory, the top-most directory in the tree, contains the ../ directory. Trying to go up from the root directory will just land you back in the root directory however.

A few other handy shortcuts are below. To move up two directories at once: 

  $ cd ../../                      (go up two directories)

To move to straight to your home directory: 

  $ cd                            (go directly to your home directory)
  $ pwd
  $ /Users/bob

To move to a directory within your home directory: 

  $ cd ~/project                  (go to a subdirectory of your home directory)
  $ pwd
  $ /Users/bob/project

To move to the last directory you were in, prior to the last invocation of cd: 

  $ pwd
  $ /Users/bob
  $ cd /Users/mary/projects/one
  $ pwd
  $ /Users/mary/projects/one
  $ cd -                          (go to the directory prior to the last cd)
  $ pwd
  $ /Users/bob

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