Working with files and directories
OverviewTeaching: 20 min
Exercises: 10 minQuestions
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
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
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
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!).
If you are in
cd .. to get back to the
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
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.
To cancel this print of
829-0.txt, or indeed any ongoing processes in the Unix shell, hit
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 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,
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
to see if there exists an option to specify the number of lines to get
head -n 20 will print 20 lines).
Another way to navigate files is to view the contents one screen at a time.
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
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.
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
33504-0.txt (Remember your friend Tab? Type
3 followed by Tab to
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
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 *.pdfresults 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
$ 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
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
cpcommand takes two arguments: the old name and the new name. How would you make a copy of the file
gulliver-backup.txt? Try it!
cp gulliver.txt gulliver-backup.txt
Renaming a directory
Renaming a directory works in the same way as renaming a file. Try using the
mvcommand to rename the
mv firstdir backup
Moving a file into a directory
If the last argument you give to the
mvcommand is a directory, not a file, the file given in the first argument will be moved to that directory. Try using the
mvcommand to move the file
mv gulliver-backup.txt backup
This would also work:
mv gulliver-backup.txt backup/gulliver-backup.txt
The wildcards and regular expressions
?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.)
?wildcard matches the regular expression
*wildcard matches the regular expression
historycommand to see a list of all the commands you’ve entered during the current session. You can also use
Ctrl + rto 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
enterto 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 + cto 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.txtthat 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.
echocommand 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
pwdcommand you have learned earlier today. You do this by enclosing a shell command in
), for instance
$(pwd). Now, try out the following:
echo "Finally, it is nice and sunny on" $(date). Note that the output of the
datecommand 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?
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
The shell can be used to copy, move, and combine multiple files