Introduction to the Command-Line

by Elizabeth Patitsas, University of Toronto, December 2012

Contents

A command-line interface is a way of interacting with a computer through textual commands, rather than using the point-and-click graphical approach you are probably used to. Command-line interfaces are actually much older than graphical approaches, and despite the higher learning curve, are much more powerful. You can do more things on the command-line, you can automate them, you can be more precise, and you receive more precise feedback from the computer in response.

In CS 190, we'll be using command-line interfaces throughout the term; this document serves as a beginner's guide. As your code is expected to run on the ECF machines, we will begin with how to access the ECF machines remotely.

Before starting, ensure that your ECF account works by logging in here.

1. Remote Access to the ECF Machines

All of the code you will be asked to write for your labs/assignments is expected to compile on the ECF machines. Because C is a platform-dependent language, it is often the case that C code written on your personal computer will not behave the same as it would on the ECF machines.

Fortunately, you can access the ECF machines remotely, through a protocol known as ssh (or "secure shell"). To use ssh, however, you will need a Unix terminal available, so that is where we'll begin.

Depending on your operating system, this will either be straightforward (Linux/Mac), or a take a bit of time (Windows).

1.1. Linux

Linux is a great system for programming on, and probably the best for coding C on. Our first step will be to open up a terminal. If you've never done this before, hop on over to Starting a Terminal on Ubuntu.

Once you have it open, it's time to roll. At the command prompt, you want to type 

ssh -X USERNAME@remote.ecf.utoronto.ca

Where USERNAME is your ECF account. It will prompt you for your password; enter it and hit enter. (It will not display *s to show the letters of your password -- this is a security feature so an onlooker can't see how long your password is.)



Once logged in, you are now talking directly to an ECF machine. One thing to note is the -X in the command I showed you above -- this is optional. What it does is it allows graphical interfaces to be remotely opened on your computer.

Next, head over to Section 2 for a tutorial on using the ECF machines through the command line.

1.2. Macintosh

Getting a terminal open in Macintosh is fairly straightforward. If you've never done it before, the Terminal application is located in the Utilities folder within Applications.

A little command-line prompt will appear when you open up the terminal. In there, you want to type 

ssh -X USERNAME@remote.ecf.utoronto.ca

Where USERNAME is your ECF account. It will prompt you for your password; enter it and hit enter. (It will not display *s to show the letters of your password -- this is a security feature so an onlooker can't see how long your password is.)

Once logged in, you are now talking directly to an ECF machine. One thing to note is the -X in the command I showed you above -- this is optional. What it does is it allows graphical interfaces to be remotely opened on your computer.

Next, head over to Section 2 for a tutorial on using the ECF machines through the command line.

1.3. Windows

There are numerous ways to set up Unix utilities such as a terminal -- but perhaps the most robust way is to install a virtual Linux distribution on your computer. This requires creating a virtual machine on your computer.

A virtual machine is a simulation of your own machine's hardware, and this simulation runs on your computer. So, when we install Linux on a virtual machine on your computer, Windows continues to be the system operating on your actual machine. The Linux installation is running on a simulation of your computer.

To set this up, you will need to download two things: VMPlayer, which is free software that creates and runs virtual machines for you; and a Linux distribution for you to install on your virtual machine. I recommend Ubuntu for those new to Linux.

To get your virtual Linux installation working, there is a step-by-step guide available here.

Once you have your virtual Linux machine working, head over to Section 1.1 and follow the information there.

2. Basic Commands

Now that you're logged into the ECF machine, there are a number of things you can ask the machine to do for you.

2.1. Files, Directories, and Programs

Let's start with typing:

ls

You'll get something that looks like:

Desktop  ECF_BACKUP_DIRS

This is a list of all the files and directories within the directory that you are currently in. You can move around between directories with the command cd (change directory); try entering

cd Desktop
ls

You'll now see the list of files and directories on your Desktop. To go back to your home directory, which is where we started, we need to go up one level in your filesystem. To do so, type:

cd ../

The ../ refers to the directory one level higher than you currently are in.

We can make our own directories. Type:

mkdir cs190

This will create a directory named "cs190". Now type:

cd cs190
ls

To see the contents of your cs190 folder; you'll see nothing gets displayed! There are currently no files in this directory. Let's make one now by opening a text editor; type:

gedit hello_world.py &

What this does is it opens a graphical file editor called gedit, to edit a new file called "hello_world.py". The & symbol tells the computer to run this process in the background -- without it, we cannot run other commands on the terminal.

When the editor starts up, type

print "hello world"

and save the file. Close gedit.

On your terminal, you can now run this Python programme with the following command:

python hello_world.py

It will display "hello world" on your terminal.

You can do a number of things to files on the command-line; for example:

• rm hello_world.py -- this will remove the file
• cp hello_world.py hello_copy.py -- this copies the file, giving it the new name "hello_copy.py"
• you can also use directory paths here; if you want to copy this file to the Desktop, then you would type: cp hello_world.py ../Desktop/hello_world.py
• mv hello_world ../Desktop/hello_world.py will move the file to the Desktop
  

Any program you can run on your computer can be opened through the terminal. For example, to run Matlab, type:

matlab

And Matlab will boot up for you. This can be quite slow! To open Matlab's text-based interface, you can instead type:

matlab -nojvm

Te text-based interface-looks like this; to make a variable a and set it to zero, you enter it like:

                            < M A T L A B (R) >
                  Copyright 1984-2010 The MathWorks, Inc.
                Version 7.10.0.499 (R2010a) 32-bit (glnx86)
                              February 5, 2010

    ----------------------------------------------------------
        Your MATLAB license will expire in 44 days.
        Please contact your system administrator or
        The MathWorks to renew this license.
    ----------------------------------------------------------
 
  To get started, type one of these: helpwin, helpdesk, or demo.
  For product information, visit www.mathworks.com.
 
>>
>> a = 0

a =

     0
>> quit()

The quit() command closes Matlab and returns you to the command-line.

2.1. GCC

GCC is the GNU C compiler, which we'll be using in this course. Type now:

gedit hello_world.c &

And paste into this new file:

#include <stdio.h>

int main()
{
    printf("Hello, World!");
}

Save the file. We can now compile it at the command line like so:

gcc hello_world.c

Which creates a binary file named "a.out" which, when executed, will run our code. To execute the file, type:

./a.out

Note the "./" in front of the name of the binary file -- this is how you execute a binary file in Unix! The program will then print "Hello, World!" for us.

Unix has many great features available to you -- we could spend an entire course learning all of its command-line utilities! But for now, we will only look at one more: the time utility.

Adding "time" before any command will tell your computer to time how long it takes to execute the command. For example:

[patitsas@remote ~/cs190]$ time ./a.out
Hello, World!
0.000u 0.001s 0:00.00 0.0%    0+0k 0+0io 0pf+0w

This can look a bit cryptic since there's a lot of information here. It begins by telling us how long the CPU spent on this command.

The time utility shows us two things: "user time" and "system time". Whenever you execute a program, some time will be spent setting up the process, sending it to the CPU, getting the output, and so on. That is known as "system time" -- if you remove that, you have "user time" -- the time that was spent actually running the program.

In this case, we spent less than 0.000 seconds setting up the program, and 0.001 seconds actually running it. (To learn more about the time command, type "man time" to open up the manual on the time utility.)

Python will be slower since it has to load an intepreter. To compare:

[patitsas@remote ~/cs190]$ time python hello_world.py
hello world!
0.006u 0.004s 0:00.01 0.0%    0+0k 0+0io 0pf+0w

Here it took 0.006 seconds to get Python running and to send the code over to the CPU -- and then the CPU spent 0.004 seconds chugging away on it. This seems pretty fast -- faster than human perception can notice -- but when you have large programs, you'll see that C performs a lot faster than Python!

3. Transferring Files

So now we have some files on the ECF machines -- how can we transfer them to our own computers?

To do so, download Filezilla. (On Ubuntu you can open a terminal and type "sudo apt-get install filezilla.)

Once you have it running, enter the following details:

Host: ecf.toronto.edu
Username: your username
Password: your password
Port: 22

and then click "Quick Connect". Once connected, it will show you a listing of your files and directories on the ECF machines; you can now drag and drop files between the two computers.