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The Unix Shell

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Working With Files and Directories

Overview

Teaching: 30 min
Exercises: 20 min
Questions

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

Objectives

  • Create a directory hierarchy that matches a given diagram.

  • Create files in that hierarchy 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:

$ pwd

/Users/nelle/Desktop/data-shell

$ ls -F

data/                     fig_11_10.csv             humphrey_keeble_1976.pdf
fig_11_01.csv             finches_01.jpg            lab_notes/
fig_11_02.csv             finches_02.jpg            lab_notes.zip
fig_11_03.csv             finches_03.jpg            loewenberg_1965.pdf
fig_11_04.csv             finches_04.jpg            oldroyd_1984.pdf
fig_11_05.csv             greenleaf_et_al_1998.pdf  olsen_2017.pdf
fig_11_07.csv             habitat_01.jpg            readme for fig files.txt
fig_11_08.csv             habitat_02.jpg            van_leeuwen_2002.pdf
fig_11_09.csv             habitat_03.jpg

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., doesn’t have a leading slash), the new directory is created in the current working directory:

$ ls -F

data/                     fig_11_10.csv             humphrey_keeble_1976.pdf
fig_11_01.csv             finches_01.jpg            lab_notes/
fig_11_02.csv             finches_02.jpg            lab_notes.zip
fig_11_03.csv             finches_03.jpg            loewenberg_1965.pdf
fig_11_04.csv             finches_04.jpg            oldroyd_1984.pdf
fig_11_05.csv             greenleaf_et_al_1998.pdf  olsen_2017.pdf
fig_11_07.csv             habitat_01.jpg            readme for fig files.txt
fig_11_08.csv             habitat_02.jpg            thesis/
fig_11_09.csv             habitat_03.jpg            van_leeuwen_2002.pdf

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:

$ ls -F thesis

Let’s change our working directory to thesis using cd, then use the touch command to create a file called draft.txt:

$ cd thesis
$ touch draft.txt

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

$ 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:

$ ls

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 ..:

$ pwd

/Users/nelle/Desktop/data-shell/thesis

$ touch 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 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:

$ 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?

Solution

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

The -i option will prompt before every removal. The Unix shell doesn’t have a trash bin, so all the files removed will disappear forever. By using the -i flag, we have the chance to check that we are deleting only the files that we want to remove.

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 touch with the path thesis/draft.txt, rather than going into the thesis directory and running touch on draft.txt there.)

$ pwd

/Users/nelle/Desktop/data-shell

$ mkdir thesis
$ touch 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 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:

$ 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.

$ 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 an argument 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

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

$ mv ../analyzed/sucrose.dat ../analyzed/maltose.dat .

Recall that .. refers to the parent directory (i.e. one above the current directory) and that . refers to the current directory.

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:

$ 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.

$ rm quotes.txt
$ ls quotes.txt thesis/quotations.txt

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

  1. No. While this would create a file with the correct name, the incorrectly named file still exists in the directory and would need to be deleted.
  2. Yes, this would work to rename the file.
  3. No, the period(.) indicates where to move the file, but does not provide a new file name; identical file names cannot be created.
  4. No, the period(.) indicates where to copy the file, but does not provide a new file name; identical file names cannot be created.

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

We start in the /Users/jamie/data directory, and create a new folder called recombine. The second line moves (mv) the file proteins.dat to the new folder (recombine). The third line makes a copy of the file we just moved. The tricky part here is where the file was copied to. Recall that .. means “go up a level”, so the copied file is now in /Users/jamie. Notice that .. is interpreted with respect to the current working directory, not with respect to the location of the file being copied. So, the only thing that will show using ls (in /Users/jamie/data) is the recombine folder.

  1. No, see explanation above. proteins-saved.dat is located at /Users/jamie
  2. Yes
  3. No, see explanation above. proteins.dat is located at /Users/jamie/data/recombine
  4. No, see explanation above. proteins-saved.dat is located at /Users/jamie

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

If given more than one file name followed by a directory name (i.e. the destination directory must be the last argument), cp copies the files to the named directory.

If given three file names, cp throws an error because it is expecting a directory name as the last argument.

cp: target ‘morse.txt’ is not a directory

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’.

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.

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

$ cp *calibration.txt /backup/calibration
$ cp 2015-11-* ~/send_to_bob/all_november_files/
$ cp *-23-dataset* ~send_to_bob/all_datasets_created_on_a_23rd/

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

mv *.dat analyzed

Jamie needs to move her files fructose.dat and sucrose.dat to the analyzed directory. The shell will expand *.dat to match all .dat files in the current directory. The mv command then moves the list of .dat files to the “analyzed” directory.

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.

  • The shell does not have a trash bin: once something is deleted, it’s really gone.

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