Introduction to Unix CS 21

1
Introduction to Unix CS 21

• Lecture 6

2
Lecture Overview

• Homework questions
• More on wildcards
• Regular expressions
• Using grep
• Quiz 1

3
Homework Review

• Any questions?
• Couple of points
• Timestamps are unreliable because touch can make
  them anything you want
• The different compression algorithms work better
  on different types of data

4
More Thorough Explanation Of Wildcards For The
Shell

• Match zero or more characters of any type
• By itself, will match everything in the current
  directory
• ?
• Matches exactly one character of any type
• Character sets and range
• Will match one character in the set
• abh 0-4 a-kL-M

5
Examples
6
Examples Of ? Usage
7
Examples Of Character Range Usage
8
grep Global / Regular Expressions / Pattern

• Searches the internals of files and tries to
  match patterns
• Used to see if a file contains data you are
  looking for
• Will print out every line that contains a match
  for that pattern
• Usage grep OPTIONS pattern FILE

9
Common Flags

• -i
• Case insensitive (upper and lower cases are
  treated the same)
• -n
• Print out the line numbers
• -r
• Recursively traverse the directory
• -v
• Invert the results (show all non-matching lines)

10
Easiest grep Usage

• The easiest way to use grep is also the most
  common way to use grep
• Search files for occurrences of a string (word)
• The pattern you search for can simply be a word

11
First grep Examples
12
Regular Expressions

• grep can be used to find words that match a
  certain pattern, not just a given word
• The language of regular expressions is used to
  describe these patterns
• This includes wildcards, repetitions, and complex
  patterns

13
How grep Views Regular Expressions

• Unfortunately, greps regular expressions are
  completely different than the shell wildcards
• Some of the symbols are the same, but they are
  used in different ways
• Always use quotes () so that the wildcards are
  interpreted by grep and not the shell

14
Notation

• Beginning of the line left rooted
• End of the line right rooted
• .
• Any single character
• xy
• Any character in the set
• a-z
• Any character not in the set
• B
• Zero or more occurrences of B

15
Examples

• .
• Zero or more of any character
• Will match any pattern
• ab
• Any line that starts with a and has zero or more
  bs immediately following
• a
• abbbb
• abb

16
Working Examples
17
More Examples

• 0-9
• Any number
• 1002
• 0909
• bye
• The pattern bye located at the end of the line
• Hello and goodbye

18
Working Examples
19
One More Slide Of Examples

• g
• Match any line that does not end in g
• alpha
• Any word that contains zero or more alphabetic
  characters

20
Wait a minute
21
Inverting The Answers

• grep v this testFile
• Will find all lines that do not contain the word
  this
• Works exactly the same with regular expressions
• grep v g testFile
• Finds all lines that end in g

22
Working Example
23
grep Versus egrep

• In order to match patterns more specifically
  (instead of zero or more matches as we saw
  previously), we need egrep
• egrep stands for Extended grep

24
Additional egrep Symbols

• a?
• Zero or one occurrence of a
• a
• One or more occurrences of a
• ab
• a or b
• ()
• Used for nesting

25
Advanced Examples

• word?
• Will match wor, or word
• 01
• Will match any binary number
• 0-9 a-f
• Will match any number or any word with only a, b,
  c, d, e, or f

26
Working Example
27
More Advanced Examples

• a-z a-z? a-z
• Will match any two words that may or may not have
  a single character between them

28
Working Example
29
Example With Everything On It

• ( 0-90-9)(bc)
• Matches every line that ends with a word that
  starts with a number or begins with b followed by
  any number of cs

30
Working Example
31
Tricks To Consider

• Regular expressions will seem at first to match
  patterns that you dont want
• Think about spaces
• Think about zero occurrences
• Think about just one occurrence
• Making regular expressions do what you want is
  not easy!

32
Why Would You Ever Want To Use This?

• Most of the time, you can get by just using grep
  and searching for a specific word
• Searching for all instances of well formatted
  data requires regular expressions and egrep

33
In Lab Today

• File redirection and piping practice
• Creating regular expressions and pattern matching
  with grep and egrep
• Some applications where it will be useful