Regular Expressions!

Search for particular items within a large body of text. eg. You may wish to identify all email addresses in some content using a text editor.
Replace particular items. eg. You may wish to clean up some poorly formatted HTML by replacing all uppercase tags with lowercase equivalents in a text editor.
Validate input. eg. You may want to check that a password meets certain criteria such as, a mix of uppercase and lowercase, digits and punctuation etc in a program you are writing.
Coordinate actions. eg. You may wish to process certain files in a directory, but only if they meet particular conditions, in work you are doing on the command line.
Reformat text. eg. You may export data from one program as a text file then modify its layout so you may import it into another program using a text editor.

So Where and How do I use Regular Expressions?

Regular expressions are a feature of many pieces of software and nearly all programming languages .

Learning Regular Expressions

Break the regular expression down into it’s individual components (So for instance, in the regular expression example above it would become b and [ia]).
Speak out aloud the steps of the expression. So for the above expression I might say: “First it matches a b, followed by either an i or an a” (This step may sound silly but trust me it works. You use different parts of your brain when you speak as opposed to think internally.)
Build the regular expression incrementally, testing as you go.

Summury

pwd

Print Working Directory - ie. Where are we currently. ls
List the contents of a directory. cd
Change Directories - ie. move to another directory. Relative path
A file or directory location relative to where we currently are in the file system. Absolute path A file or directory location in relation to the root of the file system.

More About Files!

Linux is an Extensionless System

The following are common extensions:

file.exe - an executable file, or program.
file.txt - a plain text file.
file.png, file.gif, file.jpg - an image.

Linux is Case Sensitive

This is very important and a common source of problems for people new to Linux.

Spaces in names

A space on the command line is how we seperate items.

file obtain information about what type of file a file or directory is. ls -a List the contents of a directory, including hidden files. Everything is a file under Linux Even directories. Linux is an extensionless system Files can have any extension they like or none at all. Linux is case sensitive Beware of silly typos.