:::{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 {term}shell to look for an
{term}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 {term}environment variable
called PATH. Assuming you are using the default Unix bash
shell, you can see these directories by typing:
echo $PATHat 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/binThe 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/lsThis 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 {term}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
{term}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. :::