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Wednesday, August 7, 2013

C: Hello World on Linux

Hello people!

Today I'm going to describe how you go about writing, compiling and executing C code using the Linux command line (also known as shell or terminal).

You will first need to make sure you have the tools necessary to compile C code. In particular you need gcc. You can get it by installing the build-essential package. On a Debian-based Linux distribution such as Ubuntu, you'd use the following command in the terminal:

sudo apt-get install build-essential

...and then, after entering the root password (if necessary), you proceed with the installation:

To actually write the C code, you can use the vi editor. Even better, you can make sure that you have the vim (vi improved) by using the following command:

sudo apt-get install vim

Then use the following command to edit a file called hello.c (it doesn't need to exist yet):

vi hello.c

This opens the vi editor. vi is very powerful but takes a little getting used to. You start off in command mode, and just about any letter you type has a particular meaning. If you press the 'I' key, you go into insert mode, and can type code to your heart's content. Do that, and enter the following code:

#include <stdio.h>

int main(int argc, char ** argv)
    printf("Hello world!\n");

    return 0;

Over here we're first including the stdio.h library, which allows us to do input/output (I/O) such as outputting text to the terminal. The actual code goes into the main() function, which returns an integer. You can ignore the argc and argv bits, and just use this as a template for your code, for the time being. We actually write something to the terminal using the printf() function. The only thing that might seem a little special here is the \n thing - it's an escape sequence that outputs a newline.

Once you're done typing the above, press ESC in vi to go back to command mode. Then type :wq and press enter - this saves the file (w) and quits (q).

Back in the terminal, type the following command to compile the code in hello.c:

gcc hello.c -o hello

The -o part means that the next argument (in this case "hello") is the name of the executable to be produced by gcc. On Linux, executables don't need to have extensions such as .exe, although it's perfectly fine to include them to make it easier to recognise them.

Finally, type ./hello and press ENTER to run the program. You should see "Hello world!" as the output, as above.

Note: just in case you can't run the program because of some permissions, try the following command:

chmod 777 hello

Great! This simple tutorial showed how to write C code using the vi editor, compile it using the gcc program, and execute it from within a Linux shell. Come back for more tutorials!

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