Problem

You want to move files around using a graphical interface like you can on a macOS-based machine or Windows PC.

Solution

Use the File Manager.

You can find this program on your Main menu, in the Accessories group (Figure 3-1).

Using the File Manager, you can drag a file or directory from one directory to another or use the Edit menu to copy a file from one location and paste it to a second. This operates in much the same way as the Windows File Manager or macOS Finder.

Figure 3-1. The File Manager

Discussion

The lefthand side of the File Manager shows the folder structure.

The central area displays the files in the current folder, which you can navigate using the buttons in the toolbar or by typing a location in the file path area at the top.

You can right-click a file to open a menu presenting options you can use on that file (Figure 3-2).

Figure 3-2. Right-clicking a file opens a menu with more options

You can also select more than one file at a time for copying or dragging by holding down the Ctrl key while you select files, or you can select a range of files by selecting one file and then holding down the Shift key while you select the end of the list of files that you want to select.

See Also

See also Recipe 3.5.

Problem

You want to copy a file from your Raspberry Pi onto a USB flash drive.

Solution

Insert the USB flash drive into a USB port, and the dialog shown in Figure 3-3 should appear. Select OK to open it in the File Manager.

Figure 3-3. The removable-media dialog box

The drive will be mounted in /media/pi followed by the name of the flash drive (in my case, UNTITLED). To copy a file from your home folder, drag it onto the folder representing your flash drive, as shown in Figure 3-4.

Figure 3-4. Copying a file by dragging it onto a USB flash drive

Windows, macOS, and Linux all have their own disk formats. The USB flash drive should be formatted as FAT32 or exFAT for maximum compatibility with macOS and Windows computers. exFAT supports larger disk sizes than FAT32.

Discussion

After your USB flash drive is mounted on the Raspberry Pi’s filesystem, you can also copy files using the command line. The following example copies the file test.txt to the flash drive:

$ cd /home/pi
$ cp test.txt /media/pi/UNTITLED/

In this example, cd is the command to change directory, and cp is the copy command. These commands are explained more fully in Recipe 3.4 and Recipe 3.5.

See Also

For general information on using the File Manager, see Recipe 3.1.

To copy files from the command line, see Recipe 3.4.

Problem

When using a Raspberry Pi, you need to issue text commands in a Terminal.

Solution

At the top of the Raspberry Pi desktop, select the Terminal icon (it looks like a black computer monitor), or, on the Start menu in the Accessories group, select the Terminal menu option (Figure 3-5).

Figure 3-5. Opening the Terminal

Discussion

When the Terminal starts, it is set to your home directory (/home/pi).

You can open as many Terminal sessions as you want. It is often useful to have a couple of sessions open in different directories so that you don’t need to constantly switch directories using cd (Recipe 3.4).

When using the Terminal, everything is case sensitive. That is, if you are using a command, you must use the correct case when you’re typing. For example, the ls command that you will meet in the next recipe must be written as lowercase ls and not LS or Ls or lS. Similarly, all filenames are case sensitive, so files named picture.jpg and Picture.jpg are two different files.

See Also

In the next section (Recipe 3.4), you will look at navigating the directory structure using the Terminal.

Problem

You need to know how to change directory and move about the filesystem using the Terminal.

Solution

The main command used for navigating the filesystem is cd (change directory). After cd, you need to specify the directory that you want to change to. This can be either a relative path to a directory within your current directory or an absolute path to somewhere else on the filesystem.

To see what the current directory is, use the command pwd (print working directory).

Discussion

Try out a few examples. Open a Terminal session, and you should see a prompt like this:

pi@raspberrypi: ~ $

The prompt that you will see after each command (pi@raspberrypi: ~ $) is a reminder of your username (pi) and your computer name (raspberrypi). The ~ character is shorthand for your home directory (/home/pi). So at any point you can change your current directory to your home directory as follows:

$ cd ~

You can confirm that the command did indeed set the directory to the home directory by using the pwd command:

$ pwd
/home/pi

If you want to move up one level in the directory structure, you can use the special value .. (two dots) after the cd command, as shown here:

$ cd ..
$ pwd
/home

As you might have deduced by now, the path to a particular file or directory is made up of words separated by a /. So the very root of the entire filesystem is /, and to access the home directory within /, you would refer to /home/. Then, to find the pi directory within that, you would use /home/pi/. You can omit the final / from a path.

Paths can be absolute (starting with a / and specifying the full path from the root), or they can be relative to the current working directory, in which case they must not start with a /.

You will have full read and write access to the files in your home directory, but when you move into the places where system files and applications are kept, your access to some files will be restricted to read-only. You can override this (Recipe 3.12), but some care is required.

Check out the root of the directory structure by entering the commands cd / and ls, as shown in Figure 3-6.

Figure 3-6. Listing the contents of a directory

The ls command (list) shows us all of the files and directories below (/) the root directory. You will see that there is a home directory listed, which is the directory you have just come from.

Now change into one of those directories by using the commands shown in Figure 3-7.

Figure 3-7. Changing directory and listing the contents

You will see that the files and folders have some color coding. Files are displayed in various colors, whereas directories are dark blue.

Unless you particularly like typing, the Tab key offers a convenient shortcut. If you start typing the name of a file, pressing the Tab key allows the autocomplete feature to attempt to complete the filename. For example, if you’re going to change directory to network, type the command cd netw and then press the Tab key. Because netw is enough to uniquely identify the file or directory, pressing the Tab key will autocomplete it.

If what you have typed is not enough to uniquely identify the file or directory, pressing the Tab key another time will display a list of possible options that match what you have typed so far. So if you had stopped at net and pressed the Tab key, you would see something like Figure 3-8.

Figure 3-8. Autocompletion using the Tab key

You can provide an extra argument after ls to narrow down the things that you want to list. Change directory to /etc and then run the following:

$ ls f*
fake-hwclock.data  fb.modes  fstab  fuse.conf

fonts:
conf.avail  conf.d  fonts.conf  fonts.dtd

foomatic:
defaultspooler  direct  filter.conf

fstab.d:
pi@raspberrypi /etc $

The * character is called a wildcard. In specifying f* after ls, we are saying that we want to list everything that begins with an f.

Helpfully, the results first list all the files within /etc that begin with f, and then the contents of all the directories in that folder beginning with f.

A common use of wildcards is to list all files with a certain extension (e.g., ls *.docx).

A convention in Linux (and many other operating systems) is to prefix files that should be hidden from the user by starting their name with a period. Any so-named files or folders will not appear when you type ls unless you supply ls with the option -a (all).

For example:

$ cd ~
$ ls -a
.                                  Desktop               .pulse
..                                 .dillo                .pulse-cookie
Adafruit-Raspberry-Pi-Python-Code  .dmrc                 python_games
.advance                           .emulationstation     sales_log
.AppleDB                           .fltk                 servo.py
.AppleDesktop                      .fontconfig           .stella
.AppleDouble                       .gstreamer-0.10       stepper.py.save
Asteroids.zip                      .gvfs                 switches.txt.save
atari_roms                         indiecity             Temporary Items
.bash_history                      .local                thermometer.py
.bash_logout                       motor.py              .thumbnails
.bashrc                            .mozilla              .vnc
.cache                             mydocument.doc        .Xauthority
.config                            Network Trash Folder  .xsession-errors
.dbus                              .profile              .xsession-errors.old

As you can see, the majority of the files and folders in your home directory are hidden.

See Also

See also Recipe 3.14.

Problem

You need to rename a file by using a Terminal session.

Solution

Use the mv command to rename files and directories.

Discussion

The mv (move) command is used in a similar way to the cp command, except that the file or folder being moved is simply renamed rather than a duplicate being made.

For example, to rename a file from my_file.txt to my_file.rtf, you use the following command:

$ mv my_file.txt my_file.rtf

Changing a directory name is just as straightforward, and you don’t need the recursive -r option you used when copying, because changing a directory’s name implicitly means that everything within it is contained in a renamed directory.

See Also

See also Recipe 3.5.

Problem

You want to view the contents of a small file without editing it.

Solution

Use the cat command or the more command to view the file.

For example:

$ more myfile.txt
This file contains
some text

Discussion

The cat command displays the whole contents of the file, even if the contents are longer than will fit on the screen.

The more command displays only one screen of text at a time. Press the space bar to display the next screen.

See Also

You can also use cat to concatenate (join together) a number of files (Recipe 3.32).

Another popular command related to more is less. less is like more except that it allows you to move backward in the file as well as forward.

Problem

You want to create a one-line file without having to use an editor.

Solution

Use the > and echo commands to redirect what you type on the command line to a file.

For example:

$ echo "file contents here" > test.txt
$ more test.txt
file contents here

Discussion

This can be useful for quickly creating a file.

See Also

To use the more command to view files without using an editor, see Recipe 3.8.

To use > to capture other kinds of system output, see Recipe 3.31.

Problem

You want to create a new directory by using the Terminal.

Solution

The mkdir command creates a new directory.

Discussion

To create a directory, use the mkdir command. Try out the following example (note that only the commands are shown, not the responses):

$ cd ~
$ mkdir my_directory
$ cd my_directory
$ ls

You need to have write permissions in the directory within which you are trying to create the new directory.

See Also

For general information on using the Terminal to navigate the filesystem, see Recipe 3.4.

Problem

You want to delete a file or directory using the Terminal.

Solution

The rm (remove) command will delete a file or directory and its contents. You should use this with extreme caution.

Discussion

Deleting a single file is simple and safe. The following example will delete the file my_file.txt from the home directory; you can use the ls command to make sure it’s gone:

$ cd ~
$ rm my_file.txt
$ ls

You need to have write permissions in the directory within which you are trying to carry out the deletion.

You can also use the * wildcard when deleting files. This example deletes all the files that begin with my_file. in the current directory:

$ rm my_file.*

You could also delete all the files in the directory by typing:

$ rm *

If you want to recursively delete a directory (that is, not just the directory itself, but all the files and directories that it contains), you can use the -r option:

$ rm -r mydir

See Also

See also Recipe 3.4.

If you are concerned about accidentally deleting files or folders, you can force the rm command to confirm by setting up a command alias (Recipe 3.36).

Problem

Some commands don’t work because you have insufficient privileges.

Solution

You need to issue commands with superuser privileges. The sudo (substitute user do) command allows you to perform actions with superuser privileges. Just prefix the command with sudo.

Discussion

Most tasks that you want to perform on the command line can usually be performed without superuser privileges. The most common exceptions to this are when you’re installing new software and editing configuration files. The apt-get command is the principal way of installing new software into Raspbian. You will meet it formally in Recipe 3.17.

Another example requiring superuser privileges is the reboot command. If you try to run it as a normal user, you will receive a number of error messages:

$ reboot
Failed to set wall message, ignoring: Interactive authentication required.
Failed to reboot system via logind: Interactive authentication required.
Failed to open /dev/initctl: Permission denied
Failed to talk to init daemon.

If you issue the same command prefixed with sudo, the command will work just fine:

$ sudo reboot

If you have a whole load of commands to run as superuser and don’t want to have to prefix each command with sudo, you can use the following command:

$ sudo sh
# 

Note how the prompt changes from $ to #. All subsequent commands will be run as superuser. When you want to revert to being a regular user, enter the command:

# exit
$

See Also

To understand more about file permissions, see Recipe 3.13.

To install software using apt-get, see Recipe 3.17.

Problem

You have seen the strange characters that accompany a filename when it is listed. You would like to know what they all mean.

Solution

To see the permissions and ownership information relating to files and directories, use the ls command with the option -l.

See Also

To change file permissions, see Recipe 3.14.

Problem

You need to change the permissions of a file.

Solution

You can use the command chmod to modify file permissions.

Discussion

Common reasons why you might want to change file permissions include needing to edit a file that is marked as read-only and giving a file execute permissions so that it can run as a program or script.

The chmod command allows you to add or remove permissions for a file. There are two syntaxes for doing this; one requires the use of octal (base 8), and the other is text based. You will use the easier-to-understand text method.

The first parameter to chmod is the change to make, and the second is the file or folder to which it should apply. This change parameter takes the form of the permission scope (+, -, = for add, remove, and set, respectively) and then the permission type.

For example, the following code will add execute (x) rights for the owner (user) of the file file2.txt:

$ chmod u+x file2.txt

If we now list the directory, we can see that the x permission has been added:

$ ls -l
total 16
-rw-r--r-- 1 pi pi    5 Apr 23 15:23 file1.txt
-rwxr--r-- 1 pi pi    5 Apr 24 08:08 file2.txt
-rw-r--r-- 1 pi pi    5 Apr 23 15:23 file3.txt
drwxr-xr-x 2 pi pi 4096 Apr 23 15:23 mydir

If we wanted to add execute permissions for the group or for other users, we would use g and o, respectively. The letter a adds the permission to everyone.

See Also

For background on file permissions, see Recipe 3.13.

See Recipe 3.15 for changing file ownership.

Problem

You need to change the ownership of a file.

Solution

You can use the command chown (change owner) to modify the ownership of a file or directory.

Discussion

As we discovered in Recipe 3.13, any file or directory has both an owner and a group associated with it. Because most users of the Raspberry Pi will just have the single user pi, we don’t really need to worry about groups.

Occasionally, you will find files on your system that have been installed with a different user than pi. If this is the case, you can change the ownership of the file by using the chown command.

To change the owner of a file, use chown followed by the new owner and group, separated by a colon, and then the name of the file.

You will probably find that you need superuser privileges to change ownership, in which case you should prefix the command with sudo (Recipe 3.12):

$ sudo chown root:root file2.txt
$ ls -l
total 16
-rw-r--r-- 1 pi   pi      5 Apr 23 15:23 file1.txt
-rwxr--r-- 1 root root    5 Apr 24 08:08 file2.txt
-rw-r--r-- 1 pi   pi      5 Apr 23 15:23 file3.txt
drwxr-xr-x 2 pi   pi   4096 Apr 23 15:23 mydir

See Also

For background on file permissions, see Recipe 3.13.

Also see Recipe 3.14 for changing file permissions.

Problem

You want to capture an image of the Raspberry Pi’s screen and save it to a file.

Solution

Use the delightfully named scrot (SCReenshOT) screen capture software.

Discussion

The simplest way to trigger a screen capture is just to enter the command scrot. This will immediately take an image of the primary display and save it in a file named something like 2019-04-25-080116_1024x768_scrot.png within the current directory.

Sometimes you want a screenshot to show a menu being opened or something that generally disappears when the window in which you are interested loses focus. For such situations, you can specify a delay before the capture takes place by using the -d option:

$ scrot -d 5

The delay is specified in seconds.

If you capture the entire screen, you can crop it later with image editing software, such as GIMP (Recipe 4.7). However, it is more convenient to just capture a specific area of the screen in the first place, which you can do by using the -s option.

To use this option, type the following command and then, with the mouse, drag out the area of screen that you want to capture:

$ scrot -s

The filename will include the dimensions in pixels of the image captured.

See Also

The scrot command has a number of other options to control things like using multiple screens and changing the format of the saved file. You can find out more about scrot from its man page by entering the following command:

$ man scrot

Problem

You want to install software using the command line.

Solution

The most frequently used tool for installing software from a Terminal session is apt-get (the Advanced Packaging Tool).

The basic format of the command, which you must run as a superuser, is as follows:

$ sudo apt-get install <name of software>

For example, to install the AbiWord word processing software, you would enter this command:

$ sudo apt-get install abiword

Discussion

The apt-get package manager uses a list of available software. This list is included with the Raspberry Pi operating system distribution that you use but is likely to be out of date. So it is a good idea to always run the following command to update the list before installing new software using apt-get:

$ sudo apt-get update

The list and the software packages for installation are all on the internet, so none of this will work unless your Raspberry Pi has an internet connection.

$ sudo rm /var/lib/dpkg/status
$ sudo touch /var/lib/dpkg/status

The installation process can often take a while because the files must be downloaded and installed. Some installations will also add shortcuts to your desktop, or the program groups on your Start menu.

You can search for software to install by using the command apt-get search followed by a search string such as abiword. This then displays a list of matching packages that you could install.

See Also

See Recipe 3.18 for removing programs that you no longer need so that you can free up space.

See also Recipe 3.21 for downloading source code from GitHub.

Problem

Having installed a whole load of programs using apt-get, you now find that you want to remove some of them.

Solution

The apt-get utility has an option (remove) that will remove a package, but it will remove only those packages that have been installed with apt-get install.

For example, if you wanted to remove AbiWord, you would use the following command:

$ sudo apt-get remove abiword

Discussion

Removing a package like this does not always delete everything, because packages often have prerequisite packages that are installed as well. To remove these, you can use the autoremove option, as shown here:

$ sudo apt-get autoremove abiword
$ sudo apt-get clean

The apt-get clean option will do some further tidying up of unused package installation files.

See Also

See Recipe 3.17 for installing packages using apt-get.

Problem

You want to download a file from the internet without using a web browser.

Solution

You can use the wget command to fetch a file from the internet.

For example, the following command fetches the file Pifm.tar.gz from https://www.icrobotics.co.uk:

$ wget http://www.icrobotics.co.uk/wiki/images/c/c3/Pifm.tar.gz
--2013-06-07 07:35:01--  http://www.icrobotics.co.uk/wiki/images/c/c3/Pifm.tar.gz
Resolving www.icrobotics.co.uk (www.icrobotics.co.uk)... 155.198.3.147
Connecting to www.icrobotics.co.uk (www.icrobotics.co.uk)|155.198.3.147|
:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 5521400 (5.3M) [application/x-gzip]
Saving to: `Pifm.tar.gz'

100%[==================================================>] 5,521,400    601K/s

2018-06-07 07:35:11 (601 KB/s) - `Pifm.tar.gz' saved [5521400/5521400]

If your URL contains any special characters, it is a good idea to enclose them in double quotes. This example URL is from Recipe 4.6.

Discussion

You will find instructions for installing software that rely on using wget to fetch files. It is often more convenient to do this from the command line rather than use a browser, find the file, download it, and then copy it to the place you need it.

The wget command takes the URL to download as its argument and downloads it into the current directory. It’s typically used to download an archive file of some type but will also download any web page.

See Also

For more information on installing with apt-get, see Recipe 3.17.

Problem

Sometimes Python libraries and another software are hosted on the GitHub website or other online Git repository. You need to be able to fetch them onto your Raspberry Pi.

Solution

To use code in Git repositories, you need to use the git clone command to make your own copy of the files.

For example, the following command will download all of the source code examples from this book into a new folder:

$ git clone https://github.com/simonmonk/raspberrypi_cookbook_ed3.git

Along with the URL for the code to clone, there is a web page that you can visit with a browser. If you go to the GitHub web page, you will find a web page that looks like Figure 3-12.

Clicking the “Clone or download” button allows you to copy the repository’s URL and then paste it after the command git in your Terminal session.

Discussion

There is a difference between Git and GitHub. Git is the software and GitHub is one of many websites hosting code that you push to GitHub using Git. In fact you can actually host your own Git repository on a Raspberry Pi if you want to. However, there are benefits to using a Git-based website such as GitHub or GitLab:

• Your code is stored in the cloud, so if your disk (or SD card) breaks, you won’t lose the code.

• The code is publicly visible, so other people can look at it and use it, and if they find things wrong with it they might even offer up fixes for you.

• You can include documentation about your project for all to see in the README file.

Figure 3-12. The GitHub repository page for this book

If you are working on a Raspberry Pi project that you think others might be interested in, I would recommend using GitHub or GitLab to host your code. There is a little bit to learn, but it’s worth the effort.

Another advantage to using Git (whether locally or with a service like GitHub) is that every time you do a chunk of work on a project, you will push that work up to the master copy of your code. This does not replace the code that was already there but is instead stored as a new edition. You can at any time recover earlier versions of the code, should you make a mistake.

I host all of the code from my books and other projects on GitHub. These are the steps I take when I make a new repository:

1. Go to your home page on GitHub (you’ll need to create an account) and click the + button and select the New Repository option.
2. Give the repository a name and short description.
3. Check the option “Initialize this repository with a README.”
4. Select a license—I select MIT for no better reason than my great respect for that august center of learning, reasoning that if it has a license for sharing my work, it’s probably a good one.
5. Click “Create repository.”
6. Now, back on your computer (Raspberry Pi or other), open a command prompt (Recipe 3.3).
7. Run the command git followed by the URL of the repository. This will create a folder for the project. Any files that you write in this folder will eventually be saved to GitHub.

$ git add .
$ git commit -m "message about what you changed or added"
$ git push

The first of these commands adds all of the changed or new files to the list of files to be committed. The commit command gives you an option to explain what’s new in the changes being committed. Finally, the push command pushes the changes up to GitHub. At this point, you are prompted for your GitHub username and password.

You will find GitHub to be a rich source of Python and other code for use with the Raspberry Pi. This is especially true when it comes to software interfaces to different types of hardware such as displays and sensors.

See Also

Learn more about Git and the Git hosting services GitHub and GitLab.

For information on downloading this book’s program code and other files relating to this book see Recipe 3.22.

Problem

You want to download all the source code and other files relating to this book.

Solution

You can either clone the files from GitHub as described in Recipe 3.21 or, as we will describe here, get the downloads as a single ZIP archive file from GitHub.

A good starting point for getting the book’s downloads is to go to the book’s web page using the browser on your Raspberry Pi. Here, in addition to a link to the book’s code hosted on GitHub, you will find errata and other information about the book.

So whether you begin at the website or directly at the GitHub page, when you click the “Clone or download” button, you will see an option to Download ZIP. You can see this in Figure 3-12 in Recipe 3.21.

Click the Download ZIP option. Chromium saves this to your Downloads folder. Click the down arrow next to the downloaded ZIP file (Figure 3-13) and then select the option “Show in folder.”

Figure 3-13. Downloading a ZIP archive for this book’s files

This opens a File Manager window in the Downloads folder. Find the ZIP file that has just been downloaded (Figure 3-14).

Figure 3-14. Finding the ZIP archive file in Downloads

Double-click the ZIP file to open the Xarchiver tool on the ZIP file (Figure 3-15) and then click the “Extract files” icon.

Figure 3-15. Using the Xarchiver tool to extract the files for this book

In the dialog that appears (Figure 3-16), change the path to which the extracted folder is to be saved to /home/pi and then click Extract.

Figure 3-16. Changing the path for the extracted files

After the files have been extracted, there will be a new folder in your home directory containing all the downloads for the book.

Discussion

If you now use the File Manager to see what’s in your home directory, you will find a folder called raspberry_pi_cookbook_ed3-master (Figure 3-17).

Figure 3-17. The extracted files

The folder PiBakery contains PiBakery configurations from Recipe 1.8. The python folder contains all the Python programs used in the book.

See Also

For more information on using Git and GitHub, see Recipe 3.21.

Problem

You want to arrange for a script or program to start automatically every time the Raspberry Pi reboots.

Solution

Debian Linux, on which most Raspberry Pi distributions are based, uses a dependency-based mechanism for automating the running of commands at startup. This is a little tricky to use and involves creating a configuration file for the script or program that you want to run, which will reside in a folder called init.d.

Discussion

The following example shows you how to run a Python script in your home directory. The script could do anything, but in this case, the script runs a simple Python web server, which is described further in Recipe 7.17.

Here are the steps involved:

1. Create an init script.

2. Make the init script executable.

3. Tell the system about the new init script.

First, create the init script. You need to create this in the folder /etc/init.d/. The script can be called anything, but in this example, we call it my_server.

Create the new file by using nano with the following command:

$ sudo nano /etc/init.d/my_server

Paste the following code into the editor window and save the file. This is a lot to type, so if you are reading a paper copy of this book, you can copy and paste the code from this web page; just scroll down until you find this chapter and recipe:

### BEGIN INIT INFO
# Provides: my_server
# Required-Start: $remote_fs $syslog $network
# Required-Stop: $remote_fs $syslog $network
# Default-Start: 2 3 4 5
# Default-Stop: 0 1 6
# Short-Description: Simple Web Server
# Description: Simple Web Server
### END INIT INFO

#! /bin/sh
# /etc/init.d/my_server

export HOME
case "$1" in
  start)
    echo "Starting My Server"
    sudo /usr/bin/python /home/pi/myserver.py  2>&1 &
  ;;
stop)
  echo "Stopping My Server"
  PID=`ps auxwww | grep myserver.py | head -1 | awk '{print $2}'`
  kill -9 $PID
  ;;
*)
  echo "Usage: /etc/init.d/my_server {start|stop}"
  exit 1
;;
esac
exit 0

This is quite a lot of work to automate the running of a script, but most of it is boiler-plate code that is the same for every service. To run a different script, just work your way through the script, changing the descriptions and the name of the Python file that you want to run.

The next step is to make this file executable for the owner, which you do by using this command:

$ sudo chmod +x /etc/init.d/my_server

Now that the program is set up as a service, you can use the following command to test that everything is OK before you set it to autostart as part of the boot sequence:

$ /etc/init.d/my_server start
Starting My Server
Bottle v0.11.4 server starting up (using WSGIRefServer())...
Listening on http://192.168.1.16:80/
Hit Ctrl-C to quit.

Finally, if that runs OK, use the following command to make the system aware of the new service that you have defined:

$ sudo update-rc.d my_server defaults

See Also

For a simpler approach to making a program run automatically, see Recipe 3.23.

For more information on changing file and folder permissions, see Recipe 3.13.

Problem

You want to run a script once each day or at regular intervals.

Solution

Use the Linux crontab command.

To do this, the Raspberry Pi needs to know the time and date and therefore needs a network connection or a real-time clock; see Recipe 12.13.

Discussion

The command crontab allows you to schedule events to take place at regular intervals. This can be daily or hourly, and you can even define complicated patterns so different things happen on different days of the week. This is useful for backup tasks that you might want to run in the middle of the night.

You can edit the scheduled events by using the following command:

$ crontab -e

If the script or program that you want to run needs to be run by a superuser, prefix all the crontab commands with sudo (Recipe 3.12).

The comment line (starting with a #) indicates the format of a crontab line. The digits are, in order, minute, hour, day of month, month, and day of week and are followed by the command that you want to run.

If there is a * in the relevant position, that means every; if there is a number there instead, the script runs only at that minute/hour/day of the month.

So, to run the script every day at 1 a.m., you would add the line shown in Figure 3-18.

Figure 3-18. Editing crontab

By specifying a range of day numbers say 1–5 (Monday to Friday) in the day of week column, the script will run only at 1am on those days, as demonstrated here:

0 1 * * 1-5 /home/pi/myscript.sh

If your script needs to be run from a particular directory, you can use a semicolon (;) to separate multiple commands, as shown here:

0 1 * * * cd /home/pi; python mypythoncode.py

See Also

You can see the full man page documentation for crontab by entering this command:

$ man crontab

Problem

You want to find a file that you know is on the system somewhere.

Solution

Use the Linux find command.

Discussion

Starting with a directory specified in the command, the find command will search for a file that you specify and, if it finds the file, display its location.

For example:

$ find /home/pi -name gemgem.py
/home/pi/python_games/gemgem.py

You can start the search at various points on the tree, even at the root of the entire filesystem (/). A search of the entire filesystem will take a lot longer and will also produce error messages. You can redirect these error messages by adding 2>/dev/null to the end of the line.

To search for the file throughout the entire filesystem, use the following command:

$ find / -name gemgem.py 2>/dev/null
/home/pi/python_games/gemgem.py

Note that 2>/dev/null redirects output that would make it difficult to see the file when it was eventually found. You can find out more about redirection in Recipe 3.31.

You can also use wildcards with find as follows:

$ find /home/pi -name match*
/home/pi/python_games/match4.wav
/home/pi/python_games/match2.wav
/home/pi/python_games/match1.wav
/home/pi/python_games/match3.wav
/home/pi/python_games/match0.wav
/home/pi/python_games/match5.wav

See Also

The find command has a number of other advanced features for searching. To see the full man page documentation for find, use this command:

$ man find

Problem

You want to be able to repeat commands on the command line without having to type them again.

Solution

Use the up arrow and down arrow keys to select previous commands from the command history, and use the history command with grep to find older commands.

Discussion

You can access the previous command you ran by pressing the up arrow key. Pressing it again will take you to the command before that, and so on. If you overshoot the command you wanted, the down arrow key will take you back in the other direction.

If you want to cancel without running the selected command, use Ctrl-C.

Over time, your command history will grow too large for you to use the arrow keys to find a command that you used ages ago. To find a command from way back, you can use the history command:

$ history
    1  sudo nano /etc/init.d/my_server
    2  sudo chmod +x /etc/init.d/my_server
    3  /etc/init.d/my_server start
    4  cp /media/4954-5EF7/sales_log/server.py myserver.py
    5  /etc/init.d/my_server start
    6  sudo apt-get update
    7  sudo apt-get install bottle
    8  sudo apt-get install python-bottle

This lists all of your command history and is likely to have far too many entries for you to find the command you want. To remedy this, you can pipe (see Recipe 3.33) the history command into the grep command, which will display only results matching a search string. So, for example, to find all the apt-get (Recipe 3.17) commands that you’ve issued, you can use the line:

$ history | grep apt-get
    6  sudo apt-get update
    7  sudo apt-get install bottle
    8  sudo apt-get install python-bottle
   55  history | grep apt-get

Each history item has a number next to it, so if you find the line you were looking for, you can run it using ! followed by the history number, as shown here:

$ !6
sudo apt-get update
Hit http://mirrordirector.raspbian.org wheezy InRelease
Hit http://mirrordirector.raspbian.org wheezy/main armhf Packages
Hit http://mirrordirector.raspbian.org wheezy/contrib armhf Packages
.....

See Also

To find files rather than commands, see Recipe 3.26.

Problem

The Raspberry Pi can run a bit slow sometimes, so you want to see what’s hogging the processor.

Solution

Use the Task Manager utility, which you’ll find on the Main menu, in the Accessories program group (Figure 3-19).

Figure 3-19. The Task Manager

The Task Manager allows you to see at a glance how much CPU and memory are being used. You can also right-click a process and select the option to kill it from the pop-up menu that appears.

The bar graphs toward the top of the window display the total CPU usage and memory usage. The processes are listed below that, and you can see the CPU share each is taking.

Discussion

If you prefer to do this type of thing from the command line, use the Linux top command to display very similar data about processor and memory usage and which processes are using the most resources (Figure 3-20). You can then use the kill command to kill a process. You will need to do this as a superuser.

In this case, you can see that the top process is a Python program that uses 97% of CPU. The first column shows its process ID (2447). To kill this process, enter this command:

$ kill 2447

It is quite possible to kill some vital operating system process this way, but if you do, powering off your Pi and turning it back on again will restore things to normal.

Figure 3-20. Using the top command to see resource usage

Sometimes you might have a process running that is not immediately visible when you use top. If this is the case, you can search all the processes running by using the ps command and piping (Recipe 3.33) the results to the grep command (Recipe 3.27), which will search the results and highlight items of interest.

For example, to find the process ID for our CPU-hogging Python process, we could run the following command:

$ ps -ef | grep "python"
pi        2447  2397 99 07:01 pts/0    00:00:02 python speed.py
pi        2456  2397  0 07:01 pts/0    00:00:00 grep --color=auto python

In this case, the process ID for the Python program speed.py is 2447. The second entry in the list is the process for the ps command itself.

See Also

See also the man pages for top, ps, grep, kill, and killall. You can view these by using the man command followed by the name of the command for which you want information, as shown here:

$ man top

Problem

You have downloaded a compressed file and want to uncompress it.

Solution

Depending on the file type, you will need to use the tar command or the gunzip command.

Discussion

If the file that you want to uncompress just has the extension .gz, you can unzip it using the command:

$ gunzip myfile.gz

You also often find files (called tarballs) that contain a directory that has been archived with the Linux tar utility and then compressed with gzip into a file with a name like myfile.tar.gz.

You can extract the original files and folders out of a tarball by using the tar command:

$ tar -xzf myfile.tar.gz

If the file is a ZIP archive, you can use the File Manager and Xarchiver tools, as shown in Recipe 3.22.

See Also

You can find out more about tar from its man page, which you can access by using the command man tar.

Problem

You’ve plugged in a USB device and want to make sure Linux recognizes it.

Solution

Use the lsusb command. This lists all of the devices attached to the USB ports on your Raspberry Pi:

$ lsusb
Bus 001 Device 002: ID 0424:9512 Standard Microsystems Corp.
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 001 Device 003: ID 0424:ec00 Standard Microsystems Corp.
Bus 001 Device 004: ID 15d9:0a41 Trust International B.V. MI-2540D [Optical mouse]

Discussion

This command informs you as to whether a device is connected, but it will not guarantee that the device is working correctly. There might be drivers to install or configuration changes to make for the hardware.

See Also

For an example of using lsusb when attaching an external webcam, see Recipe 4.3.

Problem

You want to quickly create a file with some text or record a directory listing into a file.

Solution

Use the > command to redirect output that would otherwise appear in your Terminal after you run the command.

For example, to copy a directory listing into a file called myfiles.txt, do the following:

$ ls > myfiles.txt
$ more myfiles.txt
Desktop
indiecity
master.zip
mcpi

Discussion

You can use the > command on any Linux command that produces output, even if you are running, say, a Python program.

You can also use the opposite (<) command to redirect user input, although this is not nearly as useful as >.

See Also

To use cat command to join together a number of files, see Recipe 3.32.

Problem

You have a number of text files, and you want to join them into one big file.

Solution

Use the cat command to concatenate a number of files into one output file.

For example:

$ cat file1.txt file2.txt file3.txt > full_file.txt

Discussion

Joining files is the real purpose of the cat command. You can supply as many filenames as you like, and they will all be written to the file that you specify. If you do not redirect the output, it will just appear in your Terminal window. If they are big files, this process might take some time!

See Also

See also Recipe 3.8, in which cat is used to display the contents of a file.

Problem

You want to use the output of one Linux command as the input to another command.

Solution

Use the pipe command, which is the bar symbol (|) on your keyboard, to pipe the output of one command to another.

For example:

$ ls -l *.py | grep Jun
-rw-r--r-- 1 pi pi 226 Jun  7 06:49 speed.py

This example will find all the files with the extension py that also have Jun in their directory listing, indicating that they were last modified in June.

Discussion

At first sight, this looks rather like output redirection using > (Recipe 3.31). The difference is that > will not work if the target is another program. It will work only for redirecting to a file.

You can chain together as many programs as you like, as shown here, although this isn’t something you will do often:

$ command1 | command2 | command3

See Also

See Recipe 3.28 for an example of using grep to find a process, and Recipe 3.27 to search your command history using pipe and grep.

Problem

You want to run a command, but you don’t want the output filling up your screen.

Solution

Redirect the output to /dev/null using >.

For example:

$ ls > /dev/null

The dev directory contains operating system devices, including things like serial ports. Within this directory, a special device (the null device) is defined that simply discards everything sent to it.

Discussion

This example illustrates the syntax but is otherwise pretty useless. A more common use is when you’re running a program and the developer has left a lot of trace messages in its code, which you don’t really want to see. The following example hides superfluous output from the find command (see Recipe 3.36):

$ find / -name gemgem.py 2>/dev/null
/home/pi/python_games/gemgem.py

See Also

For more information about redirecting standard output, see Recipe 3.31.

Problem

You want to run a program while also working on some other task.

Solution

Run the program or command in the background using the & command.

For example:

$ python speed.py &
[1] 2528
$ ls

Rather than wait until the program has finished running, the command line displays the process ID (the second number) and immediately allows you to continue with whatever other commands you want to run. You can then use this process ID to kill the background process (Recipe 3.28).

To bring the background process back to the foreground, use the fg command:

$ fg
python speed.py

This reports the command or program that is running and then waits for it to finish.

Discussion

Output from the background process will still appear in the Terminal.

An alternative to putting processes in the background is to just open more than one Terminal window.

See Also

For information on managing processes, see Recipe 3.28.

Problem

You want to create aliases (shortcuts) to commands that you use frequently.

Solution

Edit the file ~/.bashrc using nano (Recipe 3.7), and then move to the end of the file and add as many lines as you want, like this:

alias L='ls -a'

This creates an alias called L that, when entered, will be interpreted as the command ls -a.

Save and exit the file using Ctrl-X and Y, and then type the following command to update the Terminal with the new alias:

$ source .bashrc

Discussion

Many Linux users set up an alias for rm like the following so that it confirms deletions:

$ alias rm='rm -i'

This is not a bad idea, as long as you do not forget when you use someone else’s system who doesn’t have this alias set up!

See Also

For more information about rm, see Recipe 3.11.

Problem

You want to manually set the date and time on your Raspberry Pi because it does not have an internet connection.

Solution

Use the Linux date command.

The date and time format is MMDDhhmmYYYY, where MM is the month number; DD is the day of the month; hh and mm are the hours and minutes, respectively; and YYYY is the year.

For example:

$ sudo date 010203042019
Wed  2 Jan 03:04:00 GMT 2019

Discussion

If the Raspberry Pi is connected to the internet, as it boots up it will automatically set its own time using an internet time server.

You can also use date to display Coordinated Universal Time (UTC) time by entering date on its own:

$ date
Fri 19 Jul 10:59:08 BST 2019

See Also

If you want your Raspberry Pi to maintain the correct time even when there is no network, you can use a real-time clock (RTC) module (Recipe 12.13).

Problem

You want to know how much free space there is on the SD card.

Solution

Use the Linux df (disk filesystem) command:

$ df -h
Filesystem      Size  Used Avail Use% Mounted on
rootfs          3.6G  1.7G  1.9G  48% /
/dev/root       3.6G  1.7G  1.9G  48% /
devtmpfs        180M     0  180M   0% /dev
tmpfs            38M  236K   38M   1% /run
tmpfs           5.0M     0  5.0M   0% /run/lock
tmpfs            75M     0   75M   0% /run/shm
/dev/mmcblk0p1   56M   19M   38M  34% /boot

The -h option shows the sizes using the kB, MB, and GB symbols rather than the number of bytes.

Discussion

Looking at the first line of the results, you can see that there is 3.6 GB of storage on the SD card, of which 1.7 GB is used.

When you run out of disk space, you are likely to get unexpected bad behavior, such as error messages saying that a file could not be written.

See Also

You can find the man page for df by using the command man df.