[rfc791.ORG]
August Ada Byron was born December 10, 1815 the daughter of poet Lord Byron. Five weeks after Ada was born, Lady Byron asked for a seperation from Lord Byron, and was awarded sole custody of Ada, who she brought up to be a mathematician and scientist. Ada heard Charles Babbage's ideas for the Analytical Engine in November, 1834. Luigi Menabrea wrote a summary of what Babbage described, and published an article in French about the development. Ada, in 1843, married to the Earl of Lovelace, and the mother of three, translated Menabrea's article. When she showed Babbage her translation, he suggested she add her own notes, which turned out to be about three times the length of the original article. Letters between Charles and Ada flew back and forth. In her article, published in 1843, Lady Lovelace predicted that such a machine might be used to compose complex music, to produce graphics, and would be used for both practical and scientific use. Ada suggested to Babbage writing a plan for how the engine might be used to calculate Bernoulli numbers. This plan is now considered as the first "computer program". A software language developed by the US DOD was named "ADA" in her honor in 1979.

[ rfc791.ORG : Unix Help : The unix file system ]

Every operating system includes a facility to store documents you create, programs configuration information, etc. This information is stored in logical units called files, and the facility that stores them is called the file system.

The unix file system has provisions for a special kind of file called a directory. Directories are special because, although they cannot contain any data themselves, they can contain other files (including other directories). If you are familiar with the Windows or Macintosh operating systems, you may know about directories already, although you may refer to them as folders. Windows and MacOS borrowed the idea of directories from unix.

Every process in unix has a current working directory. This includes your shell. Files you create will be stored in your current working directory, unless you explicitly specify for them to be stored elsewhere. When you first log in to a unix system, your current working directory is your home directory, a special directory set aside for you to put your files in.

To find out what your shell's current working directory is, you can use the pwd (print working directory) command.
Monolith[100]% pwd
/home/brad
This tells me that my current working directory is the directory called `brad' inside the directory called home. This happens to be my home directory.

To find the names of the files in your current working directory, you can use the ls command.
Monolith[101]% ls
the		contents	of		my		home
directory	is		enormous	but		normally
all		the		files		in		it
would		list		here
To change your current working directory you can use the `cd' command.
Monolith[102]% cd /usr/bin
This changes my working directory to the directory `bin' inside the directory `usr'. I could also have typed:
Monolith[103]% cd /usr
to get to the `usr' directory, and
Monolith[104]% cd bin
to get to the bin directory inside the current working directory, which is now `usr' (because of the last command I typed). The entire filesystem starts at the root directory (/). This is why I put a slash in front of `usr', but not in front of `bin': I wanted to change my working directory to the bin directory inside the usr directory inside the root directory. /usr/bin happens to be where most of the programs that come with the unix distribution are.

Each directory contains two special entries . and .., which refer to the current directory and the parent directory (the parent is the directory that contains the current directory). So to change your working directory to the directory that contains your current working directory, you could type
Monolith[106]% cd ..
To get back to my home directory, I could type
Monolith[106]% cd /home/brad
or I could simply type
Monolith[107]% cd
When you save a file, you choose a name for that file. The file system stores the data you're saving and marks it with the filename you chose. It also marks the file with the date and time you modified it. If the filename you chose contains no slashes, the file is stored in your current working directory. Otherwise it's stored in the directory you've specified. For example: Two commands exist for copying and moving files around in the file system, cp and mv.
Monolith[108]% cp rock.mp3 /fileshare/mp3/
this copies the file rock.mp3 from the current working directory to the mp3 directory inside the fileshare directory.
Monolith[109]% mv rock.mp3 /fileshare/mp3/
This moves the file rock.mp3 from the current working directory to the mp3 directory inside the fileshare directory. rock.mp3 no longer exists in the current working directory.
Monolith[110]% cp /fileshare/mp3/rock.mp3 .
This copies the file rock.mp3 from the mp3 directory inside the fileshare directory to "." (the current working directory).

Filenames are case sensitive in unix, unlike MacOS and Windows, so MyFile, myFile, myfile, MyFiLe and MYFILE are all different files. A filename must be unique to the directory the file is in.

A file can have multiple names in the file system. You can create a new name for a file by using the ln command.
Monolith[111]% ln myphonebook /fileshare/bradsphonebook
Creates a file in /fileshare called bradsphonebook that contains the same data as the file myphonebook in the current working directory.
You can also create soft links, which are a special kind of file that link to another file (like Windows shortcuts or MacOS aliases). These are usually preferrable to hard links (the type mentioned before) because you can't hard-link a directory, nor can you hard link a file on one disk to another.
Monolith[112]% ln -s /fileshare .
This creates a softlink to the directory /fileshare in the current working directory (also called fileshare, since I didn't specify a different name).

Unix-style file systems store several bits of information besides file names and modification dates: You can see these attributes by using the -l command line option to ls:
Monolith[113]% ls -l
total 33
drwxr-xr-x   2 root  wheel    512 Oct  4 08:59 fs
-rw-r--r--   1 root  wheel   9012 Oct  2 16:49 index.html
-rw-r--r--   1 brad  users  21137 Oct  2 13:42 kenden.jpg
Monolith[114]%
The first entry is for `fs'. The `d' in the drwxr-xr-x means that `fs' is a directory. The `rwx' means `fs' is readable, writable, and executable by the owning user. The `r-x' right after that means `fs' is readable and executable, but not writable, by the owning group. The final `r-x' means `fs' is readable and executable, but not writable by everyone else.

These attribute bits actually mean something different on files versus directories:

Attribute Regular Files Directories
Read File can be opened and copied. Contents of directory can be listed (eg, with ls).
Write File can be overwritten/modified or deleted. Contents of directory can be altered.
Execute File can be executed (a program or shell script). Files inside directory (that have read permission enabled) can be opened.

Following that, the `root' means the file is owned by the user `root'. `root' is the system administrator on any given unix system. The group that owns the file comes next, `wheel'. Following that is the last modification date and time. Unlike DOS and Windows, unix does not store the creation date and time of a file.
All these attributes (with the exception of the modification time) can be modified. To change the owner of a file, you can use the `chown' command.
Monolith[115]% chown brad.users kenden.jpg
This makes the file `kenden.jpg' owned by user `brad' and group `users'. You can only change the ownership of a file if you are root.

You can also change permissions of files using the `chmod' command. Adding and removing permissions on a file is quite easy using this command. The first parameter to chmod tells chmod who to add or remove permissions for, and following a + or -, what permissions to add or remove. In this first parameter, `u' means owning user, `g' means owning group, and `o' means other users. r, w, and x mean read, write and execute, respecively.
Monolith[116]% chmod o-w kenden.jpg
This removes write permission on the file `kenden.jpg' for all users other than the owner and the members of the owning group.
Monolith[117]% chmod u+rw kenden.jpg
This adds read and write permission for the owner of the file.
Monolith[118]% chmod g+r-w kenden.jpg
This adds read permission and removes write permission for members of the owning group.

If you happen to be well versed in the octal numbering system, you can change the modes of files using octal numbers:
Monolith[119]% chmod 0644 kenden.jpg
This makes kenden.jpg readable and writable by the owner, and read-only to everyone else.

The second octal digit (the six) is the permissions for the owner. The third (the first four) is for the owning group, and the fourth (the secnd four) is for everyone else. If you think about which bits are set in each number, it makes sense:
octalbinarymeaning
0100000 001 000 000file executable/ directory browsable by owner (unreadable executable files aren't very useful)
0200000 010 000 000writable by owner
0300000 011 000 000writable and executable by owner
0400000 100 000 000readable by owner
0500000 101 000 000readable and executable by owner
0600000 110 000 000readable and writable by owner
0700000 111 000 000executable and readable and writable by owner
0050000 000 101 000readable and executable by group
0005000 000 000 101readable and executable by everyone else
0644000 110 100 100readable and writable by owner, read only for everyone else
and so on and so forth ...

The first octal digit (the zero) is for special purpose stuff like making directories sticky (ie, anyone can write to them but noone can delete anyone else's files or change them), or making programs set-user-id (meaning that the program runs with the permissions of the owner). See chmod(1) in the manual for more details.