Introduction to Tools and Techniques in Computer Science

Compiling and running code

Franklin Bristow

Starting your terminal and connecting

You spent a lot of time in your terminal in “Remote connections and command-line file management”, and we’re going to continue to spend a lot of time in our terminal in this module. So: open your terminal and connect to Aviary. If you forget how to connect to a remote computer (and it’s totally OK if you do!), refer back to the notes on connecting to a remote computer.

Downloading files from the internet

Sometimes you need to download files from the internet so that you can work with them on the command line:

  • Your course instructor has given you a template you can use to start your assignment.
  • You’re doing work on analyzing genetic data from bacteria and you need the data to work with.
  • Your current project uses a program that reads a configuration file, but the configuration file is very long or hard to type out.

In all of these cases it’s possible for you to open your web browser (e.g., Microsoft Edge, Google Chrome, Mozilla Firefox), go to the page with the URL, click the URL, download the file to your Downloads folder, then transfer the file to the remote computer with something like scp, but doing that is tedious and time consuming (you have to download the file to your computer, then transfer it again to another computer).

Thankfully it’s straightforward to download files from the internet directly to the remote computer using a command-line program wget.

You can use a program like wget to download a file from the internet using its address. Addresses of files on the internet look like https://example.com/file.txt.

Here’s a link to a .zip file that contains many versions of a “hello world” program:

https://toolsntechniques.ca/topic03/hello.zip

Here’s how you would download this file to a remote computer directly on the command line:

wget "https://toolsntechniques.ca/topic03/hello.zip"

Congratulations 🎉! You just downloaded a file directly to a remote computer without downloading it to your computer first!

You were given the address directly in this example, but how do you get addresses from web sites in general?

  • Sometimes you are told the address directly, just like in this example.
  • You may find a link to the file that you want on a web page. You can copy a link from a web page by right clicking on the link, then selecting (something similar to) “Copy link”.

You unfortunately can’t download all files this way. One example of a file that you can’t trivially download directly to a remote computer would be files that are stored on a course web page on UM Learn. More generally: files that are on web sites that require you to log in before you can see the files.

Unzipping that zip file

You might have noticed that the file you just downloaded to the remote computer is a .zip file.

Your operating system will automatically unzip a zip file when you download it, but we’re going to need to manually unzip this file. You can use a command-line program called unzip to unzip a zip file.

Find the file that you just downloaded (you put it into an appropriate location in your folder structure, right? If not: mv the file you downloaded into the right place!).

In the same directory as the zip file (i.e., if you type ls, you should see the zip file in the directory), you can use unzip and tell unzip the name of the file it should unzip:

unzip hello.zip

Compile and run programs

We’re not going to be doing any programming in this course, but we do want to be able to compile and run code on the command line. The file that you just downloaded and unzipped contains implementations of “Hello, world!” in a variety of programming languages. You’re not expected to write in any of these languages, and you’re not necessarily expected to read any of these programming languages. All we want to do is compile them (when necessary) and run them on the command line. You will (maybe) learn these programming languages in future courses, learn them in the workplace, or learn them on your own time.

We’re going to be looking at three main types of languages:

  1. Languages that require you to compile the code, then run the resulting program like a normal program (compiled languages).
  2. Languages that require you to compile the code, then run the resulting program using a different program (compiled, but “virtual machine”-based or VM-based languages)
  3. Languages that you give to a program to execute (“interpreted” or “scripting” languages).

We’re going to be looking at only one or two examples of each of these languages, but there are many more kinds of each type of language.

You will revisit building programs in COMP 1006 when you look at the command-line environment, when we actually go through the process of building more complex applications with build systems and dependency management, but for now we’re just going to get programs compiled and run them.

All the examples are in the folder that you unzipped called hello. Change into that directory now with cd hello.

Compiled languages

Change into the compiled directory and list its contents. This directory contains a hello world program that’s written in the C programming language.

C programs require you to compile the code using a compiler before you can run them. There are two C compilers that are available on Aviary: gcc and clang. Which one you pick isn’t actually that important for the purposes of this course, they are effectively interchangeable.

You can compile a C program with clang (or gcc) by running a command:

clang hello.c -o hello # or replace `clang` with `gcc`

The hello after the -o is the name of the program that you just compiled. You can run the program by saying “run the program from this directory”:

./hello # ./ means "from this directory"

Hooray 🎉! You just compiled and ran your first C program.

Compiled Virtual Machine-based (VM-based) languages

Change back to the main hello directory, then change into the compiled-vm directory.

Remember how you can use special directory names like .. to go back or up a directory? You can combine this special directory name with others, and can go back and then to a different directory in a single command.

### Doing this with 2 commands
cd ..
cd compiled-vm

### Doing this with 1 command
cd ../compiled-vm # go back, then to the `compiled-vm` directory

Remember to use tab completion to auto-complete these folder names!

You may have already written, compiled, and run Java programs (or Processing programs), but you probably did this entirely within an integrated development environment (IDE) like the Processing IDE, DrJava, Eclipse, IntelliJ, …

You can also compile and run Java programs on the command line using the javac and java commands.

You first have to compile (with javac), then separately run your Java program (with java) on the command line.

javac Hello.java
java Hello # the same name as the class in the .java file.

Hello, Java ☕!

Interpreted or scripting languages

The last kind of language we’re going to look at are languages that are run by a program. An example of this kind of language is Python.

Change back to the main hello, then change into the interpreted directory.

You can run the Python “interpreter” (a program that will read your Python program and then do the statements) on the command line using the python command.

Once you’re in a directory with a .py file, you can tell the Python interpreter to run that program:

python hello.py

Hello, Python 🐍!