Creating Things
***************
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.
Making Directories
^^^^^^^^^^^^^^^^^^
We now know how to explore files and directories, but how do we create
them in the first place? Let's go back to Nelle's home directory,
``filesystem/users/nelle``, and use ``ls -F`` to see what it contains:
::
$ pwd
::
/Users/SWC/Downloads/filesystem/users/nelle
::
$ ls -F
::
creatures/ molecules/ pizza.cfg
data/ north-pacific-gyre/ solar.pdf
Desktop/ notes.txt writing/
Let's create a new directory called ``thesis`` using the command
``mkdir thesis`` (which has no output):
::
$ mkdir thesis
As you might (or might not) guess from its name, ``mkdir`` means "make
directory". Since ``thesis`` is a relative path (i.e., doesn't have a
leading slash), the new directory is made below the current working
directory:
::
$ ls -F
::
creatures/ north-pacific-gyre/ thesis/
data/ notes.txt writing/
Desktop/ pizza.cfg
molecules/ solar.pdf
However, there's nothing in it yet:
::
$ ls -F thesis
Let's make a file
^^^^^^^^^^^^^^^^^
Let's change our working directory to ``thesis`` using ``cd``, then run
a text editor called Nano to create a file called ``draft.txt``:
::
$ cd thesis
$ 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 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:
.. image:: img/nano-screenshot.png
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``:
::
$ ls
::
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
``filesystem/users/nelle`` using ``cd ..``:
::
$ pwd
::
/Users/SWC/Download/filesystem/users/nelle/thesis
::
$ nano draft.txt
$ ls
::
draft.txt
::
$ cd ..
If we try to remove the entire ``thesis`` directory using ``rm thesis``,
we get an error message:
::
$ rm thesis
::
rm: cannot remove `thesis': 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 thesis
::
rmdir: failed to remove `thesis': 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 ``thesis`` we must first
delete the file ``draft.txt``:
::
$ rm thesis/draft.txt
The directory is now empty, so ``rmdir`` can delete it:
::
$ rmdir thesis
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 thesis # Warning! This can set you six years behind.
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.
More Practice
^^^^^^^^^^^^^
Let's create that directory and file one more time. (Note that this time
we're running ``nano`` with the path ``thesis/draft.txt``, rather than
going into the ``thesis`` directory and running ``nano`` on
``draft.txt`` there.)
::
$ pwd
::
/users/nelle
::
$ mkdir thesis
::
$ nano thesis/draft.txt
$ ls thesis
::
draft.txt
``draft.txt`` isn't a particularly informative name, so let's change the
file's name using ``mv``, which is short for "move":
::
$ mv thesis/draft.txt thesis/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 ``thesis/draft.txt`` to
``thesis/quotes.txt``, which has the same effect as renaming the file.
Sure enough, ``ls`` shows us that ``thesis`` now contains one file
called ``quotes.txt``:
::
$ ls thesis
::
quotes.txt
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 thesis/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 ``thesis`` is empty:
::
$ ls thesis
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
::
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 thesis/quotations.txt
$ ls quotes.txt thesis/quotations.txt
::
quotes.txt thesis/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 ``thesis`` that we didn't delete:
::
$ ls quotes.txt thesis/quotations.txt
::
ls: cannot access quotes.txt: No such file or directory
thesis/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
Nelle's home directory is ``/home/nelle``, then ``~/data`` is
equivalent to ``/home/nelle/data``. This only works if it is the
first character in the path: ``here/there/~/elsewhere`` is *not*
``/home/nelle/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.
- Nano is a very simple text editor—please use something else for real
work.
Exercises
^^^^^^^^^
What is the output of the closing ``ls`` command in the sequence shown
below?
::
$ pwd
/home/jamie/data
$ ls
proteins.dat
$ mkdir recombine
$ mv proteins.dat recombine
$ cp recombine/proteins.dat ../proteins-saved.dat
$ ls
Suppose that:
::
$ ls -F
analyzed/ fructose.dat raw/ sucrose.dat
What command(s) could you run so that the commands below will produce
the output shown?
::
$ ls
analyzed raw
$ ls analyzed
fructose.dat sucrose.dat
What does ``cp`` do when given several filenames and a directory name,
as in:
::
$ mkdir backup
$ cp thesis/citations.txt thesis/quotations.txt backup
What does ``cp`` do when given three or more filenames, as in:
::
$ ls -F
intro.txt methods.txt survey.txt
$ cp intro.txt methods.txt survey.txt
The command ``ls -R`` lists the contents of directories recursively,
i.e., lists their sub-directories, sub-sub-directories, and so on in
alphabetical order at each level. The command ``ls -t`` lists things by
time of last change, with most recently changed files or directories
first. In what order does ``ls -R -t`` display things?