02-create.md

Creating Files and Directories

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Objectives

• Create a directory hierarchy that matches a given diagram.
• Create files in that hierarchy using an editor or by copying and renaming existing files.
• Display the contents of a directory using the command line.
• Delete specified files and/or directories.

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We now know how to explore files and directories, but how do we create them in the first place? Let's go to your SWC lessons folder, and use ls -F to see what it contains:

$ ls -F

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

$ mkdir my_scripts

As you might (or might not) guess from its name, mkdir means "make directory". Since my_scripts is a relative path (i.e., doesn't have a leading slash), the new directory is made below the current working directory:

$ ls -F

However, there's nothing in it yet:

$ ls -F my_scripts

Let's change our working directory to my_scripts using cd, then run a text editor called Nano to create a file called draft.txt:

$ cd my_scripts
$ touch 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 almost anyone can drive it anywhere without training, but please use something more powerful for real work. On Unix systems (such as Linux and Mac OS X), many programmers use Emacs or Vim (both of which are completely unintuitive, even by Unix standards), or a graphical editor such as Gedit. On Windows, you may wish to use Notepad++.

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, then use Control-O to write our data to disk:

Once our file is saved, we can use Control-X to quit the editor and return to the shell. (Unix documentation often uses the shorthand ^A to mean "control-A".) 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:

Let's tidy up by running rm draft.txt:

$ rm draft.txt

This command removes files ("rm" is short for "remove"). If we run ls again, its output is empty once more, which tells us that our file is gone:

$ ls

Deleting Is Forever

Unix doesn't have a trash bin: when we delete files, they are unhooked 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.

Let's re-create that file and then move up one directory to your SWC folder using cd ..:

$ pwd
$ touch draft.txt
$ ls
$ cd ..

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

$ rm my_scripts
rm: cannot remove `my_scripts': Is a directory

This happens because rm only works on files, not directories. The right command is rmdir, which is short for "remove directory". It doesn't work yet either, though, because the directory we're trying to remove isn't empty:

$ rmdir my_scripts
rmdir: failed to remove `my_scripts': Directory not empty

This little safety feature can save you a lot of grief, particularly if you are a bad typist. To really get rid of my_scripts we must first delete the file draft.txt:

$ rm my_scripts/draft.txt

The directory is now empty, so rmdir can delete it:

$ rmdir my_scripts

With Great Power Comes Great Responsibility

Removing the files in a directory just so that we can remove the directory quickly becomes tedious. Instead, we can use rm with the -r flag (which stands for "recursive"):

$ rm -r my_scripts

This removes everything in the directory, then the directory itself. If the directory contains sub-directories, rm -r does the same thing to them, and so on. It's very handy, but can do a lot of damage if used without care.

Let's create that directory and file one more time. (Note that this time we're running nano with the path my_scripts/draft.txt, rather than going into the my_scripts directory and running nano or 'vim' on draft.txt there.)

draft.txt isn't a particularly informative name, so let's change the file's name using mv, which is short for "move":

$ mv my_scripts/draft.txt my_scripts/quotes.txt

The first parameter tells mv what we're "moving", while the second is where it's to go. In this case, we're moving my_scripts/draft.txt to my_scripts/quotes.txt, which has the same effect as renaming the file. Sure enough, ls shows us that my_scripts now contains one file called quotes.txt:

$ ls my_scripts

Just for the sake of inconsistency, mv also works on directories - there is no separate mvdir command.

Let's move quotes.txt into the current working directory. We use mv once again, but this time we'll just use the name of a directory as the second parameter to tell mv that we want to keep the filename, but put the file somewhere new. (This is why the command is called "move".) In this case, the directory name we use is the special directory name . that we mentioned earlier.

$ mv my_scripts/quotes.txt .

The effect is to move the file from the directory it was in to the current working directory. ls now shows us that my_scripts is empty:

$ ls my_scripts

Further, ls with a filename or directory name as a parameter only lists that file or directory. We can use this to see that quotes.txt is still in our current directory:

$ ls quotes.txt

The cp command works very much like mv, except it copies a file instead of moving it. We can check that it did the right thing using ls with two paths as parameters—like most Unix commands, ls can be given thousands of paths at once:

$ cp quotes.txt my_scripts/quotations.txt
$ ls quotes.txt my_scripts/quotations.txt

To prove that we made a copy, let's delete the quotes.txt file in the current directory and then run that same ls again. This time it tells us that it can't find quotes.txt in the current directory, but it does find the copy in my_scripts that we didn't delete:

$ ls quotes.txt my_scripts/quotations.txt
ls: cannot access quotes.txt: No such file or directory
my_scripts/quotations.txt

Another Useful Abbreviation

The shell interprets the character ~ (tilde) at the start of a path to mean "the current user's home directory". For example, if my home directory is /home/april, then ~/data is equivalent to /home/april/data. This only works if it is the first character in the path: here/there/~/elsewhere is not /home/april/elsewhere.

Key Points

• Unix documentation uses '^A' to mean "control-A".
• The shell does not have a trash bin: once something is deleted, it's really gone.

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