IT 244: Introduction to Linux/Unix

IT 244: Introduction to Linux/Unix
Class 8

Today's Topics

Reading Assignment

The reading assignment for this week is chapter 3 of Sobell, The Utilities.

You will find a list of all reading assignments here.

There is a link to this page on the class web page.

Homework 4

I have posted Homework 4 here.

As usual, it will be due next Sunday at 11:59 PM.

Tips and Examples

Be Sure You Know Where You Are

Whenever you are working at the command line ...
you always have a current directory
It is important that you always know your current directory
If you think you are in one directory ...
but you really are in another ...
you can get into trouble
Every semester I get at least one email from a student ...
saying some file has disappeared
Linux will never delete a file ...
unless you tell it to
The file is always somewhere ...
but it may not be where you think it is
Often the file is in their home directory
When you think a file has disappeared ...
look for it in other directories
Another problem can arise if you are in the wrong directory
If you try to save a file ...
and get error message that you don't have permission ...
it means you are in the wrong directory
The owner of a directory decides who can do what ...
in that directory
If you are in a directory where you do not have write permission ...
you won't be able to create or change anything
A text editor like nano will open a file ...
whatever directory you are in
But you won't be able to save it ...
unless you have write permission in that directory

Review

The Hierarchical Filesystem

Unix uses a hierarchical filesystem
This means there is one directory at the top ...
called the root directory
The root directory is indicated by a simple slash character, /
All other directories are contained within the root directory ...
or one of its many subdirectories
This structure is called a tree because it looks like a tree turned upside down
A hierarchical filesystem also resembles a family tree
So we often use terms that describe family members ...
when talking about directories
The directory up one level from your current directory is called the parent directory
All directories above the current directory are called ancestors
All directories inside the current directories are called child directories
All directories below the current directory can be called descendants
All directories and files within the same parent directory are called siblings

Unix Files and Directories

Files are linear arrangements of data ...
on a storage device
There are several types of files
- Program files
- Text files
- Data files
- Configuration files
Directories are simply containers that hold files
Unix tends to treat everything is sees as a file
Unix even considers devices, such as printers, as files
Directories are files too, as far as Unix is concerned
You cannot run cat, more or less on a directory ...
or on a device
The information that directory files contain ...
can only be accessed by system programs and system calls

Filenames

When you ask Unix for a file you must give it two pieces of information
- The name of the file
- The location of the file in the hierarchical file system
Every file has a filename
The maximum number of characters permitted in a filename ...
varies from one Unix to another
Most Unix flavors allow file names of up to 255 characters
It is best to keep filenames short
This makes typing and remembering them easier
Never use a space in a file or directory name
This is a bad idea
Use an underscore, _ , instead of a space in file names
To avoid problems, only use the following characters in file names
- Uppercase letters (A-Z)
- Lowercase letters (a-z)
- Digits (0-9)
- Underscore _
- Dash -
- Period .
You cannot have two files with the same name ...
in the same directory

Case Sensitivity

Unix is case sensitive
This means that "Foo", "foo" and "FOO" ...
are three different things ...
as far as Unix is concerned
Unix utility and program names are always lowercase
Some operating systems do not distinguish ...
between UPPERCASE and lowercase characters
Windows is such a system
Make life easy for yourself
Use only lowercase characters in Unix filenames

Filename Extensions

Extensions are strings of characters ...
that appear at the end of the filename ...
after a period, .
Extensions are not recognized by the Unix filesystem
As far as Unix is concerned ...
they are just legal characters ...
that are part of the filename
Some Unix programs expect their files ...
to have certain extensions
For example, the C compiler, gcc ...
expects the filenames of source code files to end in .c
The Java compiler, javac ...
expects Java source files to have .java ...
at the end of the filename
The extensions are required by these program ...
not by Unix

Current Directory

The way a Unix command works ...
depends, somewhat, on your environment
One of the most important parts of your environment ...
is your current directory
The pwd (print working directory) command ...
will always tell you your current directory

Your Home Directory

Whenever you log in to a Unix host ...
you will always find yourself in your home directory
This a directory that belongs to your Unix account only
You have full control of everything inside this directory
If you use cd with no arguments ...
it will take you to your home directory
```
$ cd
    
$ pwd
/home/tsoro
```
Your home directory contains a number of hidden files ...
which customize your environment ...
like .plan
On most Unix systems, home directories are found inside the /home directory
On a Mac, home directories appear inside /Users
The name of your home directory ...
is the same as your Unix username

Any file or directory in the filesystem ...
will be one of four positions ...
relative to your current directory
It can be inside your current directory
It can be below your current directory
It can be above your current directory
It can be off to the side of your current directory
In this last case, you must go up ...
before you can go down to reach this file

New Material

Hidden Filenames

A file whose filename begins with a period, . ...
is a "hidden" or "invisible" file
ls does not display these files unless you use the -a option
Two special hidden files . and .. appear in every directory
```
$ ls  -a
.  ..  hw3  work
```
We'll discuss them shortly

Startup Files

The files .login and .bash_profile are startup files
Startup file contain Unix commands that are run ...
just before you get the first shell prompt
These customize your Unix environment
They only work when placed inside your home directory
They can set variables which can help you with your work
We'll talk more about this in a future class

The . and . . Directory Entries

Every directory has at least two entries: . and . .
When a new directory is created, these entries are always there
. stands for the current directory
. . stands for the parent directory ...
the directory immediately above your current directory
```
$ pwd
/home/tsoro
    
$ cd ..
    
$ pwd
/home
```

. is most often used in two circumstances

To run a program in your current directory
To move or copy a file to your current directory

$ ls
it244  notes.txt  work  work2
    
$ cd  work
    
$ ls
bletch.txt  foo.txt
    
$ cp  ../notes.txt  .
    
$ ls
bletch.txt  foo.txt  notes.txt

Pathnames

Every file has a pathname ...
which is used to access the file
A pathname has two components
- A path to reach the file
- The name of the file
A pathname is like an address on a letter ...
a name ...
and directions to get there
The name of the file is always at the end of a pathname
What comes before the filename ...
is the path to the directory that holds the file
A path is the list of directories you must go through ...
to get to the file
When you see a slash, / , to the right of a name
it means the name refers to a directory
When a path consists of several directories ...
a slash, / , separates the directory names
There are two types of pathnames
- Absolute
- Relative

Absolute Pathnames

The directory at the top of the filesystem ...
is called the root directory
The root directory is represented by a single slash, /
It can stand alone ...
or appear as the first character before a directory name
An absolute path is a list of directories ...
starting with the root directory ...
and ending with the directory that holds the file
When you add the filename to the end of an absolute path ...
you have an absolute pathname
The absolute pathname of my startup file, .bash_profile ...
in my home directory ...
is /home/tsoro/.bash_profile
This means that my home directory, tsoro ...
is under the directory named home ...
which is under the root directory, /
The advantage of an absolute pathname ...
is that it can be used from any part of the filesystem
It does not depend on where you are ...
at a given moment
The disadvantage is that absolute pathnames tend to be long
It is easy to make a mistake when typing an absolute pathname
An absolute pathname always begins with either a slash, / ...
or a tilde, ~

Tilde, ~, in Pathnames

There is one form of absolute path that is short
This is the tilde character, ~
Tilde stands for the absolute address of your home directory
This means you can use tilde, ~ ...

anywhere you would normally use a path to your home directory

$ pwd
/home/tsoro/it244
    
$ cd ~
    
$ pwd
/home/tsoro

When you put a tilde in front of a Unix username ...
it stands for the home directory of that account
```
$ cd ~tsoro
    
$ pwd
/home/tsoro
```

Relative Pathnames

Absolute pathnames are useful ...
because you can use them anywhere
But they are long and easy to mistype
It is easier to use relative pathnames
In a relative pathname ...
the path starts from your current directory
In an absolute pathname ...
the path starts from the root directory, /
The only difference between an absolute ...
and a relative path ...
is where they start
An address on a letter is like an absolute pathname

If you address a letter to me at

Glenn Hoffman
40 Central Street
Somerville, Massachusetts 02143
USA

you can mail it to me anywhere in the world ...
and it will get to me
If someone asked me the directions to the men's room ...
from the Web Lab ...

I would say

Go out the door
Turn left
Take you first door on the right

These directions only work from the Web Lab
This is like a relative path
While all absolute pathnames start with a slash, /, or a tilde, ~
relative pathnames never do
The absence of a slash, / , or a tilde, ~
at the start of a pathname ...
means it is a relative pathname
As far as Unix is concerned it makes no difference ...
whether you use and absolute or relative pathname
Relative pathnames are more convenient ...
and are most often used
There are four types of relative pathnames
- When the file is in your current directory
- When the file is in a subdirectory of your current directory
- When the file is in a directory that is above your current directory
- When the file is in a directory that is neither above or below of your current directory

Relative Pathnames in Your Current Directory

Using a relative pathname with a file or directory ...
inside your current directory ...
is easy
The relative pathname is simply the name of the file or directory
The "path" part of the relative pathname is nothing
because the file or directory you want ...

is already inside your current directory

$ ls
notes.txt  it244  work  work2
    
$ cat notes.txt 
foo
foo
foo to you

Notice that all I need to get the file ...
is the file name
It's the same for directories

Relative Pathnames in a Subdirectory

Things get a little more complicated ...
when you are dealing with a file in a subdirectory
In this case you must list every directory ...
between your current directory ...

and the file you want

$ ls work
bletch.txt  foo.txt  notes.txt
    
$ cat work/notes.txt 
foo
    foo
    foo to you

You must use a slash, / ...
to separate the name of each directory ...
from the name of the file or directory ..
that comes after it
Notice that there is nothing ...
no slash, /, or tilde, ~ ...
before the name of the first directory in the path

Relative Pathnames above the Current Directory

Every directory has a . . entry
that refers to the directory immediately above it
If I wanted to list the contents of the parent directory ...
I would use
```
ls ..
```
You can use many . . entries ...
to refer to a directory several levels up
Use one . .
for each directory up the chain of directories in the path ...
with a slash, /, between each . .

Relative Pathnames Neither above Nor below the Current Directory

What if the file is neither above nor below?
Here you have to go up to a common ancestor directory ...
and then go down to the directory that holds the file
The path starts with one or more . . ...
and keeps going up ...
until you get to a directory that is an ancestor ...
of both your current directory ...
and the file you are trying to reach
Once you get to the common ancestor ...
you go down to the directory that holds the file

Standard Directories

In the early days of Linux ...
each distribution stored its important files ...
in different directories
This made it hard to document programs
It also made things hard for developers ...
working with different flavors of Linux
Fortunately, reason has since prevailed
We now have something called the FHS
It stands for Linux Filesystem Hierarchy Standard
It has been adopted by most major Linux distributions
The standard does not say ...
where to find different kinds of files
Instead it says that if you have a certain directory ...
what files you can find in that directory
You can see a listing for the FHS on page 96 ...
of the current version of Sobell ...
or page 91 of the previous edition
Most Linux distributions do not use all these directories
But when they do, they use them for the type of files ...
specified by the standard

Your Home Directory versus /home

Most Unix accounts have a home directory ...
though not all
One of the Ubuntu commands that creates an account ...
has an option to not create a home directory ...
for that account
On most Unix and Linux systems ...
all home directories are contained in the directory /home ...
which is directly under the root directory ...
written as /
On the Mac, home directories are contained in /Users ...
also directly under the root directory
Do not confuse /home with your home directory

The Two Uses of /

/ is used in two different ways ...
when writing pathnames
When / appears at the beginning of a pathname ...
it refers to the root directory ...
the directory at the top of the filesystem
The root directory is the only directory ...
that is not contained in another directory
Here is an example of this use of /
```
ls /home
```
This is the absolute address of home ...
which holds the home directories of all users
When the / is not at the beginning of a pathname ...
it is used a separator ...
a character to mark the boundary between a directory ...
and the name of a file ...
or another directory
Here is an example of this use of /
```
ls it244/hw
```
This is the relative pathname of the hw directory ...
inside the it244 directory
A pathname can use the / in both ways
```
ls /home/tsoro
```
Here the first / means the root ...
while the second is used to separate the home directory ...
from my home directory, tsoro

The Two Special Uses of .

Another character that has more then one use ...
when writing pathnames ...
is .
When . appears alone ...
it means the current directory
So the following command
```
cp /home/tsoro/test.txt .
```
copies a file to the current directory
When . appears as the first character ...
in a filename or directory name ...
the ls command will not list these directory entries ...
unless you use the -a option
These "invisible" files are used ...
to configure Linux ...
and Linux programs
Examples are .forward and .plan
Unlike / ...
. can be used ...
as an ordinary character in a file or directory name
```
$ touch a.b.c.d....

$ ls
a.b.c.d....
```

IT 244: Introduction to Linux/Unix
Class 8

Today's Topics

Tips and Examples

Review

New Material

Reading Assignment

Homework 4

Tips and Examples

Be Sure You Know Where You Are

Review

The Hierarchical Filesystem

Unix Files and Directories

Filenames

Case Sensitivity

Filename Extensions

Current Directory

Your Home Directory

Navigating the Hierarchical File Systems

New Material

Hidden Filenames

Startup Files

The . and . . Directory Entries

Pathnames

Absolute Pathnames

Tilde, ~, in Pathnames

Relative Pathnames

Relative Pathnames in Your Current Directory

Relative Pathnames in a Subdirectory

Relative Pathnames above the Current Directory

Relative Pathnames Neither above Nor below the Current Directory

Standard Directories

Your Home Directory versus /home

The Two Uses of /

The Two Special Uses of .

Attendance

Class Quiz