A command shell can be thought of as a combination of (a) a text-based version of a file manager and (b) a place where one can type commands to be executed. At any given time it is open to a specific folder (or directory) on your computer. We call the folder that is currently open in the shell the working directory. The pictures below show a command shell and a graphical file manager with the same working directory.
| Terminal window | Finder window | |
|---|---|---|
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Every computer user has what is known as a home directory. This is the folder that has your name. In MacOS, it is the folder you typically see when you ask for a new Finder window. Whenever you open a new command shell, it always starts in the home directory. As your homework and lab assignments will often be in different folders, the first thing you need to learn about the command shell is how to change directories. That is the focus of the tutorials below.
On the Macintosh, the command shell is called the Terminal. It can be found in the Applications > Utilities folder as shown below. We recommend also putting it in your Dock immediately.
When you start up the Terminal, you will get some message about the “last login” (a holdover of the days in which Terminals were used to connect machines over the network) followed by a line with a cursor that looks like a box. The left side of the line will depend on your settings, but the last symbol will likely be either a $ or a >. This symbol is called the prompt, and it is a cue for you to type something into the Terminal.
To get the Terminal to do something, simply type in a command, and hit Return. The shell will then process the command, either doing something or printing out an error message. When done, it will present the prompt again, ready for you to type in a new command.
As we mentioned above, the Terminal works a lot like the Finder. At any given time it is open to a specific folder (or directory) on your computer, which we call the working directory.
When working on a Python assignment, you want to make sure that the working
directory is the directory that contains the .py files you are
currently editing. Many a student has found themselves editing a .py
file while testing one (of the same name) in a different folder. You might
be tempted to just put everything in your home directory, but this will cause the folder to get very cluttered as the semester progresses.
Because you often need to change your working directory, the three most
important commands to know in the Terminal are pwd, ls,
and cd. Typing in pwd displays the current working directory.
The command ls lists the contents (files and folders) in the
working directory. An example of these two commands is shown below.
The cd command is an abbreviation for “change directory”. It is
how you move from one folder to another. When you type this command
into the Terminal, you must give it two things: the command cd
and the name of the folder you wish to go to. Using the example above,
suppose you wish to switch the working directory to Desktop. Then you
would type
cd Desktop
Try this out and then type ls. See the difference?
There are a couple of important tricks to know about the cd command.
The simplest form cd can only move to a folder that is “sees” (e.g. is a folder inside the working directory). If you change to directory (such as Desktop), you can no longer see the original directory (your home directory); it is outside of the current working directory. So how do you back-out if you go into a folder by mistake?
The solution is that there is a special folder called ... This refers
to the folder that contains the current one. Type
cd ..
and see what happens. If you typed it just after moving into the Desktop folder (from the previous example), then you should be back in your home directory.
Combining cd .. with regular uses of the cd command are enough to
allow you to move up and down the directory hierarchy on your computer.
It is also possible to type cd by itself, without a directory name. If you do
this, it will immediately put you back in your home folder. This is very helpful
should you ever get lost while using the Terminal.
If you are new to the Terminal, you might find yourself quickly getting tired of all the
typing that you have to do. Particularly when you have a directory with a very long name.
A slight misspelling and you have to start all over again.
Fortunately, MacOS has tab completion to speed things up. Go to your home directory and type (but do not hit Return)
cd Desk
Now hit the tab key. See what happens? It completes the work “Desk” to “Desktop”, because it is the only thing in your home folder that starts with “Desk” (if you actually do have something else in your folder that starts with “Desk”, this example will not work).
As another example type (but do not hit Return)
cd D
and hit tab again. There are at least two things in your home directory that start with D: Desktop
and Documents. MacOS does not know which one to complete to, so it lists the possibilities for you.
Tab autocompletion only works when the Terminal has enough information to uniquely pick one option
from the current folder. Try doing this again with
cd De
What happens?
Suppose you are currently in the your home directory and you want to move to the folder
“iTunes” which is inside of “Music”. You could do this with two cd commands. But to
do it with a single command, you just connect the folders with a /, as follows:
cd Music/iTunes
When you combine this with .., you can do some rather clever tricks. Suppose you are
currently in the Desktop directory, and you want to move in the Documents directory
(which is contained in your home directory). You can do this with the command
cd ../Documents
We refer to these expressions as paths. They are are a “path” from the working directory to the directory that you want to go to.
The paths that we have shown you are more properly called relative paths. They show how to get from the working directory to your new directory. The correct path to use depends on exactly which directory is the current working directory.
Absolute paths are paths that do not depend on the working directory. In MacOS (and
all Unix systems), absolute paths start with a /. This / represents
the root directory that contains everything else. For example, if you wanted
to go to your Applications directory (which is just inside the root directory), you
would type
cd /Applications
Absolute paths are very important when you are trying to navigate to a different disk drive.
In MacOS, when you plug in a new disk drive it is added to the /Volumes folder
(note the / indicating that Volumes is just inside the root folder). Suppose you
have a Kingston USB drive from the Campus store named KINGSTON. To view
the contents of this drive in the terminal, type
cd /Volumes/KINGSTON
To drive home the difference between relative and absolute paths, create a folder called
“Applications” in your home directory. Make sure the terminal is in the home directory (go
home by typing cd by itself) and type
cd Applications
Look at the contents with ls. Now go back to the home directory again and type
cd /Applications
Look at the contents with ls. See the difference?
The Terminal breaks up the commands that you type in by spaces. That means that if you have a
folder with spaces in the name, it will break it up into references to two different folders.
For example, suppose you have a folder called “Python Examples”, and you type
cd Python Examples
You will (likely) get an error saying that the folder “Python” does not exist.
To solve the problem, put the directory in quotes. The following should work correctly.
cd "Python Examples"
If you are changing multiple directories then you need to put the entire path in quotes (not just the folder). For example, if “Python Examples” were on the Desktop, you would type
cd "Desktop/Program Files"
Alternatively, you can represent a space using the escape character \
which we talked about in class. For example, the following should also work correctly:
cd Python\ Examples
If you use Tab Completion a lot, you will notice that this is the prefered way of handling spaces.
If you take a folder and drag and drop it onto the Terminal, it will fill the window with the absolute pathname of that folder. Therefore, to quickly move the Terminal to a a specific folder, do the following:
CS 1110cd followed by a space.This trick works on Windows and Linux as well. However, MacOS has an even faster trick that is unique to its operating system. Simply take the folder icon and drop it onto the Terminal icon (in your Dock), and it will open a new Terminal window with that folder as its working directory.
The Terminal allows you to do everything that Finder can do (and more). You can use the Terminal to make folders, move files, and delete files. However, none of this is necessary for you to learn. For this class, you never need to understand how to do anything other than navigate directories. You can do everything else in the Finder (or some other program) if you wish.
To make a new folder or directory, use the command mkdir followed by the name of the
new folder. For example, type:
mkdir MyFolder
The new folder will appear in the current working directory.
You can also delete a directory with the rmdir command. For example, to delete the
folder you just made, type
rmdir MyFolder
The Terminal will only delete empty directories. If there is anything in a directory, it will not let you delete it. You have to delete the contents first.
You move files with the mv command. The way this command works is that you give it
two file names. It searches for a file with the first file name; once it finds it, it
makes a copy with the new file name and then deletes the original.
For example, suppose you wanted to rename the file test.py to
assignment3.py. Then you would type
mv test.py assignment3.py
(this by the way, illustrates why paths cannot have spaces in them).
If the second filename is path to a file, then it will move the the file into the correct
directory. For example, suppose you now wanted to move assignment3.py to the
Desktop (which is a folder in the current working directory), and rename it
completed.py. Then you would type
mv assignment3.py Desktop/completed.py
If you want to keep the name as assignment3.py, you could shorten this to
mv assignment3.py Desktop
In this case, the Terminal will move assignment3.py into Desktop, but keep the
name of the file unchanged.
The mv command will always delete the original (name of) the file when it is done.
Sometimes you want to make a copy of a file. We do that with the cp command. Suppose
that assignment3.py is in the working directory and we want to put a copy on the
Desktop without deleting the original. Then you would type
cp assignment3.py Desktop/assignment3.py
Files are deleted with the rm command. In our running example, to delete the file
assignment3.py, you would type
rm assignment3.py
Be very careful with this command. It completely erases the file. It does not move the file your Trash. You cannot retrieve a file deleted this way.