grep
  
  All I will say is that it depends on your definition of 'hacking'. The
  following will increase your power in working with a Unix based system.
  
  Grep is from a family of commands: grep, egrep, and fgrep. They all search 
  the named input files (or standard input if no files are named) for lines
  containing a match to the given pattern. Each of the grep commands are
  basically the same, the only real difference is that egrep uses a slightly
  different syntax for its pattern matching, whereas fgrep uses fixed
  strings. There is also another member to the grep family, and that is
  zgrep. Zgrep is used to search compressed files and is invoked the same way
  as grep. In this text I will be detailing grep, and I feel that it is easier
  to learn and understand by seeing examples, so I hope to provide alot of
  usefull ones :)
  
  For examples I will be using a list of Bauhaus songs. Just cut and paste
  the following to a file and name it bauhaus.txt
  
  ----cut here----
  The passion of lovers
  Bela Lugosi's dead
  She's in parties
  Ziggy stardust
  Wasp
  Hope
  King Volcano
  The sanity assassin
  Terror couple hill colonel
  ----cut here----
  
  The syntax for grep is as follows:
  
    grep [options] pattern [file]
    
  Usefull options:
  
    -c counts number of matching lines
    -i ignore caps
    -n includes the line number
    -s suppress error messages
    -v lines NOT mattching the pattern
    
  A simple example:
  
    #grep -c Z bauhaus.txt
    1
    
  The above statement counts how many lines contain the letter Z (case
  sensitive) and displays the result. If I typed the following, it will
  display the lines:
  
    #grep Z bauhaus.txt
    Ziggy stardust

  With the added option -v, lines NOT matching will be counted:
  
    #grep -vc Z bauhaus.txt
    8
    
  and displayed:
  
    #grep -v Z bauhaus.txt
    The passion of lovers
    Bela Lugosi's dead
    She's in parties
    Wasp
    Hope
    King Volcano
    The sanity assassin
    Terror couple hill colonel
    
  displayed and line numbered:
  
    #grep -vn Z bauhaus.txt
    1:The passion of lovers
    2:Bela Lugosi's dead
    3:She's in parties
    5:Wasp
    6:Hope
    7:King Volcano
    8:The sanity assassin
    9:Terror couple hill colonel
    
  Options can be mixed like any other command.
    
  Regular expressions are used to provide grep with expressions whcih set
  locations of patterns and ranges of characters (all regular expressions
  must be quoted). The hat (^) means start of line, and the dollar ($) means
  the end of the line.
  
  To display lines ending with 's'
  
    #grep 's$' bauhaus.txt
    The passion of lovers
    She's in parties
  
  To display lines not ending in 's' and also number them:
  
    #grep -vn 's$' bauhaus.txt
    2:Bela Lugosi's dead
    4:Ziggy stardust
    5:Wasp
    6:Hope
    7:King Volcano
    8:The sanity assassin
    9:Terror couple hill colonel
  
  The full stop (.) represents a single character wildcard. eg the following
  will display any line that has any character before the 'e':
  
    #grep '.e' bauhaus.txt
    The passion of lovers
    Bela Lugosi's dead
    She's in parties
    Hope
    The sanity assassin
    Terror couple hill colonel
    
  More examples:
  
    #grep -i '.L' bauhaus.txt     - any case, with any character/s before 'L'
    #grep 'V.....o' bauhaus.txt   - V, any 7 characters, then o
  
  The square brackets ([]) specify any one of the characters enclosed. eg, to
  display the lines beginning with 'T', 'W' or 'Z':
  
    #grep '^[TWZ]' bauhaus.txt
    The passion of lovers
    Ziggy stardust
    Wasp
    The sanity assassin
    Terror couple hill colonel
    
  For a range of characters, use a hyphen:
  
    #grep '^[A-J] bauhaus.txt
    Bela Lugosi's dead
    Hope
    
  More examples:
  
    #grep '^[A-Za-z0-9] bauhaus.txt - all letters / numbers
    #grep '[0-9]$' bauhaus.txt - ending with a number
    #grep -v '[a-m]$' bauhaus.txt - lines that dont end with a-m
    
  When the hat (^) is used in the square brackets it means 'not'. eg the 
  following will show lines not beginning with 'A' to 'G':
  
    #grep '^[^A-G]' bauhaus.txt
    The passion of lovers
    She's in parties
    Ziggy stardust
    Wasp
    King Volcano
    The sanity assassin
    Terror couple hill colonel
    
  A wildcard can also be used (*). eg the following will display lines
  beginning with 'T' and ending with 's'
  
    #grep '^T.*s$' bauhaus.txt
    The passion of lovers

  The following will display lines beginning with 'M' to 'Z' and ending
  in 's' or 't':
  
    #grep '^[M-Z].*[st]$' bauhaus.txt
    The passion of lovers
    She's in parties
    Ziggy stardust


  The above was just an introduction to grep, there is a myrid of other 
  statements, redirections (>>) and piping (|) that can be done using it.
  From the above, you should now be able to do alot of sorting, extracting,
  and removing from logs ALOT easier now ;) 

  (grep -v <ip> /var/log/messages >> /var/log/messages.2)