Working With Files and Directories

Overview:

Teaching: 25 min

Exercises: 10 min

Questions

  • How can I create, copy, and delete files and directories?
  • How can I edit files?

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.
  • Delete, copy and move specified files and/or 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 our data-shell directory on the Desktop and use ls -F to see what it contains:

In [1]:
%%bash2 --dir ~/data/data-shell
pwd

/home/nbuser/library/data/data-shell

If after working through the exercises at the end of the previous lesson, you are not in your data-shell directory, navigate to the correct location.

In [2]:
%%bash2
ls -F

creatures/
data/
molecules/
north-pacific-gyre/
notes.txt
pizza.cfg
solar.pdf
thesis/
writing/

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

In [3]:
%%bash2
mkdir thesis

As you might 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 created in the current working directory:

In [4]:
%%bash2
ls -F

creatures/
data/
molecules/
north-pacific-gyre/
notes.txt
pizza.cfg
solar.pdf
thesis/
writing/

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 they 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 whitespaces: Whitespaces can make a name more meaningful but since whitespace is used to break arguments on the command line it is better to avoid them in names of files and directories. You can use - or _ instead of whitespace.
  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 whitespace or another non-alphanumeric character, you should surround the name in quotes ("").

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

In [5]:
%%bash2
ls -F thesis

quotations.txt

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

In [6]:
%%bash2
cd thesis
# nano draft.txt
# If you want to run nano, use the interactive terminal, it cannot be used in a notebook
# You can instead use the %%file magic if you want to continue working in the notebook.
# The '#' indicates a comment in bash so the command is not executred. 
# When you run at the command line you should just type: 'nano draft.txt'
In [7]:
%%file draft.txt
This is my file

Writing 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 workshop. On Unix systems (such as Linux and Mac OS X), 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). alttext 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:

In [8]:
%%bash2
ls

draft.txt
quotations.txt

Creating Files a Different Way

We have seen how to create text files using the nano editor. Now, try the following command in your home directory:

cd                  # go to your home directory
touch my_file.txt
  1. What did the touch command do? When you look at your home directory using the GUI file explorer, does the file show up?
  2. Use ls -l to inspect the files. How large is my_file.txt?
  3. When might you want to create a file this way?

Solution

Returning to the data-shell directory, let's tidy up the thesis directory by removing the draft we created:

In [9]:
%%bash2
#cd thesis
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:

In [10]:
%%bash2
ls

quotations.txt

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 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 /Users/nelle/Desktop/data-shell using cd ..:

In [11]:
%%bash2
pwd
#nano draft.txt

/home/nbuser/library/data/data-shell/thesis
In [12]:
%%file draft.txt
Some text

Writing draft.txt
In [13]:
%%bash2
ls

draft.txt
quotations.txt
In [14]:
%%bash2
cd ..

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

In [15]:
%%bash2
rm thesis

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

To really get rid of thesis we must also delete the file draft.txt. We can do this with the recursive option for rm:

In [16]:
%%bash2
rm -r thesis

Using rm Safely

What happens when we type rm -i thesis/quotations.txt? Why would we want this protection when using rm?

(Hint: This won't work in a notebook!)

Solution

With Great Power Comes Great Responsibility

Removing the files in a directory recursively can be a very dangerous operation. If we're concerned about what we might be deleting we can add the "interactive" flag -i to rm which will ask us for confirmation before each step

rm -r -i thesis
rm: descend into directory 'thesis'? y
rm: remove regular file 'thesis/draft.txt'? y
rm: remove directory 'thesis'? y

This removes everything in the directory, then the directory itself, asking at each step for you to confirm the deletion.

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.)

In [17]:
%%bash2
pwd

/home/nbuser/library/data/data-shell
In [18]:
%%bash2
mkdir thesis
#nano thesis/draft.txt
In [19]:
%%file thesis/draft.txt
More text!

Writing thesis/draft.txt
In [20]:
%%bash2
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":

In [21]:
%%bash2
mv thesis/draft.txt thesis/quotes.txt

The first argument 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:

In [22]:
%%bash2
ls thesis

quotes.txt

One has to be careful when specifying the target file name, since mv will silently overwrite any existing file with the same name, which could lead to data loss. An additional flag, mv -i (or mv --interactive), can be used to make mv ask you for confirmation before overwriting.

Just for the sake of consistency, mv also works on directories

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 argument 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.

In [23]:
%%bash2
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:

In [24]:
%%bash2
ls thesis

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

In [25]:
%%bash2
ls quotes.txt

quotes.txt

Moving to the Current Folder

After running the following commands, Jamie realizes that she put the files sucrose.dat and maltose.dat into the wrong folder:

ls -F
analyzed/ raw/
ls -F analyzed
fructose.dat glucose.dat maltose.dat sucrose.dat
cd raw/

Fill in the blanks to move these files to the current folder (i.e., the one she is currently in):

mv ___/sucrose.dat  ___/maltose.dat ___

.

Solution

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 arguments - like most Unix commands, ls can be given multiple paths at once:

In [26]:
%%bash2
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.

In [27]:
%%bash2
rm quotes.txt
ls quotes.txt thesis/quotations.txt

thesis/quotations.txt

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.

What's In A Name?

You may have noticed that all of Nelle's files' names are "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.

Renaming Files

Suppose that you created a .txt file in your current directory to contain a list of the statistical tests you will need to do to analyze your data, and named it: statstics.txt

After creating and saving this file you realize you misspelled the filename! You want to correct the mistake, which of the following commands could you use to do so?

  1. cp statstics.txt statistics.txt
  2. mv statstics.txt statistics.txt
  3. mv statstics.txt .
  4. cp statstics.txt .

Solution

Moving and Copying

What is the output of the closing ls command in the sequence shown below?

pwd
/Users/jamie/data
ls
proteins.dat
mkdir recombine
mv proteins.dat recombine/
cp recombine/proteins.dat ../proteins-saved.dat
ls
  1. proteins-saved.dat recombine
  2. recombine
  3. proteins.dat recombine
  4. proteins-saved.dat

Solution

Copy with Multiple Filenames

For this exercise, you can test the commands in the data-shell/data directory.

In the example below, what does cp do when given several filenames and a directory name?

mkdir backup
cp amino-acids.txt animals.txt backup/

In the example below, what does cp do when given three or more file names?

ls -F
amino-acids.txt  animals.txt  backup/  elements/  morse.txt  pdb/  planets.txt  salmon.txt  sunspot.txt
cp amino-acids.txt animals.txt morse.txt

.

Solution

Wildcards

* is a wildcard. It matches zero or more characters, so *.pdb matches ethane.pdb, propane.pdb, and every file that ends with '.pdb'. On the other hand, p*.pdb only matches pentane.pdb and propane.pdb, because the 'p' at the front only matches filenames that begin with the letter 'p'.

? is also a wildcard, but it only matches a single character. This means that p?.pdb would match pi.pdb or p5.pdb (if we had these two files in the molecules directory), but not propane.pdb. We can use any number of wildcards at a time: for example, p*.p?* matches anything that starts with a 'p' and ends with '.', 'p', and at least one more character (since the ? has to match one character, and the final * can match any number of characters). Thus, p*.p?* would match preferred.practice, and even p.pi (since the first * can match no characters at all), but not quality.practice (doesn't start with 'p') or preferred.p (there isn't at least one character after the '.p').

When the shell sees a wildcard, it expands the wildcard to create a list of matching filenames before running the command that was asked for. As an exception, if a wildcard expression does not match any file, Bash will pass the expression as an argument to the command as it is. For example typing ls *.pdf in the molecules directory (which contains only files with names ending with .pdb) results in an error message that there is no file called *.pdf. However, generally commands like wc and ls see the lists of file names matching these expressions, but not the wildcards themselves. It is the shell, not the other programs, that deals with expanding wildcards, and this is another example of orthogonal design.

Using Wildcards

When run in the molecules directory, which ls command(s) will produce this output?

ethane.pdb methane.pdb
  • ls *t*ane.pdb
  • ls *t?ne.*
  • ls *t??ne.pdb
  • ls ethane.*

Solution

More on Wildcards

Sam has a directory containing calibration data, datasets, and descriptions of the datasets:

2015-10-23-calibration.txt
2015-10-23-dataset1.txt
2015-10-23-dataset2.txt
2015-10-23-dataset_overview.txt
2015-10-26-calibration.txt
2015-10-26-dataset1.txt
2015-10-26-dataset2.txt
2015-10-26-dataset_overview.txt
2015-11-23-calibration.txt
2015-11-23-dataset1.txt
2015-11-23-dataset2.txt
2015-11-23-dataset_overview.txt

Before heading off to another field trip, she wants to back up her data and send some datasets to her colleague Bob. Sam uses the following commands to get the job done:

cp *dataset* /backup/datasets
cp ____calibration____ /backup/calibration
cp 2015-____-____ ~/send_to_bob/all_november_files/
cp ____ ~/send_to_bob/all_datasets_created_on_a_23rd/

Help Sam by filling in the blanks.

Solution

Organizing Directories and Files

Jamie is working on a project and she sees that her files aren't very well organized:

ls -F
analyzed/  fructose.dat    raw/   sucrose.dat

The fructose.dat and sucrose.dat files contain output from her data analysis. What command(s) covered in this lesson does she need to run so that the commands below will produce the output shown?

ls -F
analyzed/   raw/
ls analyzed
fructose.dat    sucrose.dat

.

Solution

Copy a folder structure but not the files

You're starting a new experiment, and would like to duplicate the file structure from your previous experiment without the data files so you can add new data.

Assume that the file structure is in a folder called '2016-05-18-data', which contains a data folder that in turn contains folders named raw and processed that contain data files. The goal is to copy the file structure of the 2016-05-18-data folder into a folder called 2016-05-20-data and remove the data files from the directory you just created.

Which of the following set of commands would achieve this objective? What would the other commands do?

cp -r 2016-05-18-data/ 2016-05-20-data/
rm 2016-05-20-data/raw/*
rm 2016-05-20-data/processed/*

rm 2016-05-20-data/raw/*
rm 2016-05-20-data/processed/*
cp -r 2016-05-18-data/ 2016-5-20-data/

cp -r 2016-05-18-data/ 2016-05-20-data/
rm -r -i 2016-05-20-data/

.

Solution

Key Points

  • cp old new copies a file.
  • mkdir path creates a new directory.
  • mv old new moves (renames) a file or directory.
  • rm path removes (deletes) a file.
  • * matches zero or more characters in a filename, so *.txt matches all files ending in .txt.
  • ? matches any single character in a filename, so ?.txt matches a.txt but not any.txt.
  • Use of the Control key may be described in many ways, including Ctrl-X, Control-X, and ^X.
  • The shell does not have a trash bin: once something is deleted, it's really gone.
  • Depending on the type of work you do, you may need a more powerful text editor than Nano.

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