A Short Course in Unix

Connecting to a Unix Machine
The Command Line
Your Home Directory
Current Directory
Listing the Contents of a Directory
The Hierarchical Filesystem
Knowing Where You Are in the Filesystem
Changing Your Location in the Filesystem
Getting Lost
Command Options
Running a Command Again
Unix is Case Sensitive
Fixing Command Line Mistakes
Command Line Editing - Arrow Keys
Command Line Editing - Control Key Sequences
Command Line Editing Summary
Creating a Text File
Copying
Creating Directories
Removing a File
Removing a Directory
Moving a File or Directory
"Invisible" Files and Directories
. (dot) and .. (dot dot) in a Directory
Paths and Pathnames
Two Kinds of Paths
Absolute Pathnames
Tilde, ~
Relative Pathnames
Relative Pathnames inside Your Current Directory
Relative Pathnames below Your Current Directory
Relative Pathnames above Your Current Directory
Relative Pathnames Neither above Nor below the Current Directory

Connecting to a Unix Machine

To connect to the Unix machines at UMB you need to use software that implements the SSH protocol
SSH stands for secure shell
It allow a remote machine to establish a communications channel to a Unix machine
What you type at the keyboard is encrypted and sent over the Internet the the Unix machine ...
and the output of your command is sent back to you the same way
Software that implements the SSH protocol is called an ssh client
PuTTY is a nice ssh client for Windows that can be downloaded for free here
See Connecting to a Unix machine from Windows for more information
If you have a Mac or Linux machine you already have an ssh client
It is called ssh and must be run from the command line ...
inside a terminal program
See Connecting to a Unix machine from MacOS
If you need to copy files to or from a Unix machine ...
you can download Filezilla here
See this page for more information

The Command Line

After you log in to your Unix account your ssh client will show you a window ...
into which you can type Unix commands
This environment is called the command line
When you are at the command line you are interaction with a special kind of program ...
called a shell
The shell allows you to type the name of the command you want to run ...
and then runs that command for you
The shell we are using is called bash and it has many useful features
The shell tells you it is ready to run another command ...
by printing text on the left hand side of the screen
This text, which usually ends with a $ is called a prompt and it means that Unix is ready for you to enter a command
For the sake of simplicity we will use the prompt $ in all examples here

Your Home Directory

Whenever you log in to a Unix machine you will find yourself in your home directory
A directory is the same thing as a folder on Windows or MacOS
Your home directory has the same name as your Unix account name
It is located under the directory named home ...
which is directly under the root directory, / ...
at the top of the filesystem
Your Unix account is the only account that can make any changes in your home directory
You can create files in your home directory to customize your Unix environment
For example, if you create a text file named .forward in your home directory ...
containing an email address ...
any mail sent to your Unix account will be forwarded to that address

Current Directory

Whenever you log in to a Unix system ...
you are always inside some directory
The directory in which you find yourself is your current_directory
It is important to know your current directory ...
because where you are determines what you can do

Listing the Contents of a Directory

Usually the first thing you do when you find yourself in a new directory ...
is to list the contents of that directory to see what it holds
You do this with the Unix command ls
Most Unix commands are short and have a name that suggests their meaning
ls stands for list
You run a Unix command by typing its name on the command line
If you type ls on the command line and nothing more ...
it will list the contents of your current directory
```
	$ ls
memo.txt  notes.txt
```
If you want to run ls on some other directory ...

you must follow ls with the name of that directory

	$ ls  /courses
cs105  cs411  cs611  cs656      cs768
cs108  cs420  cs612  cs664      cs799
cs109  cs430  cs615  cs670      cs999
...

The value that follow a Unix command on the command line is called an argument
The argument tells the command what to work on
Some command take more than one argument
Some commands take no arguments

The Hierarchical Filesystem

A filesystem lets you store and organize file ...
on some hardware device ...
like a hard drive, SSD or thumb drive
All files are stored in directories that let you organize your files
Directories are called "folders" on Windows and MacOs
The Unix filesystem is hierarchical
This means directories can be stored inside other directories
It also means that there is one special directory at the top
This special directory is called the root directory
On Unix there is one root directory which is written as /
On Windows machines each hard drive has its own root
The arrangement of a root directory at the top of the filesystem ...
and directories containing files and other directories ...
is called a hierarchical filesystem

Knowing Where You Are in the Filesystem

It is always important to know where you are in the filesystem
Most commands take arguments that are files or directories
To know how to specify a file or directory ...
you usually have to know your current directory
There are two ways to do this
You can run the pwd (print wworking directory) command
```
	$ pwd
/courses/it244/s18
```
The other way is to look at your prompt
The normal prompt on the UMB Unix machines has three parts
- The account name
- The name of the Unix machine
- Your current location
Everything from the beginning of the line to the @
is the account name
```
	ghoffman@itserver6:/courses$
```
The machine name lies between @ and :
```
	ghoffman@itserver6:/courses$
```
Your current location lies between : and $
```
	ghoffman@itserver6:/courses$
```

Changing Your Location in the Filesystem

When you first log in to Unix you will be in your home directory
But that is not where you will be doing your course work

Every class taught by the CS Department has a directory inside /courses

	$ ls  /courses
cs105  cs411  cs611  cs656      cs768
cs108  cs420  cs612  cs664      cs799
cs109  cs430  cs615  cs670      cs999
...

Although you can access any other file or directory ...
wherever you are in the filesystem ...
it is usually more convenient to go to the directory ...
that has the files you want to work with
To change your current directory use the cd command
You follow this command with the directory you want to move to
If you wanted to move to the /courses directory you would type
```
	$ cd  /courses
```
It is always a good idea to run ls right after cd ...
to make sure you are in the right directory

Getting Lost

The command line takes some time to get used to
In the beginning you may find yourself in the wrong directory
Whenever you find yourself in a directory that does not have the files you need ...
the best thing to do is go back you your home directory
The easiest way to do this is to run the cd command ...
without any argument
```
	cd
```
This will always take you to your home directory ...
wherever you are in the filesystem

Command Options

Most commands take at least one argument ...
but some command take no arguments
The argument tells the command what to work on
Arguments are not the only things that can appear after the name of the command ...
on the command line
Commands can also have options
which appear right after the command
An option changes the way a command works
For example, ls without options simply lists the contents of a directory
```
	$ ls tmp
empty.txt  ex23.py  notes
```

If you run ls with the -l option it will provide more information on each entry in the directory

	$ ls -l tmp
total 8
-r--r--r-- 1 ghoffman faculty    0 Jul 19 13:57 empty.txt
-rwxr-xr-x 1 ghoffman faculty 1457 Jul 19 16:03 ex23.py
drwxr-xr-x 2 ghoffman faculty 4096 Jul 19 12:56 notes

Running a Command Again

Unix makes it easy to run an old command again
If you hit the up arrow key, ↑, Unix will bring the last command back to the command line
If you keep hitting ↑ you will see older commands
If you go too far back and want to move to newer commands hit the down arrow, ↓

Unix is Case Sensitive

In Unix upper case letters and lower case letters are different characters
All Unix commands are lower case

If you try to run a command using upper case characters you will get an error

	$ LS
The program 'LS' is currently not installed. To run 'LS' please ask your administrator to install the package 'sl'

In general, you don't use upper case characters on the command line

Fixing Command Line Mistakes

Using the command line is hard because the slightest mistake will cause an error
Fortunately, Unix makes fixing such mistakes easy ...
because it allows you to change a command on the command line itself
This is called command line editing
A Unix command only runs after you hit Enter on a Window machine ...
or Return on a Mac
This means you can change the command before you run it
If you have already run the command and seen an error message ...
you can bring the command back using ↑ ...
correct the mistakes ...
and run the command again

Command Line Editing - Arrow Keys

The easiest way to fix a mistake is to use the arrow keys
You can use the right and left arrow keys, → and ← ...
to change where you are on the command line
Move to the point where the error occurs ...
and then add or delete text as needed
Once you are done hit Enter or Return
You do not have to be at the end of the line to run a command

Command Line Editing - Control Key Sequences

On the lower left corner of your keyboard you will find a Control Key
You can use this key, along with a letter, change the command line
Some of these commands move the place where you can enter text ...
called the text insertion point ...
to a new position on the line
If you hold down the control key while pressing the A key ...
it will move you to the beginning of the line
When I say the A key, I do not mean you have the hold the shift key
I mean the key with A on its face
This combination of holding the control key while typing a letter ...
is a control key sequence
The control E sequence moves you to the end of the line
Control K removes all the text from your current position ...
to the end of the line
Control U removes all text from your current position ...
to the start of the line
Any text you have removed can be brought back ...
to any place on the command line ...
by using Control Y
To undo the last change you made to the command line use control _ ( underscore )
Note that you have to hold down the shift key to type _

Command Line Editing Summary

Command	Meaning
Control A	Move to the beginning of the line
Control E	Move to the end of the line
Control U	Remove everything from the text entry point to the beginning of the line
Control K	Remove everything from the text entry point to the end of the line
Control _	Undo the last command line change
→	Move the text entry point one character to the left
←	Move the text entry point one character to the right
↑	Recall the previous command line entry in the history list
↓	Recall the following command line entry in the history list

Creating a Text File

To create a text file in Unix you need to use a Unix text editor
Most installation of Unix have three editors
- emacs
- vi
- nano
Both emacs and vi are full featured editors ...
that take some time to learn
nano is a simple editor that does not have many features
emacs tends to be used by people who write code
vi tends to be used by system administrators
Each is very different from the other ...
so if you learn one ...
it becomes hard to use the other
If you do not have time to learn emacs or vi ...
start with nano
To create a file using nano write nano ...
followed by the name of the file
```
	$ nano  hello.py
```
Commands in nano use control key sequences
- Control O saves a file
- Control X quits nano
You will find more information on nane here
To change a text file write nano followed by the name of the file
```
	$ nano  hello.py
```

Copying

To make a copy of a file you use cp which takes two arguments
The first argument is the name of the file you want to copy
If you are making a copy in the same directory ...
the second argument is the name of new file
```
	$ cp  hello.py  hello2.py
```
If you want to make the copy in another directory ...
the second argument is the name of the directory
```
	$ cp  hello.py  project1
```
You can also copy a directory and all it contains ...
if you use the -r option
```
	$ cp  -r  project1  new_project
```

Removing a File

To remove a file use the rm command
The command is followed by the name of the file to be removed
```
	$ rm  memo.txt
```
Once file had been deleted it cannot be restored
There is no undelete command in Unix
rm cannot remove a directory without a special option

Creating Directories

Directories are the way we organize information in a file system
Directories are called folders on Windows and MacOS
A directory can contain files and other directories
You cannot have two things with the same name ...
inside the same directory
To create a directory used the mkdir command ...
followed by the name of the new directory
```
	$ mkdir  temp
```

Removing a Directory

To remove a directory use the rmdir command
Follow this command with the name of the directory to be removed
```
	$ rmdir  temp
```
rmdir cannot remove a directory if it has anything inside it
You must delete the contents of a directory befor deleting the directory itself

Moving a File or Directory

To move a file or directory from one place to another ...
use the mv command
mv takes two arguments ...
what you want to move ...
and where you want to move it to
```
	$ mv  memo.txt  temp
```
You also use mv when you want to change the name of a file or directory
```
	$ mv memo.txt  note.txt
```

"Invisible" Files and Directories

Any file or directory whose name begins with . (dot) ...
will not appear when you run ls

To see them, you have to run ls with the -a option

	$ ls
it116  it244

$ ls -a
.  ..  .bash_history  .bash_profile  it116  it244

Unix uses files and directories whose begins with . (dot) ...
to configure your Unix environment
For example, .bash_porfile contains Unix commands ...
that are run right after you login
You can write Unix commands in this file to customize Unix for your needs

. (dot) and .. (dot dot) in a Directory

Every time you create a directory, two entries, . (dot) and .. ...
are automatically created inside the new directory
. (dot) refers to the directory itself
.. refers to the directory directly above this new directory
When every you want to go to the directory directly above your current directory ...
you run cd ..
. (dot) is used when you want to copy a file from another directory ...
into your current directory
```
	$ cp  notes/note_1.txt  .
```

Paths and Pathnames

Most Unix commands take a file or a directory for their arguments
If you want to run a command on a file or directory that is inside your current directory ...
you just use the name of the file or directory
But what if the file or directory is not inside your current directory?
Although a name can appear only once in a given directory ...
you can use that name in other directories
This means that you can identify a file or directly in any filesystem ...
with two pieces of information
- The name
- The location
We specify the location with a path
A path is a list of directories starting at some point ...
and ending with the directory that holds the file
To separate the name of each directory in the path ...
use a slash, /, character
Windows uses a different character, the backslash, \, ...
to separate directory names
To uniquely specify any file or directory in the filesystem ...
you write the path ...
followed by the name of the file or directory
This is called a pathname

Two Kinds of Paths

Unix let's you create two kinds of paths
Each of these paths starts at a different place
An absolute path starts at the root directory
A relative path starts in your current directory
You can use either path when creating a pathname
It does not matter which you chose
They each have advantages and disadvantages
Absolute pathnames work wherever you are in the filesystem ...
but they tend to be long and complicated
Relative pathnames tend to be short and simple
but they only work inside one directory
If you are trying to use a file or directory that is far away ...
or if you are trying to send a pathname to someone else ...
use an absolute pathname
If you want to refer to something close by ...
use a relative pathname
An absolute pathname is like the address on a letter
You can send a letter to me anywhere in the world using the following address
```
	Glenn Hoffman
40 Central Street
Somerville, Massachusetts  02143
USA
```
A relative pathname is like directions from one room to another

If you wanted to know how to get to the CS office I would say

	Turn left when you leave my office
When you come to the corridor turn right
Walk all the way down he corridor
Just before you reach the end of the corridor you will see a window
That is the CS office

Relative pathnames only work in very specific places

Absolute Pathnames

The path part of an absolute pathname starts at the root directory ...
which we write as /
If I am someplace other than my home directory ...
and want to run a Unix command on a file in that directory ...
I can use an absolute pathname
The name of my home directory is ghoffman
All home directories on our Unix machines are contained in the directory home ...
which is directly under the root directory, /
The absolute path
to my home directory is
```
	/home/ghoffman
```
This path lists three directories
- r coot (/)
- home
- ghoffman
If I want to run a Unix command on the file .bash_profile in my home directory
I would create an absolute pathname by writing the absolute path ...
a slash, / ...
and the name of the file

Notice that the slash character, / is used in two different ways
When is appears as the first thing in a path ...
it means the root directory
In every other position it separates the name of a directory ...
from what comes next

Tilde, ~

The character tilde, ~, has special meaning in Unix
You will find this character on the upper right corner of the keyboard
Whenever you type this character on the command line ...
Unix will replace this character ...
with the absolute pathname of your home directory ...
before it runs the command
```
	$ echo  ~
/home/ghoffman
```
If you write a ~followed by a Unix username ...
Unix will replace it with the absolute pathname ...
of that user's home directory
```
	$ echo ~it244gh
/home/it244gh
```
~ is a special kind of absolute pathname ...
that only works with home directories

Relative Pathnames

You can use an absolute pathname anywhere
But many absolute pathnames are very long ...
so most people use relative pathnames
The path part of a relative pathname always starts with your current directory
When you are in one directory and want to run a command ...
on a file or directory in a different directory ...
that file or directory can be in one of four places ...
relative to your current position
- Inside your current directory
- Below your current directory
- Above your current directory
- Off to the side of your current directory
This means that there are four types of relative pathnames
The diagram below shows the four possibilities

The blue foo1.txt file is inside your current directory
The red foo2.txt file is below your current directory
The green foo3.txt file is above your current directory
The brown foo4.txt file is off to the side of your current directory

Relative Pathnames inside Your Current Directory

When you want to work with a file or directory inside your current directory ...
the path part of the pathname is nothing
You are already in the directory that hold what you want to work with ...
so the pathname is simply the name of the file or directory
This is the type of pathname we use most often
If I wanted to run cat on the file notes.txt ...
inside my current directory ...
I would write
```
	$ cat notes.txt
```
The diagram below illustrates the situation

Relative Pathnames below Your Current Directory

What if you wanted to run a Unix command on a file or directory ...
that was below your current directory?
In other words, a file or directory that was itself inside a directory ...
contained in your current directory?
In this case the path part of the pathname would be a list of every directory ...
between your current directory ...
and the file or directory you needed
If there were more than one directory in the path ...
you would insert a / between each directory name
I wanted to run cat on the file notes.txt ...
inside the directory work ...
I would write
```
	$ cat work/notes.txt
```
The diagram below illustrates the situation

Relative Pathnames above Your Current Directory

If you want to run a Unix command on a file or directory above your current directory ...
you need a different approach
Every time you create a new directory ...
Unix automatically creates .. (dot dot) inside it
This refers to the parent directory
The path part of the pathname will have one .. ...
followed by a / ...
for each directory above your current directory
If I wanted to run cat on the file notes.txt ...
which was two directories above my current directory ...
I would write
```
	$ cat ../../notes.txt
```
The diagram below illustrates the situation

Relative Pathnames Neither above Nor below the Current Directory

There is a fourth kind of relative pathname ...
that is almost never used ...
even though it is a valid pathname ...
because it is long and complicated
If you are trying to run a Unix command on a file or directory ...
that is neither above or below your current directory ...
you could use this kind of pathname
In practice most people would use an absolute pathname instead
To create a path for this situation you actually combine two other relative pathnames
The relative pathname to point to something above your current directory ...
and the relative pathname for something below
You form the path by using .. to climb up the directory tree ...
until you get to the a directory which is above what you want ...
and then list the directories between that point and what you want
If I were in my home directory and wanted to run cat on the file notes.txt ...
which was in the directory frank ...
that was alongside my current directory ...
the path part of the pathname would start with ../ ...
to get me to the parent directory of frank
Then I would list the directory between here and notes.txt
I would write
```
	$ cat ../frank/notes.txt
```
The diagram below illustrates the situation