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Okay yesterday we learned quite a
bit of theoretical stuff about the operating system, shells, and
the theory behind the file system. Today we are going
to start getting our hands dirty and apply some of this
theory to real world commands that you can use.
Specifically, we will learn how
to move around the file system, create,
modify and delete files and directories and understand
how permissions work.
What exactly is a file system?
Well, perhaps the first question we should ask is what is a
file.
By now, most of us
will be familiar with the concept of files. In fact, we use them
every day. We save them, delete them, copy them, move them, etc.
It is almost as if files were real things on our
computer.
Well in actuality a file is only
an abstract concept (a data structure). It is a metaphor
used to describe a theoretical grouping
of dispersed bits within a computer's memory.
In actuality, files do not exist
From the perspective of the computer,
there are only flittering,
ethereal bits floating around in its memory. True, these bits do
have references to each other, but there is no actual "file
thingy" sitting in the computer somewhere.
A file is actually a pretend creature that
we have imagined to help us deal with what is otherwise a
very ethereal thing.
Of course, never underestimate
the power of make believe. It is extremely useful
to use the file metaphor because it helps people do work. The
file metaphor helps us organize things and feel comfortable in
the worldless world of bits and bytes.
| Files within the UNIX
file system can be identified by their path and name. As such,
filename standards and conventions have developed to make browsing the
file system more efficient. Some of the standards you should be familiar
with are listed below:
- A good file name
will consist of alphanumeric characters and/or punctuation characters
other than slashes. However, it is best to stick with the standard
alphanumeric name followed by a dot extension such as myfile.txt.
Though valid, try to avoid such characters as !, ?, #, *, &, |, <,
>, ', ", \, /, ^, (), {}, [], ;, :, +, =.
- Most modern UNIX systems allow filenames of
up to 256 characters, but you should try to strike a balance between the
information contained in a file name and the convenience of typing the
name in. For example, a name like "web_site_log_file_for_january.log"
might be renamed "jan98_www.log". As a side note, you probably would
not want to name the file "www_jan98.log" under the assumption that you
will have other files such as "www_feb98.log" and "www_mar98.log".
Here's why. If you put the month first, you can more easily use wild
cards to find the file. If the date comes first "ls ja*" will find
"jan98_www.log" with only 4 typed characters, whereas, if the "www"
portion came first, you would need to type out "ls www_ja*" to get the
same result.
- Since UNIX is case
sensitive, capitalization within a filename will matter. Thus,
myfile.txt and Myfile.txt are two different files. Many admins
use all capital letters or an initial cap for directories and all
lower case letters for files. When this is done, directories and files
will be very visibly distinct in directory listings using just "ls".
The directories will all be grouped together at the beginning of the
listing. One problem with this is that URLs can become confusing
to the browser who may not know about case sensitivity and may
type in a bad URL such as "http://www.yourdomain.com/news" instead of
"http://www.yourdomain.com/News" with a capital "N".
- Deciding upon good
names and a good naming standard will save you time in the long run. For
example, it is better to name a log file Jan1998.log rather than
log12.txt. Remember that it may be two years before you return to the
directory and you will need to recall what the contents of the file are.
Further, other people may need to know what the files contain as well.
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And so another crucial task performed by
the operating system is the provision of a file system to
fully elaborate the file metaphor.
A file system is also an abstract data
structure used to store information. However, a file system
stores information about files (a metaphor to hold metaphors).
Typically, the file system
uses the file cabinet metaphor to describe how files are
stored within the bowels of the computer. Individual files are
grouped into file folders called directories. Directories
may contain files or sub-directories. Sub-directories can
contain more files or more sub-directories and so on,
and so on.
Like most modern operating systems,
UNIX defines an inverted tree file system emanating from a
single "root" directory. This is
shown below
![[File System]](Images/file_system.gif)
The
structure of the UNIX file system however, has some
general rules that you can use to navigate through
it. Specifically, it defines a set of generic
directories that hold a predictable set of files.
Each system administrator may add to or delete from
these standard directories, but it is a good bet that
you will see the following hierarchy on freshly installed
UNIX systems.
![[File System]](Images/unix_fs.gif)
Lets take a look at a few of the
more important ones from the perspective of a web
technician.
The "bin" Directory
This directory contains binary files
and executables that are considered basic to the use of the
system. We are going to discuss many of these commands
tomorrow such as "ls", "mv", or "grep". For the most
part, you do not need to access this directory yourself.
However, it is a good idea that you know where it is
located.
Since these executables are considered
basic, generally, all users are granted permission to
read or execute the files in "bin". We will talk more
about permissions later.
The "lib" Directory
This directory contains libraries for
any installed compilers. If you are writing CGI scripts in
C, you may be interested in making sure the libraries you
need can be found here.
The "tmp" Directory
This directory is used to store
temporary files created by users or applications. You
are free to use this directory, but make sure you do not
store anything that cannot be deleted since this
directory is often wiped out when the system is
rebooted.
The "usr" Directory
The usr directory is where you will
probably spend most of your time. The usr directory contains
files relevant to users whereas the rest of the directories we
have discussed are more oriented towards systems issues.
For example, if you are working with
an HTTPD web server, you are likely to find it in
"/usr/local/etc/httpd". Similarly, most home pages
will be based in the "/usr/home" directory. Finally,
supporting applications such as the perl interpreter
will be found in "/usr/bin".
Hold on, what does "/usr/local/etc/httpd"
mean? Well, "/usr/local/etc/httpd" is called a "path" and
is essentially the address of some file or directory. Let's
take a closer look at paths.
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