Using the command line lets you harness the full power of your computer. This page contains an outline of concepts that you should become familiar with. However, it is not a comprehensive description of any of them, and you should look up any concept that you are unsure about.

When you are first using the command line, it will feel awkward and it may seem difficult to get things done. You will forget the command names and the required arguments and flags, and you may have to look up the same command several times before you internalize its usage. However, with just a little practice, you will become much faster at using the command line than you are at clicking buttons. You will also be able to do things that you can’t do through the standard graphical user interface.

Accessing the command line

In Mac OS, you can access the command line through the Terminal application, which provides access to a command line shell called bash. Because Mac OS is a descendant of BSD Unix, you can use all the standard Unix commands in Terminal. In the following, we use “Unix” as a generic term that includes Mac OS and Linux.

On Windows, the command line interpreter is called cmd or Command Line. The cmd commands are based on the old DOS commands and provide similar functionality to the Unix commands

Basic commands

You should familiarize yourself with these commands. You can find an abundance of tutorials online about the command line, as well as reference pages for each command.

Unix Windows Function
ls dir list files in directory
cd cd change directory
cat type print contents of file to console
mv move move file or directory
cp copy copy file or directory
mkdir md make directory
rm / rmdir del / rmdir remove file / directory
which where find full path to specified command
pwd cd print current (working) directory

Miscellaneous notes

Current directory

When working on the command line (in both Unix and Windows) you need to be aware of your current directory (sometimes called working directory). When you start up the command line shell, you begin in your home directory, i.e. your current directory starts out as your home directory. On both Unix and Windows, . (dot) refers to the current directory, and .. (dot dot) refers to the directory one level back. On Unix systems, you can refer to your home directory with the ~ (tilde) symbol, and you can jump directly back to your home directory by using cd with no additional arguments.

File paths

On Unix, directories in file paths are separated by a forward slash (/):

~/temp/hello

On Windows, directories in file paths are separated by a backslash (\):

C:\Users\kessnerd\temp\hello

Wildcards

On both Unix and Windows, you can use * (star) as a wildcard that will match any string, e.g. *.txt means “all files with extension .txt”.

PATH

Both Unix and Windows use environment variables, and in particular an environment variabled called PATH, which contains a list of directories. When the user types in a command, the operating system goes through each directory in the list, searching for the command executable, and running the executable when it finds it. If you write a command line program and want to be able to run it from any directory, you must put the executable in a directory that is in the PATH.

Scripting

A script is just another name for a program that is run through an interpreter. Command line scripts are run by the command line interpreter. A script can be as simple as a few commands. On the other hand, scripts on Unix (bash scripts) or Windows (batch files) can also use standard programming constructs such as variables, conditionals, loops, and functions. Note however that while these programming constructs are available, they are not as easy to use as in a more polished programming language like Python or C/C++/Java.