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[Intro Music]

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Let's guys now talk about

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permissions. And I'll explain you

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what is permission, and how to

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modify permissions for a particular

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file or directory. And for example,

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here in this output for this file

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'3.txt', you see that

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owner of this file is 'bogdan',

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and group is 'bogdan', and

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here are permissions. First '-' means

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that here you see file. Next, three

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characters stand for permissions

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for particular user, in this

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case owner of the file, owner is

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'bogdan'. Next three characters

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stand for permissions for other

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users in the same group, in this

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case group is 'bogdan'. And

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finally, three last characters

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stand for permissions for all

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remaining users that don't belong

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to this particular group. That's

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our permissions. Great. Let's now

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quickly clean up this home

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directory for this particular user

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that we have created before, and

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let me remove those two files,

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1.txt, and 2.txt, and this

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folder 'tmp'; and I'll do that under

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'bogdan' account. And I will be

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able to do so because now I'm owner

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of those files, here you see 'bogdan'

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everywhere. Great. Let's use

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'rm' command. 'rm 1.txt',

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'rm 2.txt', and 'rm -r',

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recursive, 'tmp'. Actually, I was able

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to use just a single 'rm' command

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with multiple arguments. Type simply

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'rm -r tmp 1.txt 2.txt'.

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Alright, now

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I don't have any unnecessary files

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here in home directory for this

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user, and I see only some hidden

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files. Great. Now let's go back to

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root account and create new

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file in home directory for this

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'bogdan' user. Let's go out of here and

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here as the root user, let's 'touch',

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new file in the '/home/bogdan'; and

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let's name it 'new-file.txt',

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like so. And of course owner of

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this file will be now 'root' user. Let's

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verify that. Yes, here it is, 'root'

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is owner of this file, and here are

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permissions for this file. Only

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owner of this file is able to write

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this file to modify it. Great. Now

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let me demonstrate you how you are

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able to modify permissions for this

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particular 'new-file.txt'

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And recap that if you will try to

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write this file under other user, for

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example 'bogdan', you will

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definitely get an error, file is not

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writeable. Because of those

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permissions, it is not possible to

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write or execute this file by any

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other user that doesn't belong to

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'root' group. Before modifying

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permissions, let me explain you how

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permissions are actually

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represented. Again, there are three

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different blocks. Here's first

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block with three characters, here's

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second block three characters, and

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last block with three characters.

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And actually, we're able to

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represent every block with three

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characters using binary format,

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with zeros and ones. And for

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example, if I want to represent

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these three characters, I could

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simply type 1, 1, and 0; read,

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write, and not execute. I could

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represent those three characters

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using such combination 100. 1 stands

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for allow read, 0 stands for deny

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write, and this 0 stands for

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deny execution. Three different

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binary numbers 0 and 1. And

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now let's try to convert those

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three binary numbers into decimal

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format. Do you know how to convert

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a binary number to decimal number?

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It is very simple. In this case, we

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need to do add powers of two

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multiplied by 0 or 1. Let me

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show you. In this case formula will

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be following 1 multiplied by 2

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to power of 2, plus 0

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multiplied by 2 to power of 1,

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plus 0 multiplied by 2 to

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power of 0 like so. And in this

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case, we will get 4. 4 is

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decimal representation of this

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binary number 100. Actually, you

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are able to use any online

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converter that will convert the

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binary to decimal for you. Let's

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quickly find such kind of converter

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binary to decimal. And here, let's

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open up, for example, this link,

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'Binary to Decimal Converter'.

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Let's enter here 100, and press

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Convert, and I get 4, decimal

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number. Great. This is how you're

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able to convert binary numbers to

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decimal numbers. Now task for you,

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please try to convert to decimal

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number, this representation, read,

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write and not execute. I wish you good luck.

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[Clock Ticking]

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All right, done. Let's

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do that together. And let me

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actually remove this formula, and

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read, write and not execute is

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actually represented like 110,

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and it will be equal to

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1 multiplied to 2 to power of

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2, plus 1 multiplied by 2 to

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power of 1, and plus 0

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multiplied by 2 to power of 0,

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like so. And the result will be

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actually 4 plus 2, equal to

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6. Great. And if there are all

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ones, I mean, read, write and

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execute, then result will be equal

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to 7, Let me modify it here, 7;

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here will be 1, and here will be

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1. And it is actually read, write

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and execute, 'rwx', like so. So you need

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to convert this representation into

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binary representation, and

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afterwards to decimal. And actually

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there are most popular combinations

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like a read, 'r', '-', '-' read,

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write, -, 'rw-'; and read write and

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execute, 'rwx'. And corresponding decimal

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numbers are 7, stands for read,

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write and execute. Let me quickly

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write it down here. I'll remove

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this part. So read, write, and

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execute, 'rwx' stands for 7, read,

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write and not execute, 'rw-', stands

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for 6. And read and

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not write and not execute, 'r--', is

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actually 4, like so. Those are

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most popular numbers, decimal

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numbers for permissions. And now

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let me show you how using this

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knowledge, we are able to modify

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permissions for a specific file.

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Let's remove this part. And here

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for modification of permissions, we

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will use command 'chmod', like

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this; and next we could supply

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number as a first argument; and

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let's type here number, let's say

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000; and here, let's type path to

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the file we want to change

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permissions, '/home/bogdan/new-file'

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like this. Let's press Enter. And

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now let's list contents of the '/home

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/bogdan' user. And now I see

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following permissions. Because 0

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in decimal represents 000 in binary

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format. That's why now there are

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actually empty permissions. And we

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are not able to read, write or

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execute this new-file.txt. Let's

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now modify permissions to following.

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Change mode, 'chmod'; and here will be, let's

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say 400. Press Enter. Let's verify

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settings now. And now I see that it

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is possible to read this file only

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by owner of this file. Great. Let's

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modify permissions again. And now

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here I'll set permission 700. That

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stands for read, write, and execute

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for a particular user, but no other

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permissions for other users. And now

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let's verify it. And actually now

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you see that permissions were

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changed to read write and execute

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and color of the file was changed

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as well. And the reason for that is

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that I have just made this file

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executable; and all executable files

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in bash shell will be marked with

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green color like you see here.

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Great. Now task for you. Please

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modify permissions for this file,

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and make it executable, writeable,

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and readable for owner; and readable

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and executable for remaining users

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in group and for other users in

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other groups. All right. Throws

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let's try to write the permissions

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we want to assign to this new-file.

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in text format. So, permissions will

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be following - read, write, execute by

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owner; read, not write, and execute by

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all other users in group; and read,

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not write and execute by all

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remaining users in other groups. And

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we already know that this section

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is represented by number 7 in

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decimal format. We have just used

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this number here, and we have not

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yet calculated this section. And if

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I represent this section in binary

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format, I will get 101, where this

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1 is four in decimal, and this

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1 is 1 in decimal, 4 +

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1, 4 + 1 is equal to

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5. And that is the permission

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for all other users in group, and

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for all other users in other

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groups. It means that final command

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will be 755, like this. Let's

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verify that. Let's list files

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and folders here. And now I see

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desired result. Now this new-file.txt

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is writeable, readable

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and executable by owner; and only

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readable and executable by all

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remaining users. And if you want to

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set full permissions, you need to

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type here 777 likes so. Great. This

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is how you're able to modify

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permissions using decimal

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representation. Also, you're able

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to add some permissions, and to

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remove some permissions using the

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following syntax. Let me show you.

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Let's go back to this command; and

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here instead of three 7s, I'll

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write following, '-x'. Let's

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press Enter. And now let's verify

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what was changed for permissions.

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And now I see that execution

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permission was removed from all

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three parts, here, here, and here. That's

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what this '-x' does. It has

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removed execution permissions for a

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particular file. All right, let's

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try to go back to this command; and

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instead of '-x', let's use '+x',

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like so. And let's have a look

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again at the settings. And now I

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see that 'x' was added again

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everywhere. All right, you are also

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able to remove or add some

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permissions for particular groups.

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For example, if you want to remove

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execution permission only for

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group, you could use following

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command. Let's go back here. And

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here will be 'g', then will be minus,

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and x; 'g-x' for group remove

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execution permission. Let's verify it.

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And now I see that 'x' was removed

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here in group part. All right,

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let's now try to remove also read

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access for group users. And here

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will be 'g-r', let's try that.

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List, and now I see that 'r' was

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removed here as well for group

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users. That's how you could very

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quickly add or remove some

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permissions if you want to. But I

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personally usually use decimal

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numbers like 777, or 700, or 755,

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that are most commonly used.

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Alright, that's all for

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permissions. And that's how you're

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able to modify permissions for

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specific files. And actually, now

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we see that this new-file.txt

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is writeable and executable by

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all users. And let's actually now

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quickly verify that. Let's change

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user to 'bogdan', list files here. I

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see new-file.txt here in this list.

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And now let's try to modify it, 'nano

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new-file.txt. And yes, I'm able

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to modify it. Because now

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permissions are lowered. That's

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what permissions do. And that's how

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you're able to modify permissions.

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And that's what actually will be

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changed when you modify

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permissions. But my personal

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recommendation to you is to be very

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careful with permissions. And don't

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assign very broad permissions that

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we'll make some files readable,

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writable and executable by all the

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users in the systems. Please assign

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only necessary permissions that

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you really need. Alright, that's

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all for this lecture. And in the

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next one, I'll tell you how to

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create executable shell script

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using Terminal. I'll see you in the

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next lecture. Bye-Bye.

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[no audio]