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Introduction to Unix – CS 21

Introduction to Unix – CS 21. Lecture 6. Lecture Overview. Homework questions More on wildcards Regular expressions Using grep Quiz #1. Homework Review. Any questions? Couple of points Timestamps are unreliable because touch can make them anything you want

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Introduction to Unix – CS 21

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  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, grep’s 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 b’s 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 • a|b • 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 • ( [0-9][^0-9]+$)|(^bc*) • Matches every line that ends with a word that starts with a number or begins with b followed by any number of c’s

  30. Working Example

  31. Tricks To Consider • Regular expressions will seem at first to match patterns that you don’t 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

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