This lab introduces you to the basic information about Linux System, compiling using the GNU g++ compiler, and to editing with the very powerful emacs editor.

Logging on

You must have an account to log on to the system. If you do not have an account on the Math/CS computer system, please ask your instructor to give you one. This is a different system than is used throughout SU; having an SU account is not enough.

Math/CS Linux filesystem structure

Unlike a simple standalone personal computer, the Math/CS filesystem has components that may exist on many different machines, but the operating system makes it look like the filesystem exists on the computer you are using. The filesystem on a Linux system is conceptually hierarchical (has a tree structure). The "root" of the tree is shown at the top of the figure below and is designated by the slash character ('/'). Every item in the filesystem is either a file or a directory (known as a "folder" in Windows-talk). The concept is essentially the same as in DOS or Windows: a directory can contain other directories and files. A file can contain data, but cannot contain other files or directories.

At any time while using the system, you can think of yourself as being "located" somewhere in the filesystem. Your current location in the filesystem is called your current working directory or cwd.

To decribe a specific location in the filesystem heirarchy, you must specify a "path." The path to a location can be defined as an absolute path from the root anchor point, or as a relative path, starting from your cwd. When specifying a path, you simply trace a route through the filesystem tree, listing the sequence of directories you pass through as you go from one point to another. Each directory listed in the sequence is separated by a slash (a forward slash /, not a DOS-type backslash \). Linux also provides the shorthand notation of "." to refer to the current location, and ".." to refer to the parent directory.

For example, the absolute path to the bin directory that is in the usr directory would be /usr/bin; we start at the root (/), walk to the usr directory, then walk to the bin directory. Notice that there is another directory named "bin". The absolute path to this one is /bin, so it's not the same as /usr/bin.

TASK 1:

1. Write down the absolute path to the directory named include on the far left of the figure.

Relative paths use the same notation, but they do not start at the root of the tree. For example, if your cwd is /usr/bin, the relative path to the /usr/include directory would be ../include. The ".." takes you one step up the tree, putting you in the /usr directory. From there you go down the tree to the /usr/include directory.

TASK 2:

1. Write down the relative path from the directory /dev to the file /bin/ls.

File and directory permissions

Remember what the meanings of r, w, and x are for directories and for files:

Viewing permissions

To see information on a file, including permissions, use the ls command, with the -l option.

For example, execute the command ls -l /bin/pwd to view file information for the executable file pwd (that prints the name of your current working directory). The output should look similar to this (there may be small differences):

-rwxr-xr-x 1 root root 13848 Jan 26 2014 /bin/pwd*

The first 10 characters describe the access permissions. The first dash indicates the type of file (d for directory, s for special file, dash for a regular file). the next three characters (rwx) describe the permissions of the owner of the file (namely the "root" user). The root user has read, write, and execute permission. The next three characters (r-x) describe the permissions for those in the group associated with the file (in this case, the group name is also "root"): read, no write, execute. The next three characters (r-x) describe the permissions for all others: read, no write, execute.

TASK 3:

1. Open an terminal window,create a directory named Cosc220 under your home directory typing mkdir Cosc220. Invoke the command ls -l /home/[your username]/Cosc220 and write down the result.
2. Invoke the command groups and write down the result.
3. Write out your answers to the following questions:
   1. Who owns this directory?
   2. What permissions does the owner have?
   3. What group is associated with this directory?
   4. What permissions do group members have?
   5. Are you in this group?
   6. What permissions do all others have?

Setting permissions

Linux allows you to set the permissions on files that you own. The command to change the file permission mode is chmod (some people pronounce it "shh-mod"). Chmod requires you to specify the new permissions you want, and specify the file or directory you want the changes applied to.

The easy way to set file permissions, is by use of the "rwx" notation to specify the type of permissions, and the "ugo" (u = user, the owner; g = group; o = others) notation to specify those the permissions apply to.

To define the kind of change you want to make to the permissions, use the plus sign (+) to add a permission, the minus sign (-) to remove a permission, and the equal sign (=) to set a permission directly.

For example, to give members of the group permission to write (modify) a file named foo, you would invoke the command chmod g+w foo

TASK 4:

1. Create a file ~/foo (the ~ notation is shorthand for your home directory). Do this by invoking the command touch ~/foo.
2. Write out your answers to the following questions:
   1. What are the permissions on the file?
   2. Who owns the file?
   3. What group is associated with the file?
   4. Are you in the group? (Invoke groups to find out).
   5. Write down the names of all the groups you are in.
3. Change the permissions on ~/foo so the owner can execute the file, group members can modify the file, and all others can read and execute the file. Write out the command(s) you used to do this.

Linux Documentation

Linux provides on-line documentation for just about every system command or function. There are basically three ways to access the documentation (not counting documentation available on the web): man (manual) pages, the info system, and apropos.

man pages

Manual pages are displayed by the man command. Man pages give in-depth information on commands and functions. The system is divided into sections. The sections are not necessarily the same on every system, but typically they are as follows:

For example, to view the manual page for the system call lseek, you would invoke man 2 lseek Actually, the "2" is optional in this case since there is only one lseek. The section number is only needed when there are multiple entities with the same name, such as time which is the name of both a user command and a system call.

TASK 5:

1. Invoke the command man ls to view the manual page for the user command ls (you could also try man 1 ls to see that the section number is optional in this case). You should see something that starts like the following:

   ---

    LS(1) User Commands LS(1)
   
   NAME ls - list directory contents
   
   SYNOPSIS ls [OPTION]... [FILE]...
   
   DESCRIPTION List information about the FILEs (the current directory by
          default).  Sort entries alphabetically if none of -cftuSUX nor
          --sort.  
   

   ---

   Every man page has this structure. The first line gives the command and the section. The name of the command is given along with a short description. The synopsis is often the most important information. It shows the forms the command might take with options and arguments. Options or arguments in square brackets, [], are optional. The description section gives a more detailed description of the command. In this example, all options and files are optional, so the command ls is enough to list the contents of the current directory.

2. There are many options available for ls but the most important ones are a,l, and R. Write down the meanings of each of these options.
3. Move to your home directory and invoke ls with no options. Now invoke ls -l. Now invoke ls -a. Write down how the output of ls differs from the output of ls -a.
4. Different entities may have the same name. For example time as a user command differs from time as a system call. Write down the NAME information for both of these time entities.
5. Of course, man itself is a user command, so it should have a man page. Write down the NAME information for the man page on man.

info

The info system gives much the same information as man, but in a different, more searchable format.

apropos

The apropos command reports on any uses of the given word in the man pages. It can be useful for finding man pages when you don't remember the exact name of a command. For example, the command apropos pwd lists all the man pages that have the word pwd in them. Try it out. Sometimes you get more than you want. For example, try apropos time.

Use the emacs editor

The emacs editor is perhaps the most powerful editor available for programmers. It's worth learning to use it since it can speed up your programming and help reduce programming errors. Emacs has a tremendous number of features, but you only need to learn a few to use it successfully. As you gain experience, you can learn more features.

• Open an terminal window, Make a new folder named Lab1 on your home directory by typing mkdir Lab1. Change the directory to Cosc220/Lab1, and type in emacs
• Examine the emacs window. It has three sections
  1. A small bar at the top ("File" "Edit" "Options" etc). When you click on one of these entries, you get a pull-down menu of options.
  2. The large central part into which you will type (it's called the text area).
  3. A small command area at the bottom (called the minibuffer). This accepts emacs commands.
• We will now create a file and enter some text into it.
• Click on the "File" entry on the top bar and choose "Open File (C-x C-f)" from the pull-down menu.
  In the SU labs, a popup window will allow you to choose the filename and directory. (In some setups, or if you use the emacs keyboard commands, there will not be a popup window, but the minibuffer area at the bottom will say "Find file:" and wait for you to enter a filename.) Either way, choose "foo.bar" as your filename.
• You should now have a blank emacs text window. Type some stuff into the text window. Right now, we don't care what you type; type a few lines of your favorite poem or song lyric.
• To save the file, click on the "File" entry and choose "Save (current buffer) (C-x C-s)". You may have noticed that the bottom information bar has two asterisks "**" in it when you modify your text window. After you save your file, the asterisks go away.

• Perhaps you noticed that some "File" menu items had remarks in parentheses such as (C-x C-f) or (C-x C-s). These are the keyboard commands that emacs accepts. You don't have to use these commands, but once you get to know them, they can really speed up your programming. In this example, "C-x C-f" means hold down the 'Ctrl' key and press 'x' then 'f'. It does the same thing as choosing "Open File" from the pull-down menu.

• Another entry to note is the "Options" entry. It allows you to turn on/off various emacs options. You should turn on the following options (in the SU labs, your settings are not saved, so you'll have to reset them each time you log on):
  • Syntax Highlighting - this causes emacs to "colorize" words it knows about. Many users like to have C++ syntax colorized.
  • Paren Matching - this causes emacs to indicate the opening parenthesis, brace, or bracket when you type the closing one. It's very useful to detect unmatched symbols in your C++ code.
  • Case-insensitive search - allows searching for words without worrying about upper or lower case.

• Notice that one of the top-line items is "Help." When you have some time, browse through the help materials to learn more about emacs. The tutorial is worth looking at.

• To exit emacs, click on the "File" entry at the top and choose "Exit emacs".

• Open a terminal window
• In your terminal window, change to the Lab1 folder you created before by typing cd Lab1
• Open another terminal window and start up emacs (as before).
• In emacs, create a C++ source file named hello.cpp in your Lab1 folder:
  • Click on the "File" entry and choose "Open File (C-x C-f)"
  • In the popup find-file window or in the minibuffer area enter /Lab1/hello.cpp
  • Emacs will interpret this file as a C++ source file because it has the '.cpp' extension. Since emacs "knows" the syntax of C++, it can do nice indentation and help you reduce syntax errors as you type. Colorization should be in effect for C++ keywords.

• Type the following C++ source code into the emacs text window. On each new line that you type, the TAB key will position the cursor at the proper place for C++ syntax. It will line up parentheses and braces. Observe how emacs positions the text. If it's not what you think it should be, there is probably a syntax error in your code (missing semicolon, too many or too few parentheses or braces, etc). Observe also that when you type a closing parenthesis or brace, emacs briefly highlights the matching opening parenthesis or brace.

  Here's the code to type:

    // hello.cpp
    // Demo for Cosc-220 lab
    // {your name]
    // Created: [ate and time]
    // Current: [todayand time]
  
    #include <iostream>
    int main() 
    {
      std::cout << "Hello from lab 1" << std::endl;
      return 0;
    } 
    

• Save the file.
• In your terminal window, type ls to be sure your hello.cpp file exists.
• In your terminal window, produce an object file named hello.o by compiling hello.cpp using the g++ compiler.

     g++ -c hello.cpp 

  If successful, you will now have an object file named hello.o in your /Lab1 folder. Type ls to make sure it's there.

• Now, link your object file to produce an executable. 

     g++ -o hello hello.o
    

  The idea is that the name that follows the "-o" is the name given to the executable.

• You should now have an executable file named according to your preference. Type ls to make sure it's there.
  To execute it, just type its name. If it's named hello, type ./hello 

TASK 7: Create a program with multiple source files 

Many of your programs in this course will entail several source files. Of course, only one file can contain a main function; the other files contain supporting code. Only trivial programs are written in a single file. Every industrial-strength software development is done using multiple files. Now's the time to learn how.

As an example, if the source code is in several files, say "file1.cpp" and "file2.cpp", along with a header file named "file2.h", they are compiled into an executable program named "myprog" using the following commands:

  g++ -c file1.cpp
  g++ -c file2.cpp
  g++ file1.o file2.o -o myprog 
  

The header file, file2.h, is not mentioned in the compilation because it is #included'd in one or more of the .cpp files. Use the emacs editor to create C++ source and header files file1.cpp, file2.cpp, and file2.h as follows:

• Open a new file named file1.cpp in your Lab1 folder by clicking "Open File (C-x C-f)" in the usual way.
• Enter the following code in the emacs text window:

    // file1.cpp
    // Contains the main function
    // Demo file for Cosc-220 Lab 1
    //
    // [your own name]
    // Created: [date and time]
    // Current: [date and time]
  
    #include <iostream>
    #include "file2.h"
  
    int main()
    {
      using namespace std;
      int i = 3;
      int sq;
  
      sq = SquareIt(i);
      cout << "Square of " << i << " is " << sq << endl;
      return 0;
    }
  

• Save this code. Click on the "File" entry and choose "Save (current buffer) (C-x C-s)"

• Now, use emacs to create file2.h in the Lab1 folder.
• Type the following code in the emacs text window (except, use your own name and today's date for current):

    // file2.h
    // Declaration of SquareIt function
    // Demo for Lab 1, Cosc-220
    //
    // [your own name]
    // Created: [date and time]
    // Current: [date and time]
  
    #ifndef FILE2_H
    #define FILE2_H
  
    int SquareIt(int);
  
    #endif
    

• Save this file.

• Compile file1.cpp to see if there are any compilation errors. In the command window, type g++ -c file1.cpp. If there are errors, return to emacs to fix them.

• Now, use emacs to create file2.cpp in your Lab1 directory.
• Type the following code in the emacs text window (except, use your own name and today's date for current):

    // file2.cpp
    // Definition of SquareIt function
    // Demo for Lab 1, Cosc-220
    //
    // [your own name]
    // Created: [date and time]
    // Current: [date and time]
  
    #include "file2.h"
  
    int SquareIt(int x)
    {
       return x * x;
    }
    

• Compile file2.cpp by typing g++ -c file2.cpp in the terminal window. Fix any errors you may have.

• This is a good time to discover that emacs has remembered all three files as well as where you left off in each. This is a tremendous advantage of emacs for programmers. Each file is stored in a separate emacs buffer and can be retrieved easily. Click on "Buffers" at the top of emacs. You will get a menu that contains each of the file names you have been editing. Choose any one of them to instantly switch to that buffer.

• In the terminal window, link the files by typing g++ -o myprog file1.o file2.o
• Execute the ./myprog executable in the terminal window.

What to Turn In

Task 1:
The absolute path to the directory named include on the far left of the figure.
Task 2:
The relative path from the directory /dev to the file /bin/ls.
Task 3:
Result from the command ls -l home/[your username]/Cosc220
Result from the command groups
Answers to the six questions.
Task 4:
Answers to the four questions
Names of all the groups you are in.
Command(s) used to change the permissions.
Task 5:
Meanings of the three options.
How the output of ls differs from ls -a.
The NAME information for time as user command and as system call.
The NAME information for man.
Task 6,7:
Printout of your file hello.cpp.
Printouts of your files file1.cpp, file2.cpp, and file2.h.
　

Last updated  08/28/2014