Lecture 1: Course Overview + The Shell (2020)

all right everyone thanks for coming in.

This is the missing semester of your CS Education;

at least that's what we chose to call

the class. if you're not here for this

class then you're in the wrong room. We

will be here for about an hour just to

set your expectations and I want to talk

to you a little bit first about why

we're doing this class. So this class

stems out of an observation that Anish

and Jose and I have made while TA-ing various

classes at MIT which is that basically

all of us computer scientists, we know

that computers are great at doing these

repetitive tasks and automating things

but we often fail to realize that there

are lots of tools that can make our own

development processes better. We can be a

lot more efficient about how we use our

computers because we can use the

computer as a tool for ourselves not

just for building websites or software

those sorts of things and this class is

an attempt to address - this is an attempt

to show you some of the tools that you

can use to great effect in your day to

day in your research and in your studies

and it's going to be a class where we

want you to teach you both how to make

the most of the tools that you already

know but also hopefully teach you about

some tools so you don't know from before

and how to combine those tools to

produce more powerful things than you

think you might be able to do with what

you know today the class is going to be

structured as a series of 11 one-hour

lectures and each one is going to cover

a particular topic you can see the

website which is also listed there for

the list of lecture topics and what date

will do each one they will mostly be

independent and so you can sort of show

up for the ones that you're interested

in but we will sort of assume that

you've been following along so that as

we get to later lectures I'm not going

to be teaching you bash all over again

for example we we are also going to post

both the lecture notes and recordings of

the lectures online exactly when we do

that we haven't established yet but it

will be after the lectures obviously the

videos have to be posted after

the class is gonna be run by me John and

Anne each sitting over there and Jose

who is not currently here but we'll be

holding tomorrow's lecture and keep in

mind that we're trying to cover a lot of

ground over the course of just 11

one-hour lectures and so we will be

moving relatively rapidly but please do

stop us if there's anything where you

feel like you're not following along if

you feel like there's something you wish

we would spend more time on just let us

know please interrupt us with questions

and also after each lecture we're going

to hold office hours on the ninth floor

of building 30 to the Stata Center of

the computer science building as if you

show up up in the ninth floor lounge

there and the gates tower then you can

come and try some of the exercises that

we give for each lecture or just ask us

other questions about things we've

talked about in lecturer or other things

about using your computer efficiently

due to the limited time that we have

available we're not going to be able to

cover all tools and full detail and so

we'll try to do is highlight interesting

tools and interesting ways to use them

we won't necessarily dig into the deep

details about how all of it works or

more elaborate use cases but if you have

questions about them please come ask us

about that too many of these tools are

tools that we have used for years and we

might be able to point you to additional

interesting things you can do with them

sort of like take advantage of the fact

that we're here this class is going to I

don't wanna say ramped up quickly but

what's going to happen over the course

of this particular lecture is that we'll

cover many of the basics that we assume

that you will know for the rest of the

semester things like how to use your

shell and your terminal and I'll explain

what those are - those who you're not

familiar with them and then will pretty

quickly ramp up into more advanced tools

and how to use them you can already see

from the lecture notes the kind of

topics that we're going to be covering

and so that brings us to today's lecture

in which we are going to cover the shell

and the shell is going to be one of the

primary ways that you interact with your

computer once you want to do more things

than what the sort of visual interfaces

you might be used to allow you to do the

visual interfaces are sort of limited in

what

they allow you to do because you can

only do the things that there are

buttons for sliders for input fields for

often these textual tools are built to

be both composable with one another but

also to have tons of different ways to

combine them or ways to program and

automate them and that is why in this

class we will be focusing on these

command line or text-based tools and the

shell is the place that you would do

most of this work so for those of you

are not familiar with the shell most

platform provides some kind of shell on

Windows this is often PowerShell but

there are also other shells available on

Windows on Linux you will find tons of

terminals these are windows that allow

you to display shells and you'll also

find many different types of shells the

most common of which is bash or the born

again shell because it's such a common

shell it is the one we're primarily

going to be covering in these lectures

if you're on Mac OS you will probably

also have bash maybe an older version of

it if you open the terminal app and so

if you want to follow along on any of

these platforms feel free but keep in

mind that most of this is going to be

sort of Linux centric in terms of how we

teach it even though most of these tools

work on all the platforms if you want to

install a terminal and a shell and you

don't know how to do it well we're happy

to show you at office hours or it's

usually very easy to just Google like

your platform plus like terminal and you

will get one now when you open a

terminal you get something that looks a

little bit like this so it will usually

have just a single line at the top and

this is what's known as the shell prompt

you can see that my shell prompt looks

like this it has my user name the name

of the machine that I'm on the current

path I'm on and we will talk about paths

a little bit later and then it's really

just sort of blinking they're asking me

for input and this is the shell prompt

where you tell the shell what you want

it to do and you can customize this

prompt a lot and when you open it on

your machine it might not look exactly

like this it might look something like

this if you've configured it a little or

it might look all sorts of different

ways we won't go too much into

customizing your shell in this

particularly

we'll do that later here we're just

going to talk about how do you use this

shell to do useful things and this is

our the main textual interface you have

to your computer's through this shell on

the shell prompt you get to write

commands and commands can be relatively

straightforward things usually it'll be

something like executing programs with

arguments what does that look like well

one program we can execute is the date

program we just type date and press

enter and then it will show you

unsurprisingly the date and time you can

also execute a program with arguments

this is one way to modify the behavior

of the program so for example there is a

program called echo and echo just prints

out the arguments that you give it and

arguments are just white space separated

things that follow the program name so

we can say hello and then it will print

hello back

perhaps not terribly surprising but this

is the very basics of arguments one

thing that you'll notice is that I said

that arguments are separated by

whitespace and you might wonder well

what if I want an argument as multiple

words you can also quote things so you

can do things like echo hello space

world and now the echo program receives

one argument that contains the string

hello world with a space well you can

also use single quotes for this and the

difference between single quotes and

double quotes will get back to and when

we talk about bash scripting you can

also just escape single characters so

for example hello world this will also

work just fine all of these rules about

how you escape and how you parse and

quote various arguments and variables

we'll cover a little bit later hopefully

you won't run into too many we oughta

tease about this just keep in mind at

least that spaces separate arguments so

if you want to do something like make a

directory called my photos

you can't just type like make directory

my photos it will create two directories

one called my and one called photos and

that is probably not what you want now

one thing you might ask is how does the

shell know what these programs are when

I type date or when I type echo how does

it know what these programs are supposed

to do and the answer to this is your

program your

computer has a bunch of built-in

programs that comes with the machine

just like you your machine my chip with

like the terminal app or it might chip

with like Windows Explorer or at my chip

with some kind of browser it also ships

with a bunch of terminal centric

applications and these are stored on

your file system and your shell has a

way to determine where a program is

located basically has a way to search

for programs it does this through

something called an invariant

environment variable an environment

variable is a variable like you might be

used to for programming languages it

turns out that the shell and the

bourne-again shell in particular is

really a programming language this

prompt that you're given here is not

just able to run a program with

arguments. You can also do things like

while loops, for loops, conditionals...

All of these - you can define

functions, you can have variables, and all

of these things you can do in the shell.

We'll cover a lot of that in the next

lecture, on shell scripting. For now,

though, let's just look at this

particular environment variable.

Environment variables are things that

are set whenever you start your shell

they're not things you have to set every

time you run your shell there are a

bunch of these that are set things like

where is your home directory what is

your username and there's also one

that's critical for this particular

purpose which is the path variable so if

I echo out dollar path this is going to

show me all of the paths on my machine

that the shell will search for programs

you'll notice that this is a list that

is colon separated it might be kind of

long and hard to read but the essentials

is that - whenever you type the name of

a program is gonna search through this

list of paths on your machine and it's

gonna look in each directory for a

program or a file whose name matches the

command you try to run so in my case

when I try to run date or echo it's

gonna walk through these one at a time

until it finds one that contains the

program called date or echo and then

it's gonna run it if we want to know

which one it actually runs there's a

command called which which lets us do

that so I can type which echo and will

tell me that if I were to run a program

called echo I would run this one it's

worth pausing here to talk about what

paths are so paths are a way to name the

location of a file on your computer on

Linux and Oh Mac OS these paths are

separated by slashes forward slashes so

you'll see here that this is in the in

the root directory so the slash at the

very beginning indicates that this is

starting from the top of the file system

then look inside the directory called

USR then look inside the directory bin

and then look for the file called echo

on windows paths like this are usually

separated by back slashes instead and

we're on Linux and Mac OS everything

lives under the root name space so all

paths start with a slash or all absolute

paths on Windows there is one root for

every partition so you might have seen

things like C colon backslash or D colon

backslash so Windows has separate sort

of file system path hierarchies for each

each Drive that you have

whereas on Linux and Mac OS these are

all mounted under one namespace you'll

notice that I said the word absolute

path and you might not know what that

means so absolute paths are paths that

fully determine the location of a file

so in this case this is saying this is

talking only about a specific echo file

and it's giving you the the full path to

that file but there are also things

known as relative paths so a relative

path is relative to where you currently

are and so the way we find out where we

currently are is you can type PWD for

present working directory present print

working directory so if I type PWD it

will print out the current path that I'm

in right so currently I'm in the home

directory under the root and then John

under that and then dev under that etc

from here I can then choose to change my

current working directory and all

relative paths are relative to the

current working directory which is

basically where you

they are in this case for example I can

do CD / home CDs change directory this

is the way that I change what my current

working directory is in this case I

change to home and I am now you'll see

my my shell prompt change to say that I

am now in home it just gives me the name

of the last segment of the path but you

can also configure your terminal to give

you the full path whenever you're

anywhere and now if I type PWD again it

will tell me I'm in slash home there are

also a couple of special directories

that exist there is dot and dot dot dot

means the current directory dot dot

means the parent directory so this is a

way that you can easily navigate around

the system for example here if I type CD

dot dot it will tell me that I am now in

slash so I'm now in the root of the file

system I was in slash home now I'm in

slash and indeed if I type PWD well it

will do that right thing and I can also

then use relative paths to go down into

the file system right so I can do CD dot

slash home and this is gonna CD into the

home directory under the current

directory right so this will bring me

back to slash home if I now tried CD dot

slash home again it will say there's no

such directory because there is no home

directory under the current directory

I'm on which I changed by doing CD right

and I can sort of see de all the way

back to the place that I was using

relative paths and I can also do things

like dot dot dot dot dot to get back to

somewhere deep in my file system this

happens to be all the way back to the

root so here there's a bin directory and

another bin there's an echo file and so

then I could do world and that runs the

echo program under bin alright so this

is a way that you can construct paths to

arbitrarily traverse your filesystem

sometimes you want to absolute paths and

sometimes you want relative ones usually

you want to use whichever one is shorter

but if you want to for example run a

program or write

a program that runs the program like

echo or date and you want it to be able

to run be run from anywhere you either

want to just give the name of the

program like date or echo and let the the

shell use the path to figure out where

it is or you want to give its absolute

path because if you gave a relative path

then if I ran it in my home directory

and you ran it in some other directory

it might work for me but not for you in

general when we run a program it is

going to be operating on the current

working directory at least by default

unless we give it any other arguments

and this is really handy because it

means that often we don't have to give

full paths for things we can just use

the name of files and in the directory

that we're currently in one thing that's

really useful is to figure out what is

in the current directory we're in so we

already saw PWD which prints where you

currently are there's a command called

LS which will show you it will list the

files in the current directory so if i

type LS here this is all the files in

the current directory right and this is

a handy way to just quickly navigate

through the filesystem you'll see that

if I sort of CD dot and then do LS it'll

show me the files in that directory

instead but with LS I can also give it

LS dot dot like I can give it a path and

then will LS that file instead of the

one that I'm currently in or LS that

directory and you can see this if I go

all the way to the root as well right

root has different files one handy trick

you might not know about here is there

are two other special things you can do

one is the tilde character this

character brings you to your home

directory so tilde always expands to the

home directory and you can do relative

paths to it so I can do tilde slash dev

slash P DOS classes missing semester and

now I'm there because tilde expanded to

slash home slash John there is also for

CD in particular a really handy argument

you can give which is - if you do CD -

it will CD to the directory you were

previously in so if I do CD - I go back

to root

if I do CD - again I go back to missing

semester so this is a handy way if you

want to toggle between two different

directories in the case of LS or in the

case of CD there might be arguments do

you don't know about right currently we

haven't really been doing anything

except giving paths but how do you even

discover that you can give a path to LS

in the first place well most programs

take what are known as arguments like

flags and options these are things that

usually start with a - one of the Hemi

is one of these is - help most programs

implement this and if you run for

example LS - help it all helpfully print

out a bunch of information about that

command and you'll see here that it says

the usage is LS and you can give some

number of options and you can give some

number of files the way to read that

usage line is triple dot means one like

zero or one or more and the square

bracket means optional so in this case

there's an optional number of options

and there's an optional number of files

and you'll see that it says what the

program does and also specifies a number

of different types of flags and options

you can give usually we call things that

are a single dash in a single letter a

flag and anything that or anything that

doesn't take a value a flag and anything

that does take a value an option so for

example - a and - all are both flags and

- C or - color R is an option one thing

you'll see under here if you scroll down

far enough is the - L flag and that's

unhelpful the - L flag the - L flag uses

a long listing format now that's

particularly helpful in and of itself

but let's see what it actually does so

if I do LS dash L it still prints the

files in the current directory but it

gives me a lot more information about

those files and this is something you'll

find yourself using quite a lot because

the additional information it gives you

is often quite handy let's look at what

some of that information

ah so first of all the D at the

beginning of some of these entries

indicate that something is a directory

so the underscore data entry here for

example is a directory

whereas for for hTML is not a directory

it's a file the following letters after

that indicate the permissions that are

set for that file so this like we saw

earlier I might not be able to open a

given file or I might not be able to CD

into a given directory and this is all

dictated by the permissions on that

particular file or directory the way to

read these is that the the first group

of three are the permissions are set for

the owner of the file all of these files

you'll see are owned by me the second

group of three characters is for the

permissions for the group that owns this

file in this case all of these files are

also known by the john group and a final

group of three is a list of the

permissions for everyone else so anyone

who's not a user owner or a group owner

this directory is perhaps kind of boring

because all of the things are owned by

me but if we do something like CD to

slash and do LS dash L you'll see that

here all of them are owned by root we'll

get back to what the root user is but

here you see some of the permissions are

a little bit more interesting the groups

of three are read write and execute what

these mean differs for files and for

directories for files it's pretty

straightforward if you have read

permissions on a file then you can read

its contents if you have write

permissions on a file then you can save

the file you can add more to it or you

can replace it entirely and if you have

execute to the X bit on a file then

you're allowed to execute that file so

if we do LS al in slash bin that's a

novel and user bin you'll see that all

of them have the execute bit set even

for people who are not the owner of the

file and this is because the echo

program for example we want everyone on

the computer to be able to execute

there's no reason to say only certain

users can run echo that doesn't really

make any make any sense

for directories though these permissions

are a little bit different so read

translates - are you allowed to see

which files are inside this directory so

think of read as lists for a directory

are you allowed to list its contents

write for a directory is whether you are

allowed to rename create or remove files

within that directory so it's still kind

of right but notice that this means that

if you have write permissions on a file

but you do not have write permissions on

its directory you cannot delete the file

you can empty it but you cannot delete

it because that would require writing to

the directory itself and finally execute

on directories is something that trips

people up a lot execute on a directory

is what's known as search and that's not

terribly helpful a name but what that

means is are you allowed to enter this

directory if you want to get to a file

if you want to open it or read it or

write it whatever you want to do

basically to CD into a directory you

must have the execute permission on all

parent directories of that directory and

the directory itself so for example for

me to access a file inside slash user

slash bin such as user bin echo I must

have executed on route I must have

execute on user and I must have execute

on bin if I do not have all those

execute bits I will not be allowed to

access that file because I won't be able

to enter the directories along the way

there are a number of other bits that

you might come across like you might see

esses or T's in these lists you might

see LS those we can talk about in office

hours if you're curious

they will mostly not matter for anything

you will do in this class but they are

handy to know about so if you're curious

about them look them up on your own or

come ask us in office hours there are

some other programs that are handy to

know about oh sorry there's one more

thing as I mentioned if you just have a

dash it means you do not have that

permission right so if it says for

example our dash X it means that you

have read and execute but you do not

have right there are some other handy

programs to know about at this point

one of them is move or the MV command so

if I CD back to missing semester here MV

lets me rename a file and rename here

takes two paths it takes the old path in

the new path this is means that move

lets you both rename a file like if you

change the name of the file but not the

directory or it lets you move a file to

a completely different directory it just

you give the path to the current file

and the path to where you want that file

to be and that can change its location

and its name so for example I can move

dot files dot MD to be food MD

unhelpfully right and similarly I can

move it back there's also the CP command

the CP or copy is very similar it lets

you copy a file CP also takes two

arguments it takes the path you want to

copy from and the path you want to copy

to and these are full paths so I could

use this for example to say I want to

copy dot files out MD - dot dot slash

food MD sure food MD and now if I do LS

dot you'll see that there's a food MD

file in that directory so CP as well

take two paths it does not have to be in

the same directory and similarly there's

the RM command which lets you remove a

file and there - you can give paths in

this case I'm removing dot dot slash

food you should be aware for removing

especially on Linux removal is by

default not recursive so you cannot

remove a directory using RM you can pass

the - our flag which lets you do a

recursive remove and then give a path

that you want to remove and it will

remove everything below it there is also

the RM dr dir command which lets you

remove a directory but it only lets you

remove that directory if it is empty so

the idea here is to sort of be a safety

mechanism for you so you don't

accidentally throw away a bunch of your

files and the final little command

that's handy to use is make there which

lets you create a new directory and as

we talked about before you don't want to

do something like this because it will

create two directories for you one

called my and one called photos if you

actually want to create a directory like

this you would either

escape the space or quote the string if

you ever want more information about how

any command to basically on these

platforms work there's a really handy

command for that as well

there is the program called man for

manual pages this program takes as an

argument the name of another program and

gives you its manual page so for example

we could do man LS and this shows us a

manual page for LS you'll notice that in

the case of LS it is fairly similar to

what we got with LS - help but it's a

little easier to navigate a little

easier to read usually towards the

bottom you will also get examples

information about who wrote it where you

can find more information and that sort

of stuff one thing that can be confusing

sometimes

at least until a recent version where

they added this three at the bottom

which says Q to quit they do not use to

say this you press Q to quit this

program it can be really hard to quit it

if you do not know that a handy keyboard

shortcut here by the way is ctrl L which

lets you clear your terminal and go back

to the top so so far we've only talked

about programs in isolation but where

much of the power of the shell really

comes through is once you start

combining different programs right so

rather than just like running CDE

running LS and etc you might want to

chain multiple programs together you

might want to interact with files and

have files operate in between programs

and the way we can do this is using this

notion of streams that the shell gives

us every program by default has I'm

gonna simplify a little and say two

primary streams it has an input stream

and an output stream by default the

input stream is your keyboard basically

the input stream is your terminal and

whatever you type into your terminal is

going to end up into the program and it

has a default output stream which is

whenever the program prints something

it's gonna print to that stream and by

default that is also your terminal this

is why when I type echo hello it gets

printed back to my terminal but the

shell gives you a way to rewire these

streams to change where the input

output of a programmer pointed the way

the most straightforward way you do this

is using the angle bracket signs so you

can write something like this or you can

write something like this the left angle

bracket indicates rewire the input for

this program to be the contents of this

file and the end angle bracket means

rewire the output of the preceding

program into this file so let's look at

an example of what that would look like

if I do echo hello I can say I want that

context the content to be stored in a

file called hello dot text and because I

gave this is a relative path right this

will construct a file in the current

directory called hello text and at least

in theory its contents should be the

word hello so if I run this notice that

nothing got printed to my output the

previous time when I ran echo hello it

printed hello

now that hello is going gone into a file

called hello text and I can verify this

by using the program called cat so cat

prints the contents of a file so I can

do cat hello dot txt and there it shows

me hello but cat is a is also something

that supports this kind of wiring so I

can say cat which by default just prints

its input it just duplicates its input

to its output I can say I want you to

take your input from Hello text what

will happen in this case is that the

shell is going to open hello dot txt

take its contents and set that to be the

input of cat and then cat is going to

just print that to its output which

since I haven't rewired it is gonna be

my terminal so this will just print

hello to the output and I can use both

of these at the same time so for example

if I want to copy a file and I don't

want to use the CP command for some

reason I can do this and in this case

I'm telling the cat program nothing at

all I'm just saying do your normal thing

right the cat program does not know

anything about this redirection but I'm

telling the shell to use hello dot txt as

the input for cat and to write anything

that cat prints - hello - dot txt again

this prints nothing to my terminal

but if

cat hello to text I get the output as I

would have expected which is a copy of

the original file there is also a double

end bracket which is append instead of

just overwrite so you'll notice that I

if I do

cat hello dot txt - hello - dot txt

again and then I cat hello2 dot txt it

still just contains hello even though it

already contained hello if I switch that

to instead be a double end bracket it

means append and if I now cat that file

it has a hello twice these are pretty

straightforward they're usually just

ways to interact with files but where it

gets really interesting is an additional

operator the shell gives you called the

pipe character so pipe is just a

vertical bar and what pipe means is take

the output of the program to the left

and make it the input of the program to

the right right so what does this look

like well let's take the example of ls /

or ls dash l / this prints a bunch of

things let's say that I only wanted the

last line of this output well there's a

command called tail and tail prints the

last n lines of its input and I can do

-n1 so this is a flag called n you

can also use dash dash lines if you want

to use it as a longer option but in this

case this is saying just print the last

line and I can wire these together so I

can say ls dash l / pipe tail -n1

and notice here that ls does not know

about tail and tail does not know about

ls they are different programs and have

never been programmed to be compatible

with one another all they know how to do

is read from input and write to output

and then the pipe is what wires them

together and in this particular case I'm

saying I want the output of LS to be the

input to tail and then I want the output

of tail to just go to my terminal

because I haven't rewired it

I could also rewire this to say I want

the output to go to ls dot text and in this

case if I cat ls dot text I would get

the appropriate output and it turns out

you can do some really neat things with this

gonna cover this a lot more in the data

wrangling lecture there will be in like

four days or something on the kind of

fancy stuff you can do when you start

building more advanced pipelines one to

give you one example we can do something

like curl - -head --silent google.com

so just to show you what that looks

like this gives me all the HTTP headers

for accessing google.com and I can pipe

that to grep a - like a --ignore-case

or just -i if I want content

length so this is gonna print the

content length header, grep is a program

that we'll talk about later they'll let

you search in an input stream for a

given keyword we can pipe that through

say the cut command which takes a

delimiter set that to be space and I

want the second field and this prints

just the content length this is sort of

a silly example right like this just

lets you extract the content length in

bytes of google.com from the command

line it's not a very useful thing to do

but you can see how by chaining these

together you can achieve a bunch of

really interesting text manipulation

effects and it turns out pipes are not

just for textual data you can do this

for things like images as well you can

have a program that manipulates a binary

image on its input and writes a binary

image to its output and you can chain

them together in this way and we'll talk

about some of those kinds of examples

later on - you can even do this for

video if you want you can stream this is

for example a great way if you have a

chromecast at home you can stream a

video file like this by having the last

program in your pipe be a chromecast

send program so you stream a video file

into it and it streams or HTTP to your

chromecast we'll take we'll talk a lot

more about this in the data wrangling

lecture but there's one more thing that

I wanted to talk to you about about sort

of how to use the terminal and a more

interesting and perhaps more powerful

way that you might be used to and this

is perhaps even going to be interesting

for the ones of you who feel like you're

already comfortable with the term

but first we need to cover a important

topic when it comes to Linux systems and

Mac OS systems in particular which is

the notion of the root user the root

user is sort of like the administrator

user on Windows and it has user IDs zero

the root user is special because it is

allowed to do whatever it wants on your

system even if a file is like not

readable by anyone or if it's not

writable by anyone root can still access

that file root is sort of a super user

that gets to do whatever they want and

most of the time you will not be

operating as the super user you will not

be root you will be a user like John or

whatever your name is and that's going

to be the user you act with because if

you were operating your computer as the

root user at all times if you ran the

wrong program they could just completely

destroy your computer and you don't want

that right but every now and again you

want to do something that requires that

you are root usually for these cases you

will use a program called sudo su do or

do as su and su in this case is Super

User so this is a way to do the

following thing as the super user

usually the way sudo works is you write

sudo and then a command like you would

normally on your terminal and it will

just run that command as if you were

root as opposed to the user you actually

are where might you need something like

this well there is a special there are

many special file system on your

computer but in particular there's one

called sysfs if you CD to slash sys

this is a whole new world this file

system is are not actually files on your

computer instead these are various

kernel parameters so the kernel is like

basically the the core of your computer

this is a way for you to access various

kernel parameters through what looks

like a file system you'll see here that

if I CD into class for example it has

directories for a bunch of different

types of devices that I can interact

with or various queues I can access or all

sorts of weird knobs internally and

because they're exposed as files it means

we can also use all the tools have been

using so far in order to manipulate them

one example of this is if you go into

sys class backlight so this backlight

directly and lets you configure the

backlight on your laptop if you have one

so I can CD in to intel backlight this

is an Intel laptop inside here you'll

see there's a file called brightness and

I can cat the brightness this is the

current brightness of my screen but not

only that I can modify this too in order

to change the brightness of my screen so

you might think that I could do let's

see what the max brightness is here okay

so it's currently set to the max

brightness you might imagine that I

could do something like if I do echo

let's do half or something

echo 500 to brightness

if I do this it says

permission denied I'm not allowed to

modify brightness because in order to

basically in order to change things in

the kernel you need to be the

administrator and you might imagine that

the way to solve this is to write sudo

echo 500 but I still get a permission

denied error but why is that it's

because as I mentioned before these

redirections of input and output is not

something the programs know about when

we piped Elison to tail tail did not

know about LS and LS did not know about

tail the pipe and the redirection was

set up by the shell so in this case

what's happening is I'm telling my shell

run the program sudo with the arguments

echo and 500 and send its output to the

file called brightness but the shell is

what is opening the brightness file it

is not the sudo program so in this case

the shell which is running as me tries

to open the brightness file for writing

and it's not allowed to do that and

therefore I get a permission down error

you might have seen this if you like

search for something end up on Stack

Overflow and it tells just run this

command and you'll see that it does

something like they give you

instructions like one two sis what's an

example net ipv4 for word for example

this is something you may have seen if

you're setting up a firewall and this

command is intended to work

because this little pound symbol

indicates run this as root this is

something that is very rarely explained

but that is what the pound symbol means

you'll see on my prompt there's a dollar

symbol instead and the dollar indicates

you are not running as root so the

question is how do I get around this

well I could switch into a root terminal

so one way to do this is to run sudo su

sudo su is saying run the following

command s root and su is a complicated

command that effectively gets you a

shell as the super user so if I do this

type of password then now you'll see

that the username at the beginning

changed from jon to root and the prompt

changed from a dollar to a pound if I

now I come in to that file if I do echo

500 to brightness my screen got a little

dimmer but you can't see it you just

have to trust me and now I didn't get an

error and this is because the shell is

now running as root it is not running as

Jon and the root user is allowed to open

this file but given our knowledge that

we have of the terminal now there's

actually a way for us to do this without

having to drop to a root shell and that

is as follows that's I guess restore it

to 1060 so do you see why this is

different here I'm telling my shell to

run the echo 1060 command which is gonna

echo 1060 and I'm telling it to run the

sudo tee brightness command and I'm

telling you to send the output of echo

into sudo tee in order to understand this

you need to know what the tee command does

the tee command takes its input and writes

it to a file but also to standard out so

tee is a convenient way if you have say a

log file that you want to like send to a

file to store for later but you also

want to see it to yourself then you can

pipe it through tee give it the name of a

file and it will write whatever its

input is both to that file and to your

screen and here I'm taking advantage of

that program I'm saying run tee as route

and have tee right into the brightness

file and so in this case the tee program

which is what is opening the brightness

file is running as root and so it is

allowed to do

if I run this it will now again you

can't see but the brightness and I've

been turned on by a laptop and I don't

get any errors and I never had to drop

into a root shell and run commands there

which can often be somewhat dangerous if

you want to explore this filesystem a

little bit more there's a lot of

interesting stuff in here if you just

sort of start browsing around you can

find all sorts of fun things so for

example we noticed that there was a fun

brightness command here I wonder what

other kinds of brightness I can set so I

can use the find command which we will

also talk about in a coming lecture I

wouldn't look on any file whose name

it's a little like brightness in the

current directory that's unhelpful maybe

they're not files did I misspell

brightness yeah why is it being annoying

Oh apparently it does not want to search

for brightness for me how well luckily

for you I know of one already handy that

there is a subdirectory called LEDs and

LEDs have brightness too what kind of

LEDs are there ooh lots of things for

example the scroll lock led now most of

you probably don't know what the scroll

lock LED is or much less what scroll

lock is you might have seen a key on

your keyboard neighbor named scroll lock

basically no one knows what it means

anymore no one really uses it for

anything it's mostly just a dead key and

also a dead LED what if you wanted to

configure it so that every time you get

email your scroll lock LED lights up

because there's no other reason why it

would light up well if we seed you into

this particular directory that has a

brightness place and it's set to zero

well what happens if I write one into it

you probably should not just be writing

random numbers into random files in this

directory because you are affecting your

kernel directly like look up what the

files do in this particular case I have

warned safety goggles and I've done my

research so now you can't tell but on my

keyboard the scroll lock LED is now lit

so now if I wrote a program that like

did some checking of mail and stuff I

could have it at the end run a program

that echoes one into this file and now I

have a way for my led to my keyboard to

indicate when I've new email at this

point you should know roughly your way

around

the terminal around the shell and and

know enough to accomplish these basic

tasks at least in theory now you

shouldn't need to use like

point-and-click interfaces to find files

anymore

there's one remaining trick you might

need and that is the ability to open a

file so far I've only really given you

ways to find files but one thing you

should know about is missing semester

xdg-open this will probably only work

on Linux on Mac OS I think it's just

called open on Windows who knows

xdg-open you give the name of a file and

it will open it in the appropriate program

so if you open if you do xdg-open an

HTML file that will open your browser

and open that file and once you have

that program in theory you should no

longer need to open like a Finder window

ever again you might want to for other

reasons but in theory you can accomplish

at all using the tools that we've

learned today this might all seem

relatively basic for some of you but as

I mentioned this is sort of the ramp-up

period of now we all know how the shell

works and a lot of what we'll be doing

in future lectures is using this

knowledge to do really interesting

things using the shell that sort of this

is learning the the interface that we're

going to be using and so it's important

we all know it we're gonna talk a lot

more in the next lecture about how to

automate tasks like this how to write

scripts that run a bunch of programs for

you and have to do things like

conditionals and loops and stuff in your

terminal and do things like run a

program until it fails which can be

handy in classes where you want to run

something until your test suite fails

for example so that's the topic for next

week's lecture did you have a question

it's what you've been demoing this

assist directory that presumably will

only work if you're running that is a

good question I don't know whether the

windows subsystem for Linux will expose

the sis file system if it does it

probably only exposes a very small

number of things it might because there

are I don't know check it out

one thing you'll see is the lecture

notes for this lecture are already

online and at the very bottom of the

file there are a bunch of exercises some

of them are relatively easy some of them

are a little bit harder and what we

encourage you to do is to take a stab at

going through them if you know this

stuff already it should go really

quickly if you don't it might teach you

a bunch of things that you might not

realize you didn't know and for the

office hours that we're gonna do right

after this lecture we will happily help

you get through all of those or if there

are other commands and you learn in the

process you want to know how to use more

efficiently and then in next lecture

which is tomorrow will basically be

assuming that you know the kind of stuff

that the exercise is going to teach you

there's also a an email address on the

website where you can send us questions

if you think of something like after the

office hours are finished are there any

questions before we end today no no all

right well we will have office hours on

the ninth floor of the gates building of

building 32 in like five minutes

sweet see you there