Lecture1–Course Overview + The Shell

Motivation

As computer scientists, we know that computers are great at aiding in repetitive tasks. However, far too often, we forget that this applies just as much to our use of the computer as it does to the computations we want our programs to perform. We have a vast range of tools available at our fingertips that enable us to be more productive and solve more complex problems when working on any computer-related problem. Yet many of us utilize only a small fraction of those tools; we only know enough magical incantations(咒语) by rote(死记硬背) to get by, and blindly copy-paste commands from the internet when we get stuck.

This class is an attempt to address this.

We want to teach you how to make the most of the tools you know, show you new tools to add to your toolbox, and hopefully instill in you some excitement for exploring (and perhaps building) more tools on your own. This is what we believe to be the missing semester from most Computer Science curricula(课程).

Class structure

The class consists of 11 1-hour lectures, each one centering on a particular topic. The lectures are largely independent, though as the semester goes on we will presume that you are familiar with the content from the earlier lectures. We have lecture notes online, but there will be a lot of content covered in class (e.g. in the form of demos) that may not be in the notes. We will be recording lectures and posting the recordings online.

We are trying to cover a lot of ground over the course of just 11 1-hour lectures, so the lectures are fairly dense. To allow you some time to get familiar with the content at your own pace, each lecture includes a set of exercises that guide you through the lecture’s key points. After each lecture, we are hosting office hours where we will be present to help answer any questions you might have. If you are attending the class online, you can send us questions at missing-semester@mit.edu.

Due to the limited time we have, we won’t be able to cover all the tools in the same level of detail a full-scale class might. Where possible, we will try to point you towards resources for digging further into a tool or topic, but if something particularly strikes your fancy, don’t hesitate to reach out to us and ask for pointers!

Topic 1: The Shell

What is the shell?

Computers these days have a variety of interfaces for giving them commands; fancyful graphical user interfaces, voice interfaces, and even AR/VR are everywhere. These are great for 80% of use-cases, but they are often fundamentally restricted in what they allow you to do — you cannot press a button that isn’t there or give a voice command that hasn’t been programmed. To take full advantage of the tools your computer provides, we have to go old-school and drop down to a textual(文本的) interface: The Shell.

Nearly all platforms you can get your hand on has a shell in one form or another, and many of them have several shells for you to choose from. While they may vary in the details, at their core they are all roughly the same: they allow you to run programs, give them input, and inspect their output in a semi-structured way.

In this lecture, we will focus on the Bourne Again SHell, or “bash” for short. This is one of the most widely used shells, and its syntax is similar to what you will see in many other shells. To open a shell prompt(提示符) (where you can type commands), you first need a terminal. Your device probably shipped with one installed, or you can install one fairly easily.

Using the shell

When you launch your terminal, you will see a prompt that often looks a little like this:

missing:~

This is the main textual interface to the shell. It tells you that you are on the machine missing and that your “current working directory”, or where you currently are, is ~ (short for “home”). The $ tells you that you are not the root user (more on that later). At this prompt you can type a command, which will then be interpreted by the shell. The most basic command is to execute a program:

1
2
3

missing:~$ date
Fri 10 Jan 2020 11:49:31 AM EST
missing:~$

Here, we executed the date program, which (perhaps unsurprisingly) prints the current date and time. The shell then asks us for another command to execute. We can also execute a command with arguments:

1 2	missing:~$ echo hello hello

In this case, we told the shell to execute the program echo with the argument hello. The echo program simply prints out its arguments. The shell parses(分析句法) the command by splitting it by whitespace, and then runs the program indicated by the first word, supplying each subsequent word as an argument that the program can access. If you want to provide an argument that contains spaces or other special characters (e.g., a directory named “My Photos”), you can either quote the argument with ' or " ("My Photos"), or escape just the relevant characters with \ (My\ Photos).

But how does the shell know how to find the date or echo programs? Well, the shell is a programming environment, just like Python or Ruby, and so it has variables, conditionals, loops, and functions (next lecture!). When you run commands in your shell, you are really writing a small bit of code that your shell interprets. If the shell is asked to execute a command that doesn’t match one of its programming keywords, it consults an environment variable called $PATH that lists which directories the shell should search for programs when it is given a command:

missing:~$ echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
missing:~$ which echo
/bin/echo
missing:~$ /bin/echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

When we run the echo command, the shell sees that it should execute the program echo, and then searches through the :-separated list of directories in $PATH for a file by that name. When it finds it, it runs it (assuming the file is executable; more on that later). We can find out which file is executed for a given program name using the which program. We can also bypass $PATH entirely by giving the path to the file we want to execute.

Navigating in the shell

A path on the shell is a delimited(界定) list of directories; separated by / on Linux and macOS and \ on Windows. On Linux and macOS, the path / is the “root” of the file system, under which all directories and files lie, whereas on Windows there is one root for each disk partition (e.g., C:\). We will generally assume that you are using a Linux filesystem in this class. A path that starts with / is called an absolute path. Any other path is a relative path. Relative paths are relative to the current working directory, which we can see with the pwd command and change with the cd command. In a path, . refers to the current directory, and .. to its parent directory:

missing:~$ pwd
/home/missing
missing:~$ cd /home
missing:/home$ pwd
/home
missing:/home$ cd ..
missing:/$ pwd
/
missing:/$ cd ./home
missing:/home$ pwd
/home
missing:/home$ cd missing
missing:~$ pwd
/home/missing
missing:~$ ../../bin/echo hello
hello

Notice that our shell prompt kept us informed about what our current working directory was. You can configure your prompt to show you all sorts of useful information, which we will cover in a later lecture.

In general, when we run a program, it will operate in the current directory unless we tell it otherwise. For example, it will usually search for files there, and create new files there if it needs to.

To see what lives in a given directory, we use the ls command:

missing:~$ ls
missing:~$ cd ..
missing:/home$ ls
missing
missing:/home$ cd ..
missing:/$ ls
bin
boot
dev
etc
home
...

Unless a directory is given as its first argument, ls will print the contents of the current directory. Most commands accept flags and options (flags with values) that start with - to modify their behavior. Usually, running a program with the -h or --help flag (/? on Windows) will print some help text that tells you what flags and options are available. For example, ls --help tells us:

1	-l use a long listing format

1 2	missing:~$ ls -l /home drwxr-xr-x 1 missing users 4096 Jun 15 2019 missing

This gives us a bunch more information about each file or directory present. First, the d at the beginning of the line tells us that missing is a directory. Then follow three groups of three characters (rwx). These indicate what permissions the owner of the file (missing), the owning group (users), and everyone else respectively have on the relevant item. A - indicates that the given principal does not have the given permission. Above, only the owner is allowed to modify (w) the missing directory (i.e., add/remove files in it). To enter a directory, a user must have “search” (represented by “execute”: x) permissions on that directory (and its parents). To list its contents, a user must have read (r) permissions on that directory. For files, the permissions are as you would expect. Notice that nearly all the files in /bin have the x permission set for the last group, “everyone else”, so that anyone can execute those programs.

Some other handy programs to know about at this point are mv (to rename/move a file), cp (to copy a file), and mkdir (to make a new directory).

If you ever want more information about a program’s arguments, inputs, outputs, or how it works in general, give the man program a try. It takes as an argument the name of a program, and shows you its manual page. Press q to exit.

1	missing:~$ man ls

Connecting programs

In the shell, programs have two primary “streams” associated with them: their input stream and their output stream. When the program tries to read input, it reads from the input stream, and when it prints something, it prints to its output stream. Normally, a program’s input and output are both your terminal. That is, your keyboard as input and your screen as output. However, we can also rewire those streams!

The simplest form of redirection is < file and > file. These let you rewire the input and output streams of a program to a file respectively:

missing:~$ echo hello > hello.txt
missing:~$ cat hello.txt
hello
missing:~$ cat < hello.txt
hello
missing:~$ cat < hello.txt > hello2.txt
missing:~$ cat hello2.txt
hello

You can also use >> to append to a file. Where this kind of input/output redirection really shines is in the use of pipes. The | operator lets you “chain” programs such that the output of one is the input of another:

missing:~$ ls -l / | tail -n1
drwxr-xr-x 1 root  root  4096 Jun 20  2019 var
missing:~$ curl --head --silent google.com | grep --ignore-case content-length | cut --delimiter=' ' -f2
219

We will go into a lot more detail about how to take advantage of pipes in the lecture on data wrangling.

A versatile and powerful tool

On most Unix-like systems, one user is special: the “root” user. You may have seen it in the file listings above. The root user is above (almost) all access restrictions, and can create, read, update, and delete any file in the system. You will not usually log into your system as the root user though, since it’s too easy to accidentally break something. Instead, you will be using the sudo command. As its name implies, it lets you “do” something “as su” (short for “super user”, or “root”). When you get permission denied errors, it is usually because you need to do something as root. Though make sure you first double-check that you really wanted to do it that way!

One thing you need to be root in order to do is writing to the sysfs file system mounted under /sys. sysfs exposes a number of kernel parameters as files, so that you can easily reconfigure the kernel on the fly without specialized tools. Note that sysfs does not exist on Windows or macOS.

For example, the brightness of your laptop’s screen is exposed through a file called brightness under

1	/sys/class/backlight

By writing a value into that file, we can change the screen brightness. Your first instinct(本能) might be to do something like:

$ sudo find -L /sys/class/backlight -maxdepth 2 -name '*brightness*'
/sys/class/backlight/thinkpad_screen/brightness
$ cd /sys/class/backlight/thinkpad_screen
$ sudo echo 3 > brightness
An error occurred while redirecting file 'brightness'
open: Permission denied

This error may come as a surprise. After all, we ran the command with sudo! This is an important thing to know about the shell. Operations like |, >, and < are done by the shell, not by the individual program. echo and friends do not “know” about |. They just read from their input and write to their output, whatever it may be. In the case above, the shell (which is authenticated just as your user) tries to open the brightness file for writing, before setting that as sudo echo’s output, but is prevented from doing so since the shell does not run as root. Using this knowledge, we can work around this:

1	$ echo 3 \| sudo tee brightness

Since the tee program is the one to open the /sys file for writing, and it is running as root, the permissions all work out. You can control all sorts of fun and useful things through /sys, such as the state of various system LEDs (your path might be different):

1	$ echo 1 \| sudo tee /sys/class/leds/input6::scrolllock/brightness

Next steps

At this point you know your way around a shell enough to accomplish basic tasks. You should be able to navigate around to find files of interest and use the basic functionality of most programs. In the next lecture, we will talk about how to perform and automate more complex tasks using the shell and the many handy command-line programs out there.

Exercises

Create a new directory called missing under /tmp.
Look up the touch program. The man program is your friend.
Use touch to create a new file called semester in missing.

Write the following into that file, one line at a time:

1 2	#!/bin/sh curl --head --silent https://missing.csail.mit.edu

The first line might be tricky to get working. It’s helpful to know that # starts a comment in Bash, and ! has a special meaning even within double-quoted (") strings. Bash treats single-quoted strings (') differently: they will do the trick in this case. See the Bash quoting manual page for more information.

Try to execute the file, i.e. type the path to the script (./semester) into your shell and press enter. Understand why it doesn’t work by consulting the output of ls (hint: look at the permission bits of the file).
Run the command by explicitly starting the sh interpreter, and giving it the file semester as the first argument, i.e. sh semester. Why does this work, while ./semester didn’t?
Look up the chmod program (e.g. use man chmod).
Use chmod to make it possible to run the command ./semester rather than having to type sh semester. How does your shell know that the file is supposed to be interpreted using sh? See this page on the shebang line for more information.
Use | and > to write the “last modified” date output by semester into a file called last-modified.txt in your home directory.
Write a command that reads out your laptop battery’s power level or your desktop machine’s CPU temperature from /sys. Note: if you’re a macOS user, your OS doesn’t have sysfs, so you can skip this exercise.

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The Missing Semester of Your CS Education
Lecture1 Course Overview+The Shell