Raspberry PI Unleashed: July 2015

Thursday, July 23, 2015

C Programming

The situation is so much better for programmers today - a cheap used PC, a linux CD, and an internet account, and you have all the tools necessary to work your way to any level of programming skill you want to shoot for."

- John D. Carmack

I've got a cheap computer with Linux on it and "A Book on C" by Al Kelley and Ira Pohl. It's time to shoot high ...

1. Program Output

Program Output

Previous | Home | Next

Gather

Read Chapter 1, section 'Program Output (p. 6-10)

Reflect

In file sea.c (omit line numbers, empty lines are only put for clarity of output)

1:  #include <stdio.h>  
2:    
3:  int main(void) {  
4:    
5:    printf("from sea to shining C\n");  
6:    
7:    return 0;  
8:  }

download code here.

Linux compiling:

 $ gcc -o sea sea.c

Running program:

 ./sea

Line 1:
Directive for compiler pre-processor which will include the content of the file stdio.h which is found in the usual place (known to the compiler). This file will allow me to use function printf and others

Line 3:
Every program starts with function main(). Function will return an int (integer) to the OS that started program (Linux here).

Line 5:
Function printf(), which is loaded with line 1 directive, will display on the screen the string (sequence of characters enclosed between quotation marks. The string will end with 'new line' character (\n).

Line 7:
Program before finishing its job, is going to return number 0 (return 0) to the OS (which is code for 'all went well').

Lines 3-8:
Body of function main() is between curly braces {}. It is the code that will be run sequentialy (top-to-bottom).

Create

Introducing some errors in the code to see how the compiler is going to react.

Error 1

1:  #include <stdio.h>  
2:    
3:  int main(void)   
4:    
5:    printf("from sea to shining C\n");  
6:    
7:    return 0;  
8:  }

During compilation and linking I get:

 $ gcc -o sea sea.c   
 sea.c: In function ‘main’:  
 sea.c:5:5: error: expected declaration specifiers before ‘printf’  
    printf("from sea to shining C\n");  
    ^  
 sea.c:7:5: error: expected declaration specifiers before ‘return’  
    return 0;  
    ^  
 sea.c:8:1: error: expected declaration specifiers before ‘}’ token  
  }  
  ^  
 sea.c:8:1: error: expected ‘{’ at end of input

What is the problem?

Error 2

1:  #include <stdio.h>  
2:    
3:  int main(void){   
4:    
5:    printf("from sea to shining C\n")  
6:    
7:    return 0;  
8:  }

During compilation and linking I get:

 $ gcc -o sea sea.c   
 sea.c: In function ‘main’:  
 sea.c:7:5: error: expected ‘;’ before ‘return’  
    return 0;  
    ^

What is the problem?

Error 3

1:  #include <stdio.h>  
2:    
3:  int main(void){   
4:    
5:    printf("from sea to shining C\n);  
6:    
7:    return 0;  
8:  }

During compilation and linking I get:

 $ gcc -o sea sea.c   
 sea.c: In function ‘main’:  
 sea.c:5:12: warning: missing terminating " character [enabled by default]  
    printf("from sea to shining C\n);  
       ^  
 sea.c:5:5: error: missing terminating " character  
    printf("from sea to shining C\n);  
    ^  
 sea.c:7:5: error: expected expression before ‘return’  
    return 0;  
    ^  
 sea.c:8:1: error: expected ‘;’ before ‘}’ token  
  }  
  ^

What is the problem?

Test

Write a program that displays the following:

first.c

Previous | Home | Next

Wednesday, July 22, 2015

CLI Explorations

Home | Terminology | Next

The list of Raspberry Pi Explorations

Home | Terminology | Next

Day 23: The Linux Command Line Ch19 Notes

Previous | Home | Terminology | Next

Chapter 19 - Regular Expressions

What are regular expressions?

Let's quote the author of "The Linux Command Line" book William E. Shotts, Jr

"Simply put, regular expressions are symbolic notations used to identify patterns in text. In some ways, they resemble the shell’s wildcard method of matching file and pathnames but on a much grander scale. Regular expressions are supported by many command-line tools and by most programming languages to facilitate the solution of text manipulation problems. However,
to further confuse things, not all regular expressions are the same; they vary slightly from tool to tool and from programming language to language. For our discussion, we will limit ourselves to regular expressions as described in the POSIX standard (which will cover most of the command-line tools), as opposed to many programming languages (most notably Perl ), which use slightly larger and richer sets of notations."

List of Regular Expressions Metacharacters
(can be escaped and treated literally with backslash)

. * ? + ^ $ [ ] { } - ( ) | \

Note:
As we can see, many of the regular-expression metacharacters are also characters that have meaning to the shell when expansion is performed. When we pass regular expressions containing metacharacters on the command line, it is vital that they be enclosed in quotes to prevent the shell from attempting to expand them.

Little explanation:

. matches any single character.

* matches zero or more of characters (including a character specified by a regular expression) that immediately precedes it.

? matches zero or one occurrences of the preceding regular expression.

+ matches one or more occurrences of the preceding regular expression.

^ matches the first character of regular expression, matches the beginning of the line.

$ matches the last character of regular expression, matches the end of the line.

[] matches any one of the class of characters enclosed between the brackets. A circumflex (^) as first character inside brackets reverses the match to all characters except newline and those listed in the class. A hyphen (-) is used to indicate a range of characters. The close bracket (]) as the first character in class is a member of the class. All other metacharacters lose their meaning when specified as members of a class.

[^bg]zip matches zip NOT preceded by bg.

{n} matches the preceding element if it occurs exactly n times.

{n,m} matches the preceding element if it occurs at least n times, but no more than m times.
{n,} matches the preceding element if it occurs n or more times.

{,m} matches the preceding element if it occurs no more than m times.

- is used to denote a range of characters (e.g. [a-z] a through z.

() matches an expression.

| acts as logical or (alternation).

\ treat following metacharacter as literal and NOT metacharacter.

The lab in next post!

Previous | Home | Terminology | Next

Tuesday, July 21, 2015

Day 22: The Linux Command Line Ch18 Notes

Previous | Home | Terminology | Next

Chapter 18 - Archive and Backup

Commands:

Compressing Programs:

gzip - Compress or expand files.
bzip2 - A block sorting file compressor.

the archiving programs:

tar - Tape-archiving utility.
zip - Package and compress files.

and the file synchronization program:

rsync - Remote file and directory synchronization.

Lab 12

List the /etc directory content and redirect output to foo.txt file. Check the size of the file.
Compress foo.txt file and check its size after compression. Use gzip. Then decompress the file.
Compress the file using bzip2 utility and check the file size.
Decompress the file.
Use foo.txt to create an archive that is going to be compressed at the same time (foo.tar.gz). Use: Verbose option.
You are about to send a file to Window user. How would you compress/decompres it?
There is an external disk mounted in /media/jaro directory. Its name is Seagate Expansion Drive. Crate a 'backup' directory on it and store a directory on it (my case 'Books').

Lab 12 Solutions

List the /etc directory content and redirect output to foo.txt file. Check the size of the file.

pi@raspberrypi ~ $ ls /etc > foo.txt; ls -l foo*
-rw-r--r-- 1 pi pi 1798 Jul 21 09:22 foo.txt
pi@raspberrypi ~ $

Compress foo.txt file and check its size after compression. Use gzip. Then decompress the file.

pi@raspberrypi ~ $ gzip foo.txt; ls -l foo*
-rw-r--r-- 1 pi pi 910 Jul 21 09:22 foo.txt.gz

pi@raspberrypi ~ $

pi@raspberrypi ~ $ gunzip foo.txt.gz

Compress the file using bzip2 utility and check the file size.

Decompress the file.

pi@raspberrypi ~ $ bzip2 foo.txt ; ls -l foo*
-rw-r--r-- 1 pi pi 1001 Jul 21 09:22 foo.txt.bz2

pi@raspberrypi ~ $

pi@raspberrypi ~ $ bunzip2 foo.txt.bz2

pi@raspberrypi ~ $

Use foo.txt to create an archive that is going to be compressed at the same time (foo.tar.gz). Use: Verbose option.

pi@raspberrypi ~ $ tar -czvf foo.tar.gz foo.txt; ls -l foo*.gz
foo.txt
-rw-r--r-- 1 pi pi 1025 Jul 21 09:38 foo.tar.gz

pi@raspberrypi ~ $

You are about to send a file to Window user. How would you compress/decompress it?

$ zip file/unzip file
$ zip -r directory/unzip directory

There is an external disk mounted in /media/jaro directory. Its name is Seagate Expansion Drive. Crate a 'backup' directory on it and store a directory on it (my case 'Books').

Notice!
The name of a drive contains spaces. You can use quotes or backslashes to enter that disk.

$ cd /media/jaro/Seagate\ Expansion\ Drive/
$ rsync -av Books/ /media/jaro/Seagate\ Expansion\ Drive/Backup

Previous | Home | Terminology | Next

Monday, July 20, 2015

Day 21: The Linux Command Line Ch17 Notes

Previous | Home | Terminology | Next

Chapter 17 - Searching for Files

Commands:

locate - Find files by name.
find - Search for files in a directory hierarchy.

xargs - Build and execute command lines from standard input.
touch - Create/Change file name.
stat - Display file/file system status

Locate utility finds files based on the database stored at:
/var/lib/mlocate/mlocate.db

This database is updated using: 'updatedb' utility.

Find searches a given directory and its subdirectories in order to find the file name. It also supports type of the file (-type f = file, -type -d = directory). For instance:

$ find ~ -type d | wc -l

will find all directories (-type d) in our home directory (~) and will list their number (| wc -l). Find is very powerful. Check for all options in the "The Linux Command Line" (p. 222-224).

Lab 11

Update database locate uses and find all files with 'passwd' name.
Using 'locate' display how many times 'passwd' was found.
In your home directory find all files with ".avi" extenstion and that are 10MB or larger in size.
Find files whose content or attributes were changed in the last 20 minutes.
Find files whose content were changed in the last 2 or less minutes.
Repeat the step 4 and 5 using only one search (hint: group both queries in two logical expressions with an -or)
Which command can follow 'find' with user defined action?
As a bonus follow the instructions as per 'A Return to the Playground' section in the book. It's awesome!

Update database locate uses and find all files with 'passwd' name.

(raspberry pi in my version did not have locate utility installed)

$ sudo apt-get install locate

$ sudo updatedb

Using 'locate' display how many times 'passwd' was found.

$ locate passwd

In your home directory find all files with ".avi" extenstion and that are 10MB or larger in size.

$ find ~ -type f -name '*.avi' -size +10M

Find files whose content or attributes were changed in the last 2 or less minutes.

$ find ~ -type f -cmin -2

Find files whose content were changed in the last 2 or less minutes.

$ find ~ -type f -mmin -2

Repeat the step 4 and 5 using only one search (hint: group both queries in two logical expressions with an -or).

$ find ~ $ -type f -cmin -2 $ -or $ -type f -mmin -2 $

Which command can follow 'find' with user defined action?

$ -exec command '{}' ';'

$ | xargs command

for instance:

$ find ~ -name '*.avi' | xargs --null ls -l

Notice!

--null option allows to interpret embedded spaces in file names to be treated as such rather than as space + another command/option

As a bonus follow the instructions as per 'A Return to the Playground' section in the book. It's awesome!

Create 100 directories as per example:

$ cd playground

$ mkdir dir-{00{1..9},0{10..99},100}

Create 100 files in each directory as per example:

$ dir-{00{1..9},0{10..99},100}/file-{A..Z}

Confirm that 100 file-A have been created (if you are in 'playground directory type the following):

$ find . -name 'file-A' | wc -l

Create file 'timestamp' for our reference:

$ cd ~

$ touch playground/timestamp

$ stat playground/timestamp

Change times on the file timestamp

$ touch playground/timestamp

$ stat playground/timestamp

Let's update some of the files:

$ find playground -type f -name 'file-B' -exec touch '{}' ';'

and find newer than timestamp:

$ find playground -type f -newer playground/timestamp

Finally, let’s go back to the bad permissions test we performed earlier and apply it to playground:

$ find playground $ -type f -not -perm 0600 $ -or $ -type d -not -perm 0700 $

Previous | Home | Terminology | Next

Friday, July 17, 2015

Day 20: The Linux Command Line Ch16 Notes Cont

Previous | Home | Terminology | Next

Chapter 16 - Networking Continued

Commands:

ftp - Internet file transfer program
wget - Non-interactive network downloader
ssh - OpenSSH SSH client (remote login program)

Lab 8 - FTP

Change directory (you can use your 'playground').
Ensure that you are in the local directory 'playground'
Using CLI, create a connection to any known public ftp server and download any file.
Make sure that you see the download progress.

Lab 8 - Solution

pi@raspberrypi ~ $ ftp ftp.microsoft.com

-bash: ftp: command not found

pi@raspberrypi ~ $

Ooops! FTP client is not installed. Let's install it.

pi@raspberrypi ~ $ sudo apt-get install ftp

Reading package lists... Done

Building dependency tree

Reading state information... Done

The following NEW packages will be installed:

ftp

0 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.

Need to get 60.6 kB of archives.

After this operation, 140 kB of additional disk space will be used.

Get:1 http://mirrordirector.raspbian.org/raspbian/ wheezy/main ftp armhf 0.17-27 [60.6 kB]

Fetched 60.6 kB in 0s (77.1 kB/s)

Selecting previously unselected package ftp.

(Reading database ... 79596 files and directories currently installed.)

Unpacking ftp (from .../archives/ftp_0.17-27_armhf.deb) ...

Processing triggers for man-db ...

Setting up ftp (0.17-27) ...

update-alternatives: using /usr/bin/netkit-ftp to provide /usr/bin/ftp (ftp) in auto mode

pi@raspberrypi ~ $

Now, let's try again! I am going to use microsoft ftp server for this lab (user: ftp, password: anything).

pi@raspberrypi ~ $ ftp ftp.microsoft.com

Connected to ftp.microsoft.com.

220 Microsoft FTP Service

Name (ftp.microsoft.com:pi): ftp

331 Anonymous access allowed, send identity (e-mail name) as password.

Password:

230-Welcome to FTP.MICROSOFT.COM. Also visit http://www.microsoft.com/downloads.

230 User logged in.

Remote system type is Windows_NT.

Now let's display the content of directory on the ftp server:

ftp> ls

200 PORT command successful.

125 Data connection already open; Transfer starting.

04-28-10 07:21PM <DIR> bussys

04-28-10 10:17PM <DIR> deskapps

04-28-10 11:14PM <DIR> developr

04-28-10 11:15PM <DIR> KBHelp

04-28-10 11:15PM <DIR> MISC

04-29-10 06:54AM <DIR> MISC1

04-29-10 08:47AM <DIR> peropsys

04-29-10 05:10PM <DIR> Products

04-29-10 05:13PM <DIR> PSS

04-29-10 05:22PM <DIR> ResKit

04-29-10 07:51PM <DIR> Services

04-30-10 08:37AM <DIR> Softlib

226 Transfer complete.

ftp>

Let's snoop around in ... ResKit directory.

ftp> cd ResKit

250 CWD command successful.

ftp> ls

200 PORT command successful.

125 Data connection already open; Transfer starting.

04-29-10 05:21PM <DIR> bork

04-29-10 05:21PM <DIR> IIS4

04-29-10 05:21PM <DIR> mspress

04-29-10 05:21PM <DIR> nt4

04-29-10 05:22PM <DIR> win2000

04-29-10 05:22PM <DIR> win98

04-29-10 05:22PM <DIR> y2kfix

226 Transfer complete.

ftp> cd y2kfix

250 CWD command successful.

ftp> ls

200 PORT command successful.

150 Opening ASCII mode data connection.

04-29-10 05:22PM <DIR> alpha

02-03-00 06:24PM 5115 readme.txt

04-29-10 05:22PM <DIR> x86

Check what's my local directory:

ftp> lcd

Local directory now /home/pi

ftp>

I need to change my local directory to 'playground'

ftp> lcd playground

Local directory now /home/pi/playground

ftp>

In order to display download progress I need to use keyword: hash

ftp> hash

Hash mark printing on (1024 bytes/hash mark).

And finally, let's download 'readme.txt' file'

ftp> get readme.txt

local: readme.txt remote: readme.txt

200 PORT command successful.

125 Data connection already open; Transfer starting.

#####

226 Transfer complete.

5115 bytes received in 0.19 secs (26.6 kB/s)

ftp> bye

221 Thank you for using Microsoft products.

The '#####' indicate download progress. With large files you will see a lot of them flying through the screen.

Lab 9 - WGET

Connect to the same server and download the same file using wget.
Use wget to download the file using the following url:

http://twimgs.com/ddj/abrashblackbook/gpbb1.pdf

In case you were interested this link has all the chapters:

http://www.drdobbs.com/parallel/graphics-programming-black-book/184404919

Lab 9 - Solution

Lab 10 - SSH

Create ssh connection to a computer using CLI.

Lab 10 - Solution

Previous | Home | Terminology | Next