tutorial.rst

Tutorial Exercises

Goal

In this tutorial, you will be looking for a word with 22 characters:

In the exercises below, the characters of the first word are hidden.

Preparations

• clone the repository or download the code as a ZIP file
• locate the exercises/ folder
• open a bash terminal (on Linux, press Ctrl + Alt + T)

1. Directories and files

1.1. Navigate directories

The first character is hidden in a file somewhere in the exercise_1/ folder. Use the commands

cd <directory_name>

(do not type the pointy brackets, just insert the directory name) and

ls

to move from one directory to the next. Look through subdirectories until you find one with the name solution_1.1 and list its contents. If you went to a wrong directory, you can go back one level by typing:

cd ..

or go back to your home folder:

cd

1.2. Show a hidden file

Some files are not visible immediately. To see them, you need the command

ls -a

The second character, is in the same directory as the first one, but in a hidden file.

1.3. Execute a program

Use cd .. to go back to the directory exercise_1/directoryB/. When listing its contents, you should see a shell script file program.sh. To find the third character, you need to execute the program. On bash, this is done by typing source and the name of the program:

source program.sh

1.4. Find out how big a file is

Go to the folder exercise_1/directoryC/. To find the fourth character, you need to find out how big the text file in the directory is. This is done with the command

ls -l

In the table the command produces, you will find the file size in bytes, the file's owner, permissions to read and modify it, and the date/time of the last modification.

To obtain the fourth character look up the file size in the Table of printable ASCII characters (the decimal numbers, not the hex ones):

ASCII Table, Public Domain

Note

When typing names of directories or files, press [TAB] after the first few characters. Unix tries to guess what you are typing.

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2. Edit text files

Please use cd .. to go back to the top directory of the tutorial material. Then, change to the directory exercise_2/.

2.1. See what is in a text file

In the directory exercise_2/, you will find a text file solution_2.1.txt. The fifth character is inside that file. To see its contents, use the command

less <filename>

2.2. Edit text files

To get character number six, you will need to create a text file in the exercise_2 directory. On Ubuntu, you can do this using the editor nano. You can start it typing the name of the program, or

nano <filename>

To exit nano, type Ctrl-X

Create a text file with the characters you have found so far.

The sixth character is the one you need to press to save a file in nano.

Note

If you want to know more about a particular command, type

man <command>

You get shown a help page that you can leave by pressing q.

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3. Copy and remove files

Please go to the directory exercise_3/.

3.1. Create a directory and copy a file to it.

To find characters seven and eight, you need to create a subdirectory named solution in exercise_3/ and copy the files from the part1/ and part2/ folders into it.

For creating directories, use the command:

mkdir <directory name>

For copying, you can use the command

cp <filename from> <filename to>

Type ls -l solution/* afterwards to see the solution.

3.2. Remove files

In the data directory, all files with an Y need to be deleted. To do so, use the command:

rm <filename>

Also, there are more files to be deleted in the data/ directory. To remove more than one file at once, you can use * as a wildcard, i.e. rm ju* will delete all of junk.txt, juniper.txt and june.docx.

To get characters nine and ten, look at the files that remain after deleting all that contain a Y.

Note

To remove an empty directory, you can use

rmdir <directory name>

The command

rm -r <directory name>

deletes a directory and everything in it.

Note

On Unix, it is not possible to undelete files!

This makes removing files with the * symbol very dangerous, because you could wipe out everything with a single command (e.g. if you type the wrong directory by accident). Backups become an even better idea after learning this command.

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4. Process text data

Please go to the directory exercise_4/.

4.1. Compare two files

There are two different versions of a quote, ai.txt, and artificial_intelligence.txt. To find out, how they differ, Unix provides the command

diff <filename1> <filename2>

Of course, you can look at the text first using less or nano. The 11th character of the solution is the single character in which the two files differ.

4.2. Sort a text file

Unix has a small program to sort text files alphabetically. It is called by

less <filename> | sort

The symbol | is called a pipe and is often used to connect Unix programs to each other. The 12th character of the solution is the first character of the last word in the alphabetically sorted file elephant.txt.

Note

To store the sorted lines in a new file, you can add an output file, like

less <filename> | sort -f > result.txt

4.3. Find words in a text file

To look for specific words in a text file, use the command

grep <word> <filename>

It produces all lines from the given file that contain the given word. The grep command is very powerful and can handle Regular Expressions.

To find the 13th character, search for the word fire in the file datascience.txt and take the first character of the output.

Note

You can search through many files at once by including a * in the filename.

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5. Unzip files

Please go to the directory exercise_5/.

5.1. Unzip an archive

Unzipping compressed files is a very basic and important task. On Unix, you often encounter .zip archives, .tar archives and .gz compressed files. For unpacking zip files, use

unzip <filename>

for .tar and .tar.gz files

tar -xf <filename>

and for .gz files,

gunzip <filename>

The 14th and 15th character of the solution are in a multiply wrapped archive in the exercise_5/ directory.

Note

To pack a directory and everything within, you can use the command

tar -cf backup.tar <directory>

To subsequently compress it, use

gzip backup.tar

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6. Command-line tools

Please go to the directory exercise_6/.

6.1. Change file access rights

Each file on Unix has separate permissions for reading r, writing w, and executing x. Displaying them with:

ls -l

There is one triplet of permissions for the owner of the file owner, one triplet for a group of users, and one for all others. The chmod command allows to change these permissions, e.g.

chmod a+x <filename>

grants all users the permission to execute a file, while chmod u-w forbids the current user (oneself) to write to the file (thereby protecting it from being deleted accidentally).

To see characters 16+17 of the solution, make the program permissions.sh executable. Then execute it with:

./permissions.sh

Note

You can grant permissions for a whole directory tree using

chmod -R a+x <directory>

6.2. How much disk space have I left?

To find out, how much disk space you have left, you can use the command

df

df lists all hard drive partitions, CD-ROMs, pendrives and some logical partitions Unix uses. All numbers are given in kilobyte (1000 byte or one 1000000th GB).

To obtain the 18th character, check out the version of the df program. Find out how to do that with:

df --help

The solution is the last character of the first authors first name.

6.3. Set an environment variable

To install some programs, it is necessary to set so-called environment variables. These can be set using the command

export <variable-name>=<value>

You can see all variables by the command

env

To obtain the 19th character, you need to use export to set the variable GIVEME to the value SOLUTION.

echo $GIVEME

Find out the characters position in the alphabet with:

echo $GIVEME | wc -c

Note

By default, changes to environment variables only affect the current terminal.

If you want to set environment variables for each console window, write the export command to the file .bashrc in your home directory (it is a hidden file).

6.4. Check whether you have internet

The easiest way to check from the Unix command line whether the internet connection works, is to send a request to a known server (e.g. https://www.academis.eu) using the command

ping <web address>

The command reports, how long a message takes back and forth to the given server. To interrupt the messages, press Ctrl+C. You can use the program

./check_ping

The 20th character is the ping option that sets the maximum number of requests sent. Check the documentation with:

man ping

6.5. Manage processes

To see what programs are running on your machine, type

top

It displays you a list of all currently active programs. Shift+P sorts them by the CPU time they are using, Shift+M by the amount of memory they are using (if you don’t see any program consuming lots of memory, start a web browser). Quit top by pressing q.

The last two characters of the solution are the first two characters of the second word in the line containing the column labels.

Note

If you want to get rid of one of the programs you started (e.g. because it crashed), you can do so by typing

kill -s 9 <pid>

You find the pid number in the first column of the top output. Of course, you may only interrupt your own programs, not those owned by root, the system administrator.