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Glossary

PATH

The list of directories in which your system will look for programs to execute. See PATH. When you type a command such as ls at the terminal prompt, this will cause your shell to look for an executable file called ls in a list of directories. The list of directories is called the system PATH. Specifically these directories are listed in the value of an environment variable called PATH. Assuming you are using the default Unix bash shell, you can see these directories by typing:

echo $PATH

at the terminal prompt, followed by the return key. This might give you output like this:

/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin:/opt/X11/bin

The shell will search this list of directories in order for an executable file called ls: first /usr/bin, then /bin, and so on. We can ask to see the full path of the program that the system finds with the which command:

$ which ls
/bin/ls

This tells us that the system did not find a ls executable file in /usr/bin, but did find one in /bin, for a full path of /bin/ls.

shell

A shell is a program that gives access to the computer operating system. It is usually a “command line interface” program that runs in a terminal, accepting strings that the user types at the keyboard. The shell program interprets the string and executes commands. The most common default shell program is bash – for Bourne-Again SHell, so-called because it is an expanded variant of an older shell program, called the Bourne shell. For example, when you open a default terminal application, such as Terminal.app in OSX or gnome-terminal in Linux, you will usually see a prompt at which you can type. When you type, the program displaying the characters and interpreting them is the shell. When you press return at the end of a line, the shell takes the completed line, and tries to interpret it as a command. See also PATH.

environment variable

An environment variable is a key, value pair that is stored in computer memory and available to other programs running in the same environment. For example the PATH environment variable, is a key, value pair where the key is PATH and the value is a list of directories, such as /usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin. In particular, the shell uses the value of the PATH environment variable as a list of directories to search for executable programs.

executable

A file is executable if the file is correctly set up to execute as a program. On Unix systems, an executable file has to have special file permissions that label the file as being suitable for execution.

file permissions

Computer file-systems can store extra information about files, including file permissions. For example, the file permissions tell the file-system whether a particular user should be able to read the file, or write the file or execute the file as a program.

voxel

Voxels are volumetric pixels - that is, they are values in a regular grid in three dimensional space - see the Wikipedia voxel entry.