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This tutorial contains some information about handling a Linux system.

1. Ubuntu

1.1. Introduction

Ubuntu Linux is a full fledged Linux system trailed for the desktop. Ubuntu builds a unique user interface and offers the users a solid choice of tools.

Unity is the default windows manager on Ubuntu. It introduced the launcher on the left side of Ubuntu and the Dash to start programs.

Press the Windows key to start the Dash. Here you can type in commands to open programs and files.

Ubuntu uses Nautilus as file explorer. Use the Dash or click on the icon in the task manager to start it.

To be able to copy the current path, press Ctrl+L.

1.2. Adding entries to the launcher

To add new entries to the launcher you can create an .desktop file and drag the file on the launcher.

1.2.1. Adding Eclipse to the launcher

To lock Eclipse to the launcher first create an eclipse.desktop file for Eclipse.

In the command-line enter:

gedit ~/.local/share/applications/eclipse.desktop

This file should include the following information:

[Desktop Entry]
Type=Application
Name=Eclipse
Comment=Eclipse Integrated Development Environment
Icon= home/vogella/eclipse/icon.xpm
Exec= home/vogella/eclipse/eclipse
Terminal=false
Categories=Development;IDE;Java;
StartupWMClass=Eclipse

Modify the icon and exec path to the path where you installed Eclipse.

Now add execution permission to eclipse.desktop in the command-line

chmod +x ~/.local/share/applications/eclipse.desktop

Finally you can add Eclipse to the launcher. To achieve this you can either use the command-line and enter

nautilus ~/.local/share/applications

select eclipse.desktop and drop it on the launcher or you can start Eclipse and then press Lock to Launcher on the Eclipse icon in the launcher.

The following example creates an entry to start Eclipse with different shortcuts for different workspaces.


[Desktop Entry]
Icon=application-x-executable
Name=eclipse
Type=Application
Exec=/home/vogella/Eclipse37/eclipse

X-Ayatana-Desktop-Shortcuts=Docu;vogella;business
 
[Docu Shortcut Group]
Name=Docu
Exec=/home/vogella/Eclipse37/eclipse -data /home/vogella/workspace/docu
TargetEnvironment=Unity

[vogella Shortcut Group]
Name=vogella
Exec=/home/vogella/Eclipse37/eclipse -data /home/vogella/workspace/vogella
TargetEnvironment=Unity

[business Shortcut Group]
Name=business
Exec=/home/vogella/Eclipse37/eclipse -data /home/vogella/workspace/business
TargetEnvironment=Unity

Useful shortcuts

The following lists a few useful shortcuts for the Unity window manager.

Table 1. Ubuntu general shortcuts
Shorcut	Description
Long press on Super (Windows key)	Opens up help for the most common keyboard shortcuts
Alt+Left Mouse Click	Allows to move the current window

1.3. Using the console

To open a console open the 'Dash' and type in 'Terminal'. Alternatively you can use the shortcut Ctrl+Alt+T.

This opens a console window which allows you to issue commands.

1.4. Using the vim command line editor

Ubuntu offers several editors which are installed by default. The most common command line editor is vim.

To install vim on your Ubuntu machine use the following command.

sudo apt-get install vim

Start vim from the command line. vim has two modes, one editing mode and other mode in which you can move within the file.

Here is a minimal command reference for vim:

i - Enter interactive mode to edit the file
Escape - Leave interactive mode
/ - Search in file
:wq - Saves the file and exists vim
:q! - Exists vim without saving

Find files

The following demonstrates the usage of the find command.

Table 2. Using the find command
Command	Description
find dir -name "pattern" 2>/dev/null	Finds all files which recursively apply to the pattern "pattern" starting from the directory "dir". The 2> sends all error messages to the null device.
find . -name '*.java' -newer build.xml -print	Search for all java files newer than the file build.xml
find . -name '*.java' -mtime +7 -print	Search for all java files newer than 7 dates
find . -name '*.java' -mtime +7 -print0	xargs -0 grep 'swt'

The find command can also be combined with the grep command. See Using the grep command.

Calculate the size of a folder

The following calculates the size (disk usage))of a folder "folder1" in megabyte.


du -sh folder1

Remove files

Use the command rm pattern to delete files. Be careful with the usage of this command. There is no way to undo deletions.


# Remove all files which ends with .class in the current directory
rm *.class
# find all files which ends with .class recursive in all directories below the current one and delete them
find . -type f -name "*.class" -exec rm -vf {} \;

Processes

To see all running processes on your system use the ps -aux command.

Change owner of files and directories

Table 3. Using the chown command
Command	Description
chown -R www-data:www-data mydir	Change recursively the owner and the group of the directory "mydir" and its subdirectories.

Creating links

You can create a soft link to a file or directory using the following command.

# Create a new soft link via
# ln -s target link

# For example
ln -s ~/workspace/e4-dev e4tools

Zipping files

To zip or unzip files on the command line you can use the following commands.

# Zip all pdf files in the ~/tmp/pdf/ diretory
zip ~/targetdir/myzip.zip ~/tmp/pdf/*.pdf


# Unzip the zip file
unzip ~/targetdir/myzip.zip

1.5. Remove tailing whitespace from files

To remove tailing whitespace from existing files you can use the following command.

find . -type f -name '*' -exec sed --in-place 's/[[:space:]]\+$//' {} \+

To create a new user via the console use the following commands. This creates the user, set his password and a home directory for the user. The -m parameter is responsible for creating the home directory of the user.

# create user called "newuser" with home directory
sudo useradd -m -s /bin/bash newuser

# assign password to the "newuser" user
sudo passwd newuser

You can create new user groups and add the user to the new group with the following command.

# see existing groups for the user
groups newuser

sudo addgroup gerrit
sudo adduser newuser gerrit

# alternative you can also use the usermod
# command but without -a this removes the user
# from existing groups

# sudo usermod -aG gerrit newuser

2.1. Giving root access

Careful: The following command allows the user to execute sudo commands (root).

# add admin rights to the user "newuser"
sudo adduser newuser admin

# alternative "newuser" can be added to the sudo group
sudo adduser newuser sudo

# afterwards to may want to lock
# the root user
# CAREFUL!!!

sudo passwd -l root

To change the default shell of the user to bash set the last entry of the corresponding user in the /usr/passwd file to /bin/bash. This is demonstrated in the following example.

testuser:x:1001:1001::/home/testuser:/bin/bash

Manually creating the home directory

In case you need to create the home directory at a later stage you can use the following command which creates the home directory and change this ownership of the directory.

# alternatively to using the -m option
# you could create the
# home directory manually
sudo mkdir /home/newuser
sudo chown newuser /home/newuser
sudo chgrp newuser /home/newuser

Deleting users

To delete a user use the following command.

# delete the user with the name "userName"
userdel userName

# delete the user and its home directory
userdel -r userName

3. Environment Variables

Table 4. Environment Variables
Command	Description
echo $VARIABLE	Prints the content of the environment variable
sudo /etc/init.d/tomcat5 start/stop	Starts / stops the tomcat server
sudo -i	Switches to root

3.1. Add a directory to the path

The PATH environment variable is where the system will look for executable files. To temporary add the /home/vogella/bin directory to the PATH use the following command.

export PATH=$PATH:/home/vogella/bin

If you want to add a directory permanently to the path, you can edit / create the file .bashrc and add the following line to the file. Every new directory in the path must start with :.


PATH=$PATH:path_to_new_directory

Open a new shell to make the changes in the .bashrc file active.

4. Important files

Table 5. Important files
File	Description
/etc/issue	Contains the Ubuntu version you are running
lsb_release -a	Prints out the Ubuntu version you are running
/etc/apt/sources.list	Contains the available sources for software installation
/usr/share/tomcat	Installation directory for tomcat
/var/www/vhosts/domain1	Contains on my v-server the user directory for a specific domain which is hosted on this server

5. Package management

On the command line Ubuntu allows to install / remove and search for packages via the following commands.

Table 6. Package management
Command	Description
sudo apt-get install paketname	Installs a package
apt-cache search openjdk	Search for all packages which contain openjdk. The found package can get installed via the "apt-get install" command.
apt-cache show eclipse	Write down the meta-data of a package, e.g., the package description and the package maintainer
sudo apt-get remove package	Removes a package but leave the configuration data active
sudo apt-get purge package	Removes a package and orphaned dependencies and its configuration files
sudo apt-get update	Update the local package list
sudo apt-get upgrade	Updates any installed packages for which an update is available. Will not install new packages or remove packages to satisfy dependencies.
sudo apt-get dist-upgrade	Install available updates for the Ubuntu release you already have installed. Also installs new packages or removes existing packages to satisfy dependencies.
dpkg -L packagename	Lists all files and their location in a package
sudo updatedb; locate javac	Updates the installation database and locates the javac command.

To search for the installed packages use the following command.


cat /var/log/dpkg.log | grep "\ install\ "

6. ShellScript

6.1. Writing a shell script

Shell scripts are short programs that are written in a shell programming language. They are interpreted by a shell process in the console. You can create these programs via a text editor and execute them in a terminal window. A bash shell scripts starts with "#!", the so-called "shebang".

Create a text file called start.sh with the following content.

#!/bin/bash
clear
echo "Hello, world."

You also have to make this file executable via:

chmod a+x start.sh

Now you can call the shell script via ./yourScript. It should print "Hello, world" to the console.

6.2. Batch renaming files

The following is an example how to recursively rename all files called "article.xml" to "001_article.xml" from the current home directory.

#!/bin/bash

find . -name "article.xml" -exec rename 's/article/001_article/;' '{}' \;

For to rename all extensions from ".xml" to ".adoc" you can use:

find . -name "*.xml" -exec rename 's/xml/adoc/;' '{}' \;

6.3. Generating a Maven build file

The following is an example in which you want to create a Maven build file for all directories in the current directory.

#!/bin/bash

echo "Hello"

GROUP_ID="com.vogella.test"
PARENT_ARTIFACT_ID="com.vogella.test.parent"

FULLDIR=`pwd`
BASEDIR=`basename $FULLDIR`
echo "Creating pom.xml for $BASEDIR"

cat > pom.xml <<- EOM
<project xmlns="http://maven.apache.org/POM/4.0.0"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
http://maven.apache.org/maven-v4_0_0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>$GROUP_ID</groupId>
<artifactId>$BASEDIR</artifactId>
<version>1.0.0-SNAPSHOT</version>
<packaging>pom</packaging>

<parent>
<groupId>$GROUP_ID</groupId>
<artifactId>$PARENT_ARTIFACT_ID</artifactId>
<version>1.0.0-SNAPSHOT</version>
</parent>

<modules>
EOM

modules=`ls -l|  grep '^d'  | awk '{ print "<module>"$9"</module>" }'`

echo "$modules" >> pom.xml


cat >> pom.xml <<- EOM
</modules>
</project>
EOM

7. systemd

systemd is used as Ubuntus default init system.

Services can be started and stopped with systemctl:

# start service
systemctl start <service-name>
# stop service
systemctl stop <service-name>
# restart service
systemctl restart <service-name>

journalctl is used to access the log files of a systemd service:

journalctl -u <service-name>

8. SDKMAN!

SDKMAN! is a tool for managing parallel versions of multiple Software Development Kits on most Unix based systems. It provides a convenient Command Line Interface (CLI) and API for installing, switching, removing and listing Candidates. It provides different versions for Java, Gradle, Maven and much more.

It is super easy to install.

curl -s "https://get.sdkman.io" | bash
source "$HOME/.sdkman/bin/sdkman-init.sh"

To see all available versions of a certain package use the sdk list packageName command.

sdk list java

To use a certain version, use the sdk list packageName version command.

sdk use java 11.0.1-open

To change the default, use sdk list packageName version.

9. Networking with Linux

9.1. SSH access to the server

The ssh command provides secure, encrypted access to a server. Use ssh IP-address or ssh your_hostname to access the server.

To upload your public ssh key which allows you to login automatically into a server use the following command.

ssh-copy-id youruser@yourserver

You can also assign a shortname for an IP address via the config file in your ~/.ssh directory.

For example, to create an alias called foo for the IP address 144.76.74.162 and to use the user called testing switch to your ~/.ssh directory, create the config file if it does not exists and enter the following.

host foo
hostname 144.76.74.161
user testing

9.2. Execute a shell script on a remote machine

Tmux allows to run multiple command line sessions that continue to run even after you log out from SSH. You can later connect to them again.

To create a new tmux session named database type this into your console:

tmux new -s database

To detach from your current session press C-b d. Your session will continue to run in the background.

To see all running background processes use the following command

tmux ls

To attach for example to the first one again use:

tmux attach -t 0

To attach to a named session just replace 0 with the respective session name:

tmux attach -t database

To scroll in tmux use Ctrl-b+[ . Now you can use the normal scroll keys to scroll. Press q to quit scroll mode.

9.3. See all open ports on your machine

You see all listeners on your machine via the following command. Port which listen to 127.0.0.1 cannot get accessed from external.

netstat -anltp | grep "LISTEN"

9.4. Firewall

Ubuntu provides a uncomplicated firewall (ufw). To install it and only allow SSH, FTP, and webtraffic use the following command.

sudo apt-get install ufw
sudo ufw allow 80/tcp
sudo ufw allow 22/tcp
sudo ufw allow 443/tcp
sudo ufw allow 21/tcp
sudo ufw enable

9.5. Network commands

The following commands give you an overview of your network connections.

Table 7. Network connections
Command	Description
lspci -nn
grep -i net
lsusb
iwconfig
ifconfig	Shows the network connections
lsmod
python -m SimpleHTTPServer	Start web server serving current directory tree at http://localhost:8000/

9.6. HTTP debugging with curl

curl is a command line tool to issue and receive http (and other) request. For example, if you want to see the HTTP output of a webpage, use the following command.


curl http://www.vogella.com
// Or
curl -G http://www.vogella.com

If you want to the HTTPrequest header (including the HTTP status codes, use the following command. This is for example nice to see if your server delivers a 404 return code for your self-defined error page.


curl -I http://www.vogella.com

You can set HTTP header information with the -h flag. For example, to request a certain MIME type use the -H’Accept:MIME' option.


curl -I http://www.vogella.com -H'Accept:text/plain'

To use curl behind a proxy.


curl -x proxy:8080 -G http://www.vogella.com

If you are using Microsoft Windows, see curl for Windows.

9.7. IRC

For IRC communication you can use the tool xchat. To install it, use "sudo apt-get install xchat".

9.8. FTP

For FTP access you can install filezilla via sudo apt-get install filezilla or map the ftp access to a virtual device.

To map the device select your desktop. Select the file menu and the entry "Connect to server".

10. Installing a FTP server

To install an easy to use and configure ftp server use the following command.

sudo apt-get install vsftpd

11. Mail server Configuring your server to send out emails

In case you want to send out emails from your PHP sites you have to configure an email client. A simple one is exim4, a setup description can be found under the following links: https://help.ubuntu.com/community/Exim4 and https://wiki.debian.org/GmailAndExim4.

sudo apt-get install exim4

Here is an example /etc/exim4/update-exim4.conf.conf file.

dc_eximconfig_configtype='smarthost'
dc_other_hostnames=''
dc_local_interfaces='127.0.0.1 ; ::1'
dc_readhost='gilbert.vogella.com'
dc_relay_domains=''
dc_minimaldns='false'
dc_relay_nets=''
dc_smarthost='smtp.gmail.com:587'
CFILEMODE='644'
dc_use_split_config='true'
dc_hide_mailname='false'
dc_mailname_in_oh='true'

You need to enter valid user credentials in the passwd.client file in the following format.

gmail-smtp.l.google.com:youruser@gmail.com:yourpassword
*google.com:youruser@gmail.com:yourpassword
smtp.gmail.com:youruser@gmail.com:yourpassword

Afterwards you need to enable the new configuration with the following command.

sudo update-exim4.conf

The log files are located in /var/log/exim4. To trace them use

tail -f /var/log/exim4/mainlog

Sometimes Google blocks the account access, because you try to login from another IP then normally. Open http://www.google.com/accounts/DisplayUnlockCaptcha and then try to send an e-mail using for example:

echo 'My Test Text' | mail -s Testmail max.mustermann@domain.com

12. MySQL

For a description of MySQL and its installation in Ubuntu see MySQL - Tutorial.

13. Apache Tomcat

13.1. Important Files

Table 8. Important Files
File	Description
/usr/share/tomcat5/	Installation directory of Tomcat
psa-webapps	Installation directory for webapps in a vhost environment
/usr/share/tomcat5/conf	Configuration Directory for Tomcat
/etc/default/tomcat5	Contains default settings for tomcat. Most important the used java version (jdk).
/var/log/tomcat5	Log files of tomcat

13.2. Important Commands

Table 9. Important Commands
Command	Description
/etc/init.d/tomcat5 restart	Restart the tomcat web server

14. PDF files

14.1. Command line tool pdftk

The "pdftk" command line tool allows to rework existing pdf files, e.g., extract pages or change the orientation of the pdf file.

You can install it via the following command.


sudo apt-get install pdftk

For example, to extract pages from a pdf document you can use the "cat" option.


# Extract certain pages from a pdf document
# "dont_ask" will override existing files without warning
pdftk Eclipse4book-20120429.pdf cat 25-27 87-91 95 output Eclipse4-Exercise.pdf dont_ask

14.2. Converting .odp file to pdf in batch

If you have LibreOffice installed, you can convert .odp files on the command line to pdf files.


soffice --nologo --invisible --convert-to pdf --outdir ./pdf *.odp

15. Command line tools for adjusting images

ImageMagick allows to convert images in batch see ImageMagick Command line Options.

For example, the following adjusts the DPI size of images to 300.


convert -units PixelsPerInch -density 300 input.png output.png

16. Command line tool for recording the screen

byzanz allows to record a specified area on the screen with different output formats and custom settings.

It can be installed with this command:

sudo apt-get install byzanz

See byzanz command line options for details about the usage.

The following example creates an animated output.gif file of the screen in the area of (x=0, y=0, width=800, height=1024) with the duration of 20 seconds.

byzanz-record --duration=20 --x=0 --y=0 --width=800 --height=1024 output.gif

Besides the gif format also flv, ogg, ogv and byzanz formats are supported.

17. UI tool for recording the screen

A simple tool to create screen recordings is called peek, which is available on GitHub: https://github.com/phw/peek

On Ubuntu it can be installed like this:

sudo add-apt-repository ppa:peek-developers/stable
sudo apt update
sudo apt install peek

18. Spell checking with hunspell

The command line tool hunspell allows you to check the spelling of a document. For example, you can check an XML document written in English called "input.xml" with the command.

# parameter -H defines that the input is an XML file
hunspell -d en_US -H -l input.xml

The following trigger an interactive session for spell checking. With (i) you can add a word to your local hunspell directory, which is located in your home directory under .hunspell_en_US in case you using the US en checker.

You can also check all words in a batch mode with the -l parameter.

# parameter -H defines that the input is an XML file
# all incorrectly spelled words are listed because of the
# -l parameter
hunspell -d en_US -H -l input.xml

19. Connection

19.1. Telnet / ssh client for Windows

To connect to your Linux system via telnet or via ssh from Windows you can use putty.

19.2. Switch to Console mode from Graphical User Interface

In case your Linux system is running under a graphical user interface (KDE, GNOME, …) and you need access to the console, use the Ctr+Alt+F1 shortcut keys to switch to the first console. To switch back to Desktop mode, use the Ctrl+Alt+F7 shortcut keys.

20. Tips

20.1. Creating a bootable USB disk with unetbootin

The default program for creating a bootable USB disk is Startup Disk Creator. This crashes frequently during the creation of a boot disk. The program unetbootin is much more stable. You can install it via the following command and start it on the command line.

sudo apt-get install unetbootin

20.2. Remote xserver

The x2go server allows to install a X-server on a remote server and access it via a client from a different machine. More information can be found on http://wiki.x2go.org/.

21. Broadway

This section describes, how you can display the Eclipse IDE or any other GTK+ application in your web browser using the GDK Broadway backend.

21.1. Start a Broadway server

In order to display Eclipse or any other application in your web browser, you need to start a Broadway server which comes with GTK+.

Simply open a Terminal window and start a Broadway server by typing the following:

start broadwayd

If you want to set a specific port, just append :offset.

start broadwayd :5

The default port is 8080 and as we chose 5 as our offset, the actual port is now 8085.

21.2. Using Broadway to run a GTK+ application

You can use any GTK3 application, like Eclipse or gedit in the browser. This demonstrate how to start such an application. Start another Terminal window and type the following:

GDK_BACKEND=broadway gedit

If you started your Broadway server with a specific port offset (e.g. to use 8085 as a port), use the following command:

GDK_BACKEND=broadway BROADWAY_DISPLAY=:5 gedit

You can now open your web browser and go to http://localhost:8080 (or 8085) and see your application running.

21.3. Starting Eclipse

GDK_BACKEND=broadway BROADWAY_DISPLAY=:5 java -jar ./plugins/org.eclipse.equinox.launcher_1.4.0.v20161219-1356.jar

21.4. Using Broadway to start a runtime Eclipse from the Eclipse IDE

Make sure, that you use Eclipse Neon or above. To now display Eclipse in your web browser, open Eclipse itself. Eclipse has the ability to start itself with the help of a new run configuration. To create one, click on the small black arrow next to the Run button and select Run Configurations…. On the left side of the newly opened window make a right-click on Eclipse Application and select New. Open the Environment tab and create two new environment variables like the following:

If you started your Broadway server with a port offset, just replace :0 with your selected offset. You can now open your web browser and go to http://localhost:8080 (or 8085) and see Eclipse running.

22. Links and Literature

vim editor reference

Setting up Tomcat and Java on Ubuntu

Tmux introduction

Guide to Tmux customization

Rescue a lost server

22.1. vogella Java example code

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Ubuntu Linux - Tips and Tricks in using Linux - Tutorial