Basics of UNIX

File and Directory Structure:

The Directory Structure of Unix/Linux based operating systems is Inverted Tree, where roots(/) are at the top and rest comes down heretically.  The file system looks like:

• / — the slash / character alone denotes the root of the virtual filesystem tree.

• /bin — stands for "binaries" and contains certain fundamental utilities, such as ls or cp, needed by all users.
• /sbin — stands for "system (or "superuser") binaries" and contains fundamental utilities, such as init, usually needed to start, maintain and recover the system.
• /etc — contains configuration files and system databases.
• /dev — stands for "devices". Contains file representations of peripheral devices.
• /home — contains the home directories for the users.
• /mnt — contains filesystem mount points.
• /lib — contains system libraries.
• /root — the home directory for the super-user root.
• /tmp — a place for temporary files. Many Unices clear this directory upon start up.
• /usr — originally the directory holding user home directories, its use has changed, and it now holds executables, libraries, and shared resources that are not system critical, like the X Window System, KDE, Perl, etc. (The name "Unix System Resources" is a post hoc backronym). However, on some UNIX systems, some user accounts may still have a home directory that is a direct subdirectory of /usr, such as the default as in Minix.
• /var — a short for "variable." A place for files that may change often.
• /proc — contains all processing data (Process information about a running operating system).
•  /opt — contains add-on software.
• /media — default mount point for removable devices.
• /srv — server data (data for services provided by system).
• /boot — contains all the important files which are required for successful booting process.
• /sys — contains information related to hardware.

For more information Please follow: http://en.wikipedia.org/wiki/Unix_directory_structure

Shortcuts of Shell

Following are the mail shortcuts that can be used in the K-shell of AIX for more productiveness:

• To navigate with the prev and next commands : ESC + j and ESC + k
• To navigate right to left and vice-versa in a command : ESC + h and ESC + l
• To inserts chars between a already written command: ESC + h/l (this is to navigate) + i (this is to insert). Remember that the INSERT key doesn’t work here on shell for inserting. 
• To auto-complete a file or directory name: ESC + \

VI editor

• VI is an Editor used to edit files in the UNIX based system. Following are the mostly used features of VI: 
• To open a file in VI editor : $vi <filename>
• Note: This will open the file if it is present in the current directory or else it will create the file if the directory is not mentioned.
• To insert in to the file once the file is open for editing : press i or press a or press INSERT Key
• To quit from the vi editor one need to come to normal mode from the insert mode. Use ESC key to come to normal mode from Insert mode.
• :q! --> to quit forcefully
• :w! --> to write the content of the last edit to the file, forcefully
• :wq! --> to write the content of the last edit to the file as well as quit, forcefully.
• Use Following commands in Normal mode:
• :set number --> to see the line numbers
• :set nonumber -->to remove the line numbering
• /<word> --> to search the word in the file

Basic Commands:

Following is the list of basic commands that can be used on almost all the UNIX based systems and enhances the productivity:

• $cd --> To come to users own home directory from any location
• $cd <directory path> --> To go to the specified directory path
• $cd .. --> To go one level up in the directory structure.
• $cd - --> To go the previous directory that one is working before working to the current directory.
• Note:Please note the SPACE between cd .. and cd –
• $touch <filename> --> Creates empty files, if the file is not present. If the file is present the access time stamp will be changed.
• $mkdir <dir-name> --> To create a directory.
• $mkdir –p <dir1/dir2/dir3> --> To create recursive directories. Using this dir2 is created in dir1 and dir3 is created in dir2.
• $rm <filename> --> To delete a file. One will be prompted yes or no when this command is used.
• $rm –rf <filename or dirname> --> To delete files or complete directories. It won’t prompt for yes or no before deleting the file or directory.

• $man <command> --> To get information about a command. For example $man grep will give all the information regarding the grep command.
• $cp <source> <destination> --> To copy a file or directory from source to destination.
• $mv <source> <destination> --> To move a file or directory form source to destination.
• $clear --> clears the prompt
• $cat <filename> --> to view the content of the file.
• $date --> prints the date on the prompt
• $echo “String” --> prints the String on the prompt.
• $grep <pattern> <filename> --> to search a pattern of strings within a filename.
• Note: The various options of grep can be easily understood using the man page ie $man grep.
• $cat <filename> | more --> To view a file which is more than one page long. RETURN (ENTER) key is used to scroll one line and SPACE key is used to scroll 1 Page.
• $more <filename> --> works same as the above command.

Shell and Shell Script

A UNIX shell is a command-line interpreter or shell that provides a traditional user interface for the UNIX based operating system and for Unix-like systems. The shell that a user is using can be found using

$echo $SHELL

/bin/ksh

This means that K-shell is used by the user.

The most widely used Shells are as follows:

• Korn Shell --> ksh
• Bourne Shell --> bsh
• Bourne Again Shell --> bash
• C- Shell --> Csh

For reading and referring to the Shell Scripting please follow the below mentioned links as it is a long topic and cannot be covered in this completely.

http://tldp.org/LDP/abs/html/index.html

Use of $

Following the table how $ can be used in a Shell Script.

$1 - $9	These variables are the positional parameters.
$0	The name of the command currently being executed.
$#	The number of positional arguments given to this invocation of the shell.
$?	The exit status of the last command executed is given as a decimal string.  When a command completes successfully, it returns the exit status of 0 (zero), otherwise it returns a non-zero exit status.
$$	The process number of this shell - useful for including in filenames, to make them unique.
$!	The process id of the last command ran in the background.
$-	The current options supplied to this invocation of the shell.
$*	A string containing all the arguments to the shell, starting at $1.
$@	Same as above, except when quoted.