Working with files and directories

Overview

Teaching: 20 min
Exercises: 10 min
Questions
  • What is the shell?

  • What is the command line?

  • Why should I use it?

Objectives
  • Create files and directories from the command line

  • Use tab completion to limit typing

  • Look at all or part of a file using the command line

  • Moving and copying files

Working with files and folders

As well as navigating directories, we can interact with files on the command line: we can read them, open them, run them, and even edit them. In fact, there’s really no limit to what we can do in the shell, but even experienced shell users still switch to graphical user interfaces (GUIs) for many tasks, such as editing formatted text documents (Word or OpenOffice), browsing the web, editing images, etc. But if we wanted to make the same crop on hundreds of images, say, the pages of a scanned book, then we could automate that cropping work by using shell commands.

We will try a few basic ways to interact with files. Let’s first move into the shell-lesson directory on your desktop.

$ cd
$ cd Desktop/shell-lesson
$ pwd
/Users/riley/Desktop/shell-lesson

Here, we will create a new directory and move into it:

$ mkdir firstdir
$ cd firstdir

Here we used the mkdir command (meaning ‘make directories’) to create a directory named ‘firstdir’. Then we moved into that directory using the cd command.

But wait! There’s a trick to make things a bit quicker. Let’s go up one directory.

$ cd ..

Instead of typing cd firstdir, let’s try to type cd f and then hit the Tab key. We notice that the shell completes the line to cd firstdir/.

Tab for Auto-complete

Hitting tab at any time within the shell will prompt it to attempt to auto-complete the line based on the files or sub-directories in the current directory. Where two or more files have the same characters, the auto-complete will only fill up to the first point of difference, after which we can add more characters, and try using tab again. We would encourage using this method throughout today to see how it behaves (as it saves loads of time and effort!).

Reading files

If you are in firstdir, use cd .. to get back to the shell-lesson directory.

Here there are copies of two public domain books downloaded from Project Gutenberg along with other files we will cover later.

$ ls -lh
total 33M
-rw-rw-r-- 1 riley staff 383K Feb 22 2017  000003160_01_text.json
-rw-r--r-- 1 riley staff 3.6M Jan 31 2017  2014-01-31_JA-africa.tsv
-rw-r--r-- 1 riley staff 7.4M Jan 31 2017  2014-01-31_JA-america.tsv
-rw-rw-r-- 1 riley staff 125M Jun 10 2015  2014-01_JA.tsv
-rw-r--r-- 1 riley staff 1.4M Jan 31 2017  2014-02-02_JA-britain.tsv
-rw-r--r-- 1 riley staff 582K Feb  2 2017  33504-0.txt
-rw-r--r-- 1 riley staff 598K Jan 31 2017  829-0.txt
-rw-rw-r-- 1 riley staff  18K Feb 22 2017  diary.html

The files 829-0.txt and 33504-0.txt holds the content of book #829 and #33504 on Project Gutenberg. But we’ve forgot which books, so we try the cat command to read the text of the first file:

$ cat 829-0.txt

The terminal window erupts and the whole book cascades by (it is printed to your terminal). Great, but we can’t really make any sense of that amount of text.

Canceling Commands

To cancel this print of 829-0.txt, or indeed any ongoing processes in the Unix shell, hit Ctrl+C

Often we just want a quick glimpse of the first or the last part of a file to get an idea about what the file is about. To let us do that, the Unix shell provides us with the commands head and tail.

$ head 829-0.txt
The Project Gutenberg eBook, Gulliver's Travels, by Jonathan Swift


This eBook is for the use of anyone anywhere at no cost and with
almost no restrictions whatsoever.  You may copy it, give it away or
re-use it under the terms of the Project Gutenberg License included
with this eBook or online at www.gutenberg.org

This provides a view of the first ten lines, whereas tail 829-0.txt provides a perspective on the last ten lines:

$ tail 829-0.txt
Most people start at our Web site which has the main PG search facility:

    http://www.gutenberg.org

This Web site includes information about Project Gutenberg-tm,
including how to make donations to the Project Gutenberg Literary
Archive Foundation, how to help produce our new eBooks, and how to
subscribe to our email newsletter to hear about new eBooks.

If ten lines is not enough (or too much), we would check man head to see if there exists an option to specify the number of lines to get (there is: head -n 20 will print 20 lines).

Another way to navigate files is to view the contents one screen at a time. Type less 829-0.txt to see the first screen, spacebar to see the next screen and so on, then q to quit (return to the command prompt).

$ less 829-0.txt

Like many other shell commands, the commands cat, head, tail and less can take any number of arguments (they can work with any number of files). We will see how we can get the first lines of several files at once. To save some typing, we introduce a very useful trick first.

Re-using commands

On a blank command prompt, hit the up arrow key and notice that the previous command you typed appears before your cursor. We can continue pressing the up arrow to cycle through your previous commands. The down arrow cycles back toward your most recent command. This is another important labour-saving function and something we’ll use a lot.

Hit the up arrow until you get to the head 829-0.txt command. Add a space and then 33504-0.txt (Remember your friend Tab? Type 3 followed by Tab to get 33504-0.txt), to produce the following command:

$ head 829-0.txt 33504-0.txt
==> 829-0.txt <==
The Project Gutenberg eBook, Gulliver's Travels, by Jonathan Swift


This eBook is for the use of anyone anywhere at no cost and with
almost no restrictions whatsoever.  You may copy it, give it away or
re-use it under the terms of the Project Gutenberg License included
with this eBook or online at www.gutenberg.org




==> 33504-0.txt <==
The Project Gutenberg EBook of Opticks, by Isaac Newton

This eBook is for the use of anyone anywhere at no cost and with
almost no restrictions whatsoever.  You may copy it, give it away or
re-use it under the terms of the Project Gutenberg License included
with this eBook or online at www.gutenberg.org


Title: Opticks
       or, a Treatise of the Reflections, Refractions, Inflections,

All good so far, but if we had lots of books, it would be tedious to enter all the filenames. Luckily the shell supports wildcards! The ? (matches exactly one character) and * (matches zero or more characters) are probably familiar from library search systems. We can use the * wildcard to write the above head command in a more compact way:

$ head *.txt

More on wildcards

Wildcards are a feature of the shell and will therefore work with any command. The shell will expand wildcards to a list of files and/or directories before the command is executed, and the command will never see the wildcards. As an exception, if a wildcard expression does not match any file, Bash will pass the expression as a parameter to the command as it is. For example typing ls *.pdf results in an error message that there is no file called *.pdf.

Moving, copying and deleting files

We may also want to change the file name to something more descriptive. We can move it to a new name by using the mv or move command, giving it the old name as the first argument and the new name as the second argument:

$ mv 829-0.txt gulliver.txt

This is equivalent to the ‘rename file’ function.

Afterwards, when we perform a ls command, we will see that it is now called gulliver.txt:

$ ls
2014-01-31_JA-africa.tsv   2014-02-02_JA-britain.tsv  gulliver.txt
2014-01-31_JA-america.tsv  33504-0.txt
2014-01_JA.tsv

Copying a file

Instead of moving a file, you might want to copy a file (make a duplicate), for instance to make a backup before modifying a file. Just like the mv command, the cp command takes two arguments: the old name and the new name. How would you make a copy of the file gulliver.txt called gulliver-backup.txt? Try it!

Answer

cp gulliver.txt gulliver-backup.txt

Renaming a directory

Renaming a directory works in the same way as renaming a file. Try using the mv command to rename the firstdir directory to backup.

Answer

mv firstdir backup

Moving a file into a directory

If the last argument you give to the mv command is a directory, not a file, the file given in the first argument will be moved to that directory. Try using the mv command to move the file gulliver-backup.txt into the backup folder.

Answer

mv gulliver-backup.txt backup

This would also work:

mv gulliver-backup.txt backup/gulliver-backup.txt

The wildcards and regular expressions

The ? wildcard matches one character. The * wildcard matches zero or more characters. If you attended the lesson on regular expressions, do you remember how you would express that as regular expressions?

(Regular expressions are not a feature of the shell, but some commands support them. We’ll get back to that.)

Answer

  • The ? wildcard matches the regular expression . (a dot)
  • The * wildcard matches the regular expression .*

Using history

Use the history command to see a list of all the commands you’ve entered during the current session. You can also use Ctrl + r to do a reverse lookup. Hit Ctrl + r, then start typing any part of the command you’re looking for. The past command will autocomplete. Hit enter to run the command again, or press the arrow keys to start editing the command. If you can’t find what you’re looking for in the reverse lookup, use Ctrl + c to return to the prompt. If you want to save your history, maybe to extract some commands from which to build a script later on, you can do that with history > history.txt. This will output all history to a text file called history.txt that you can later edit. To recall a command from history, enter history. Note the command number, e.g. 2045. Recall the command by entering !2045. This will execute the command.

Using the echo command

The echo command simply prints out a text you specify. Try it out: echo "Library Carpentry is awesome!". Interesting, isn’t it?

You can also specify a variable, for instance NAME= followed by your name. Then type echo "$NAME is a fantastic library carpentry student". What happens?

You can combine both text and normal shell commands using echo, for example the pwd command you have learned earlier today. You do this by enclosing a shell command in $( and ), for instance $(pwd). Now, try out the following: echo "Finally, it is nice and sunny on" $(date). Note that the output of the date command is printed together with the text you specified. You can try the same with some of the other commands you have learned so far.

Why do you think the echo command is actually quite important in the shell environment?

Answer

You may think there is not much value in such a basic command like echo. However, from the moment you start writing automated shell scripts, it becomes very useful. For instance, you often need to output text to the screen, such as the current status of a script.

Moreover, you just used a shell variable for the first time, which can be used to temporarily store information, that you can reuse later on. It will give many opportunities from the moment you start writing automated scripts.

Finally, onto deleting. We won’t use it now, but if you do want to delete a file, for whatever reason, the command is rm, or remove.

Using wildcards, we can even delete lots of files. And adding the -r flag we can delete folders with all their content.

Unlike deleting from within our graphical user interface, there is no warning, no recycling bin from which you can get the files back and no other undo options! For that reason, please be very careful with rm and extremely careful with rm -r.

Key Points

  • The shell can be used to copy, move, and combine multiple files