1) What is Unix?

2) How do I get online help on Unix?

Unix systems come complete with an online help system which is available for all users. This online help is stored in a database of 'man' (manual) pages which are accessed using the 'man' command.

The 'man' command can be used to get help for a specific command, or to search the help database for pages containing a given keyword. For example:

man man

will display the manual pages for the 'man' command itself, and

man -k help

To change your Unix password, type 'passwd' at the command prompt. You will first be asked to authorize the operation by entering your current password. You are then asked to enter your new password and then to enter it again to confirm that you typed it correctly. This confirmation is required because none of the passwords you type will be displayed on the screen (for reasons of security), so you will not be able to see them.

A Unix filesystem has a hierarchical structure that can be visualized as an upside-down tree composed of 'directories'. At the top of the directory hierarchy is the 'root' directory, which is referred to as '/' (forward-slash). This directory contains many subdirectories which have names like 'usr', 'bin' and 'home'. Subdirectories can themselves contain other subdirectories and each directory can contain any number of files.

Your 'home' directory, the directory that contains your own personal files, resides at a specific node within this directory structure. Its location is defined by specifiying the list of all the subdirectories that are traversed to reach it, starting from the root directory. For example, if your username was 'foobar', your home directory might be specified by the following:

/home/foobar

The leading slash represents the root directory. Then 'home' indicates that you are within the 'home' subdirectory of the root, the directory that contains all user accounts. Within the 'home' directory is a subdirectory called 'foobar' that contains the files owned by user foobar.

You can extend the directory structure further by creating your own subdirectories within your home directory which you can use to contain different kinds of files (programs, text, coursework, etc.). For example, user foobar might create a subdirectory to hold files for their CS208 module and separate subdirectories within that to contain the files for each coursework. The full specification for the directory containing the files for the first coursework might then be:

You display a list of files by using the 'ls' (list) command. The default operation of the 'ls' command (ie. the action it performs if you just type 'ls' and press 'Enter') is to display the names of all files in your current directory in alphabetical order. You can tell 'ls' to display more information about your files by adding one or more extra arguments after 'ls'. You can also list the files in other directories by specifying the directory before pressing the 'Enter' key.

For example, to display all files in a subdirectory called 'bin', type:

ls bin

To display a long file-list containing more information about each file (owner, size, creation date, etc.) in the current directory, type:

ls -l

Finally, to display a long list of the files in the 'bin' subdirectory, type:

ls -l bin

You move to another directory using the 'cd' (change directory) command. To change to a subdirectory with your current directory called 'bin', you would type:

cd bin

To change back to your previous directory (ie. the 'parent' directory of 'bin'), you can type:

cd ..

Since all directories (except the root) have a single parent directory, the two dots are used as shorthand to refer to the parent of the current directory.

You can change back to your home directory by entering 'cd' on its own with no parameters.

You can create and remove as many sub-directories as you like in your home directory by using the 'mkdir' and 'rmdir' commands. For example, to create a new directory called 'CS208' in your current directory type:

mkdir CS208

You can also create sub-directories within your sub-directories to further sub-divide your files. To create a sub-directory within the 'CS208' directory called 'cw1', you could type:

mkdir CS208/cw1

or, alternatively:

cd CS208
mkdir cw1

Sub-directories can only be removed if they are empty, that is, if they contain no files or sub-directories. You could remove the 'CS208' and 'cw1' sub-directories created above by typing:

Files can be deleted using the 'rm' (remove) command. To delete a file called 'oldfile' in the current directory, type:

rm oldfile

You can also delete multiple files in one go, an operation you should only perform with great care. The following commands delete all files in the current directory and the 'tmp' subdirectory, respectively:

rm *
rm tmp/*

Sequences of commands that you need to enter frequently can be saved in a file called a shell 'script'. You can then instruct the shell to execute the all of the commands in the script by simply typing its filename.

A script is an ordinary text-file that has 'execute' permissions set. Simple scripts can be created using 'cat':

cat >myscript
rm ~/logfile.old
mv ~/logfile ~/logfile.old
touch ~/logfile
^D

Note: The tilde (~) is an alias for your home directory and the end of the file is indicated by typing Ctrl-D (^D).

More complex scripts can be created using 'vi' or some other text editor. See 'man sh' (or ksh, csh or bash) for more information about the functionality of the shell.

Once you have created your shell script, you set the execute permissions using the chmod ('change mode') command:

chmod u+x myscript

You can then run the script by typing its name:

There are special files for configuring Unix systems. These files automatically set variables and run programs when you log into the computer, or start a Unix shell. The lines in these files are just like the commands you type on the command-line. You can add lines to these files to customize your personal Unix environment (for example, to select a different default shell) so that it is done for you automatically when you log in. Different Unix shells use different initialization files: