Lecture notes for Monday, 6/23

The first order of business is to go over the syllabus. The latest version can always be found here.

Next, we walk through the process of logging into the Silo server.

Open the program PuTTY on your computer. (If you have a different SSH client and you know how to use it, feel free to do so.)
The "Host Name" for Silo is silo.soic.indiana.edu. We want port 22 and connection type "SSH", but those are standard, so you shouldn't have to change them.
We open the connection by clicking "open".

We're now looking at a command-line interface. This means that the only way to communicate with the server is by typing in characters and the only way it can communicate with us is by sending us text. Your mouse is almost completely useless at this point.*

*In PuTTY, ou can use the mouse to select text, which is automatically copied to your computer's clipboard as long as it's selected. Silo, and any software you're running on the server don't know anything about what you have selected; it's all on your computer. If you right click on the PuTTY window, it will take whatever's in the clipboard and send that to Silo, as if you'd just typed it in. Depending on the circumstances, this may or many not accomplish what you want it to.

Enter in your IU username and hit enter. (Not tab!) Then enter in your IU passphrase. You won't see anything appear on the screen, which is a pain in the butt if you have a long and complicated passphrase, but there's not much we can do about that but complain. Backspace works, but you can't see what it's doing, so if I screw up, I usually just hit enter and try again. Hit enter again to log into Silo.

You should now be at a Linux prompt, which looks something like [username@silo ~]$. There's a lot I could say here about using Unix/Linux, but there are plenty of good references for that on the page, so right now, we'll just walk through creating a directory, creating a text file, and changing permissions.


[ewennstr@silo ~]$ ls
apache   Documents     Music         mysqld-error.log  Public     test
bin      Downloads     mysql         php-5.5.13        README     Videos
Desktop  httpd-2.2.27  mysql-5.6.19  Pictures          Templates

The command ls lists all the files and folders in the current directory. Let's make a new directory using the mkdir command and call it "homework".


[ewennstr@silo ~]$ mkdir homework
[ewennstr@silo ~]$ ls
apache   Documents  httpd-2.2.27  mysql-5.6.19      Pictures  Templates
bin      Downloads  Music         mysqld-error.log  Public    test
Desktop  homework   mysql         php-5.5.13        README    Videos

We use ls and we can see that the new directory is now there. Now let's move inside the new directory with the cd (change directory) command. This command works pretty much the same as it does in DOS or a Windows command line.


[ewennstr@silo ~]$ cd homework
[ewennstr@silo homework]$ ls

There's nothing in this directory. Let's change that. We'll use the text editor named "Pico" to open a file called "one". Since there is no file named "one" yet, it creates one for us.


[ewennstr@silo homework]$ pico one

We're now in the Pico window. In this file, I want you to put your name, your e-mail address, and a list of programming languages that you have some experience with. You can add any other message if you want. When we're done, we hit Ctrl+X (see the nice guide to commands at the bottom of the Pico screen). When it asks if we want to save, we hit Y for "yes".


[ewennstr@silo homework]$ ls
one
[ewennstr@silo homework]$ cat one
Erik Wennstrom
EWennstr@ewennstr.edu

Some programming languages I have experience with:
BASIC
Python
C++
R
Scheme
PHP
Inform 7

We check to see that the new file is there with ls and we can ask to see the contents of a text file with the cat command. So we're done with the assignment, right?

Well, not quite. See, if anyone else (in particular, your instructor and your graders) tries to access the file, they won't be allowed to because as it is, the only account that's allowed to read the file is the one we're currently logged into. We can view permissions (and other things) using the ls command by adding the -l option.


[ewennstr@silo homework]$ ls -l
total 4
-rw------- 1 ewennstr faculty 128 Jun 20 12:19 one

That -rw------- at the beginning is a key that tells us what the permissions are. Ignore the first character for now. Characters 2-4 give us the permissions that the "user" who owns the file (ewennstr, in this case) has. In this case, that's reading (r) and writing (w), but not executing (x), so we couldn't run this as a script (which is fine because it isn't a script). Characters 5-6 tell us the permissions for anyone in the "group" that the owner belongs to (faculty, in this case). Characters 7-9 tell us the permissions that any "other" account logged into Silo has. Right now, the only person who can read the file or write to it is ewennstr. Let's make it so that everyone can read the contents of the file, but not change it. We use the chmod command to "change" the "mode" of the file.


[ewennstr@silo homework]$ chmod a+r one
[ewennstr@silo homework]$ ls -l
total 4
-rw-r--r-- 1 ewennstr faculty 128 Jun 20 12:19 one

There are lots of ways to use the chmod command. (You can read about it at Wikipedia, if you like. Or you can type in the command man chmod to read the manual for the command. In fact, you can do that for any Unix command.) In this case, we added (+) the read (r) permission for all users (a). Now you can see that the group and other permissions now include the ability to read the file.

While we're at it, why don't we change the permissions for the entire "homework" directory? We go up a directory using the cd command again. ".." always refers to the parent directory of the current directory.


[ewennstr@silo homework]$ cd ..
[ewennstr@silo ~]$ ls -l
total 96
drwxr-xr-x 18 ewennstr faculty  4096 Jun 18 14:36 apache
drwx------  2 ewennstr faculty  4096 Sep 10  2010 bin
drwxr-xr-x  2 ewennstr faculty  4096 Sep  8  2011 Desktop
drwxr-xr-x  2 ewennstr faculty  4096 Sep  8  2011 Documents
drwxr-xr-x  2 ewennstr faculty  4096 Sep  8  2011 Downloads
drwx------  2 ewennstr faculty  4096 Jun 20 12:19 homework
drwx------ 12 ewennstr faculty  4096 Jun 15 15:40 httpd-2.2.27
drwxr-xr-x  2 ewennstr faculty  4096 Sep  8  2011 Music
drwx------ 18 ewennstr faculty  4096 Jun 17 16:54 mysql
drwx------ 33 ewennstr faculty  4096 May  6 06:46 mysql-5.6.19
-rw-r-----  1 ewennstr faculty 26404 Jun 17 16:54 mysqld-error.log
drwx------ 18 ewennstr faculty  4096 Jun 18 14:20 php-5.5.13
drwxr-xr-x  2 ewennstr faculty  4096 Sep  8  2011 Pictures
drwxr-xr-x  2 ewennstr faculty  4096 May 30 11:36 Public
-rw-------  1 ewennstr faculty   485 May 20 10:15 README
drwxr-xr-x  2 ewennstr faculty  4096 Sep  8  2011 Templates
drwxr-xr-x  2 ewennstr faculty  4096 Sep  8  2011 Videos

The directory homework has permissions drwx------. The first character "d" means that it's a directory. The user (ewennstr) has permission to read (see its contents), to write (add files to the directory), and to execute (which for directories mostly means to be able to use cd to change into that directory). Nobody else has any permissions. We'd like for other people to be able to see the directories contents and to access the directory with cd, so let's do that.


[ewennstr@silo ~]$ chmod a+rx homework
[ewennstr@silo ~]$ ls -l homework
total 96
drwxr-xr-x 18 ewennstr faculty  4096 Jun 18 14:36 apache
drwx------  2 ewennstr faculty  4096 Sep 10  2010 bin
drwxr-xr-x  2 ewennstr faculty  4096 Sep  8  2011 Desktop
drwxr-xr-x  2 ewennstr faculty  4096 Sep  8  2011 Documents
drwxr-xr-x  2 ewennstr faculty  4096 Sep  8  2011 Downloads
drwxr-xr-x  2 ewennstr faculty  4096 Jun 20 12:19 homework
drwx------ 12 ewennstr faculty  4096 Jun 15 15:40 httpd-2.2.27
drwxr-xr-x  2 ewennstr faculty  4096 Sep  8  2011 Music
drwx------ 18 ewennstr faculty  4096 Jun 17 16:54 mysql
drwx------ 33 ewennstr faculty  4096 May  6 06:46 mysql-5.6.19
-rw-r-----  1 ewennstr faculty 26404 Jun 17 16:54 mysqld-error.log
drwx------ 18 ewennstr faculty  4096 Jun 18 14:20 php-5.5.13
drwxr-xr-x  2 ewennstr faculty  4096 Sep  8  2011 Pictures
drwxr-xr-x  2 ewennstr faculty  4096 May 30 11:36 Public
-rw-------  1 ewennstr faculty   485 May 20 10:15 README
drwxr-xr-x  2 ewennstr faculty  4096 Sep  8  2011 Templates
drwxr-xr-x  2 ewennstr faculty  4096 Sep  8  2011 Videos

The permissions are now drwxr-xr-x, so mission accomplished!

Feel free to mess around with other Unix commands. I wouldn't alter or delete any existing files, but you can do pretty much whatever you want with files or directories you've added. You can also use the man command to see the manuals for various Unix commands.

Post-lecture wrap-up

In lecture today, you saw a glimpse of the following Unix commands:

cd
ls and ls -l
cat
mkdir
rm
chmod
pwd
cp
exit
man

We also used the Pico text editor and we talked about the directory shorthands ., .., and ~.

Remember that when you're done creating your directory and file and adjusting their permissions, you should send me an e-mail to indicate that you've completed homework assignment #1.