3 Class 3. Creating and deleting files and directories

Creating and removing directories

Content modified from the Software Carpentries

We now know how to explore files and directories, but how do we create them in the first place?

Step one: see where we are ($ pwd)  and what we already have in this directory ($ ls)

Step two: move if necessary ($ cd argument)

Once you are in the directory where you want to create your sub-directory, use the $ mkdir command.

Let’s create a new directory called thesis using the command mkdir thesis (which has no output):

$ mkdir thesis

As you might guess from its name, mkdir means ‘make directory’. Since thesis is a relative path (i.e., does not have a leading slash, like /what/ever/thesis), the new directory is created in the current working directory. You can use $ ls to verify the directory was made.

Two ways of doing the same thing

Using the shell to create a directory is no different than using a file explorer. If you open the current directory using your operating system’s graphical file explorer, the thesis directory will appear there too. While the shell and the file explorer are two different ways of interacting with the files, the files and directories themselves are the same.

Good names for files and directories

Complicated names of files and directories can make your life painful when working on the command line. Here we provide a few useful tips for the names of your files.

1. Don’t use spaces.Spaces can make a name more meaningful, but since spaces are used to separate arguments on the command line it is better to avoid them in names of files and directories. You can use - or _ instead (e.g. north-pacific-gyre/ rather than north pacific gyre/).
2. Don’t begin the name with - (dash).Commands treat names starting with - as options.
3. Stick with letters, numbers, . (period or ‘full stop’), - (dash) and _ (underscore).Many other characters have special meanings on the command line. We will learn about some of these during this lesson. There are special characters that can cause your command to not work as expected and can even result in data loss.

If you need to refer to names of files or directories that have spaces or other special characters, you should surround the name in quotes ("").

Since we’ve just created the thesis directory, there’s nothing in it yet:

Creating text files

We can run a text editor called Nano to create a file called draft.txt:

$ nano draft.txt

Which Editor?

When we say, ‘nano is a text editor’ we really do mean ‘text’: it can only work with plain character data, not tables, images, or any other human-friendly media. We use it in examples because it is one of the least complex text editors. However, because of this trait, it may not be powerful enough or flexible enough for the work you need to do after this class. On Unix systems (such as Linux and macOS), many programmers use Emacs or Vim (both of which require more time to learn), or a graphical editor such as Gedit. On Windows, you may wish to use Notepad++. Windows also has a built-in editor called notepad that can be run from the command line in the same way as nano for the purposes of this lesson.

No matter what editor you use, you will need to know where it searches for and saves files. If you start it from the shell, it will (probably) use your current working directory as its default location. If you use your computer’s start menu, it may want to save files in your desktop or documents directory instead. You can change this by navigating to another directory the first time you ‘Save As…’

Let’s type in a few lines of text. Once we’re happy with our text, we can press Ctrl+O (press the Ctrl or Control key and, while holding it down, press the O key) to write our data to disk (we’ll be asked what file we want to save this to: press Return to accept the suggested default of draft.txt).

Once our file is saved, we can use Ctrl+X to quit the editor and return to the shell.

Control, Ctrl, or ^ Key

The Control key is also called the ‘Ctrl’ key. There are various ways in which using the Control key may be described. For example, you may see an instruction to press the Control key and, while holding it down, press the X key, described as any of:

• Control-X
• Control+X
• Ctrl-X
• Ctrl+X
• ^X
• C-x

In nano, along the bottom of the screen you’ll see ^G Get Help ^O WriteOut. This means that you can use Control-G to get help and Control-O to save your file.

nano doesn’t leave any output on the screen after it exits, but ls now shows that we have created a file called draft.txt:

$ ls

draft.txt

What’s In A Name?

You may have noticed that all of the files we’ve created are named ‘something dot something’, and in this part of the lesson, we always used the extension .txt. This is just a convention: we can call a file mythesis or almost anything else we want. However, most people use two-part names most of the time to help them (and their programs) tell different kinds of files apart. The second part of such a name is called the filename extension, and indicates what type of data the file holds: .txt signals a plain text file, .pdf indicates a PDF document, .cfg is a configuration file full of parameters for some program or other, .png is a PNG image, and so on.

This is just a convention, albeit an important one. Files contain bytes: it’s up to us and our programs to interpret those bytes according to the rules for plain text files, PDF documents, configuration files, images, and so on.

Naming a PNG image of a whale as whale.mp3 doesn’t somehow magically turn it into a recording of whalesong, though it might cause the operating system to try to open it with a music player when someone double-clicks it.

Removing files

The Unix command we’ll use to delete a file is rm (short for ‘remove’):

$ rm quotes.txt

We can confirm the file has gone using ls:

$ ls quotes.txt

ls: cannot access 'quotes.txt': No such file or directory

Deleting Is Forever

The Unix shell doesn’t have a trash bin that we can recover deleted files from (though most graphical interfaces to Unix do). Instead, when we delete files, they are unlinked from the file system so that their storage space on disk can be recycled. Tools for finding and recovering deleted files do exist, but there’s no guarantee they’ll work in any particular situation, since the computer may recycle the file’s disk space right away.

Using rm Safely

The -i option will prompt before (every) removal (use Y to confirm deletion or N to keep the file). The Unix shell doesn’t have a trash bin, so all the files removed will disappear forever. By using the -i option, we have the chance to check that we are deleting only the files that we want to remove.

If you execute:

$ rm -i thesis_backup/quotations.txt

you will see the following prompt:

$ rm: remove regular file 'thesis_backup/quotations.txt'? y

Hit yes to confirm!

If we try to remove the thesis directory using rm thesis, we get an error message:

$ rm thesis

rm: cannot remove `thesis': Is a directory

This happens because rm by default only works on files, not directories.

rm can remove a directory and all its contents if we use the recursive option -r, and it will do so without any confirmation prompts:

$ rm -r thesis

Given that there is no way to retrieve files deleted using the shell, rm -r should be used with great caution (you might consider adding the interactive option rm -r -i).