Grep Pocket Reference (John Bambenek, Agnieszka Klus) (Z-Library)

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grep Pocket Reference John Bambenek and Agnieszka Klus Beijing • Cambridge • Farnham • Köln • Sebastopol • Taipei • Tokyo

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grep Pocket Reference by John Bambenek and Agnieszka Klus Copyright © 2009 John Bambenek and Agnieszka Klus. All rights reserved. Printed in Canada. Published by O’Reilly Media, Inc., 1005 Gravenstein Highway North, Se- bastopol, CA 95472. O’Reilly books may be purchased for educational, business, or sales promo- tional use. Online editions are also available for most titles (http://safari .oreilly.com). For more information, contact our corporate/institutional sales department: (800) 998-9938 or corporate@oreilly.com. Editor: Isabel Kunkle Copy Editor: Genevieve d’Entremont Production Editor: Loranah Dimant Proofreader: Loranah Dimant Indexer: Joe Wizda Cover Designer: Karen Montgomery Interior Designer: David Futato Printing History: January 2009: First Edition. Nutshell Handbook, the Nutshell Handbook logo, and the O’Reilly logo are registered trademarks of O’Reilly Media, Inc. grep Pocket Reference, the im- age of an elegant hyla tree frog, and related trade dress are trademarks of O’Reilly Media, Inc. Many of the designations used by manufacturers and sellers to distinguish their products are claimed as trademarks. Where those designations appear in this book, and O’Reilly Media, Inc. was aware of a trademark claim, the designations have been printed in caps or initial caps. While every precaution has been taken in the preparation of this book, the publisher and authors assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. ISBN: 978-0-596-15360-1 [TM] 1231511981

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Contents grep Pocket Reference 1 Introduction 1 Conceptual Overview 5 Introduction to Regular Expressions 7 grep Basics 24 Basic Regular Expressions (grep or grep -G) 27 Extended Regular Expressions (egrep or grep -E) 38 Fixed Strings (fgrep or grep -F) 41 Perl-Style Regular Expressions (grep -P) 43 Introduction to grep-Relevant Environment Variables 49 Choosing Between grep Types and Performance Considerations 54 Advanced Tips and Tricks with grep 57 References 67 Index 69 v

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grep Pocket Reference Introduction Chances are that if you’ve worked for any length of time on a Linux system, either as a system administrator or as a devel- oper, you’ve used the grep command. The tool is installed by default on almost every installation of Linux, BSD, and Unix, regardless of distribution, and is even available for Windows (with wingrep or via Cygwin). GNU and the Free Software Foundation distribute grep as part of their suite of open source tools. Other versions of grep are distributed for other operating systems, but this book focuses primarily on the GNU version, as it is the most prevalent at this point. The grep command lets the user find text in a given file or out- put quickly and easily. By giving grep a string to search for, it will print out only lines that contain that string and can print the corresponding line numbers for that text. The “simple” use of the command is well-known, but there are a variety of more advanced uses that make grep a powerful search tool. 1

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The purpose of this book is to pack all the information an ad- ministrator or developer could ever want into a small guide that can be carried around. Although the “simple” uses of grep do not require much education, the advanced applications and the use of regular expressions can become quite complicated. The name of the tool is actually an acronym for “Global Regular- Expression Print,” which gives an indication of its purpose. GNU grep is actually a combination of four different tools, each with its unique style of finding text: basic regular expressions, extended regular expressions, fixed strings, and Perl-style reg- ular expression. There are other implementations of grep-like programs such as agrep, zipgrep, and “grep-like” functions in .NET, PHP, and SQL. This guide will describe the particular options and strengths of each style. The official website for grep is http://www.gnu.org/software/ grep/. It contains information about the project and some brief documentation. The source code for grep is only 712 KB, and the current version at the time of this writing is 2.5.3. This pocket reference is current to that version, but the information will be generally valid for earlier and later versions. As an important note, the current version of grep that ships with Mac OS X 10.5.5 is 2.5.1; however, most of the options in this book will still work for that version. There are other “grep” programs as well, in addition to the one from GNU, and these are typically the ones installed by default under HP-UX, AIX, and older versions of Solaris. For the most part, the reg- ular expression syntax is very similar between these versions, but the options differ. This book deals exclusively with the GNU version because it is more robust and powerful than other versions. Conventions Used in This Book The following typographical conventions are used in this book: 2 | grep Pocket Reference

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Italic Indicates commands, new terms, URLs, email addresses, filenames, file extensions, pathnames, directories, and Unix utilities. Constant width Indicates options, switches, variables, attributes, keys, functions, types, classes, namespaces, methods, modules, properties, parameters, values, objects, events, event han- dlers, XML tags, HTML tags, macros, the contents of files, or the output from commands. Constant width italic Shows text that should be replaced with user-supplied values. Using Code Examples This book is here to help you get your job done. In general, you may use the code in this book in your programs and docu- mentation. You do not need to contact us for permission unless you’re reproducing a significant portion of the code. For ex- ample, writing a program that uses several chunks of code from this book does not require permission. Selling or distributing a CD-ROM of examples from O’Reilly books does require per- mission. Answering a question by citing this book and quoting example code does not require permission. Incorporating a significant amount of example code from this book into your product’s documentation does require permission. We appreciate, but do not require, attribution. An attribution usually includes the title, author, publisher, and ISBN. For ex- ample: “grep Pocket Reference by John Bambenek and Agnieszka Klus. Copyright 2009 John Bambenek and Agnieszka Klus, 978-0-596-15360-1.” If you feel your use of code examples falls outside fair use or the permission given here, feel free to contact us at permissions@oreilly.com. Introduction | 3

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Safari® Books Online When you see a Safari® Books Online icon on the cover of your favorite technology book, that means the book is available online through the O’Reilly Network Safari Bookshelf. Safari offers a solution that’s better than e-books. It’s a virtual library that lets you easily search thousands of top tech books, cut and paste code samples, download chapters, and find quick answers when you need the most accurate, current informa- tion. Try it for free at http://safari.oreilly.com. Comments and Questions Please address comments and questions concerning this book to the publisher: O’Reilly Media, Inc. 1005 Gravenstein Highway North Sebastopol, CA 95472 800-998-9938 (in the United States or Canada) 707-829-0515 (international or local) 707-829-0104 (fax) We have a web page for this book, where we list errata, exam- ples, and any additional information. You can access this page at: http://www.oreilly.com/catalog/9780596153601 To comment or ask technical questions about this book, send email to: bookquestions@oreilly.com For more information about our books, conferences, Resource Centers, and the O’Reilly Network, see our website at: http://www.oreilly.com 4 | grep Pocket Reference

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Acknowledgments From John Bambenek I would like to thank Isabel Kunkle and the rest of the O’Reilly team behind the editing and production of this book. My wife and son deserve thanks for their support and love as I comple- ted this project. My coauthor, Agnieszka, has been invaluable in making an onerous task of writing a book more manageable; she contributed greatly to this project. Brian Krebs of The Washington Post deserves credit for the idea of writing this book. My time at the Internet Storm Center has let me work with some of the best in the information security industry, and their feedback has been extremely helpful during the technical review process. A particular note of thanks goes out to Charles Hamby, Mark Hofman, and Donald Smith. And last, Merry Anne’s Diner in downtown Champaign, Illinois deserves thanks for letting me show up for hours in the middle of the night to take up one of their tables as I wrote this. From Agnieszka Klus First, I want to thank my coauthor, John Bambenek, for the opportunity to work on this book. It certainly has been a lit- erary adventure for me. It has opened windows of opportunity and given me a chance to peek into a world I would otherwise have not been able to. I also would like to thank my family and friends for their support and patience. Conceptual Overview The grep command provides a variety of ways to find strings of text in a file or stream of output. For example, it is possible to find every instance of a specified word or string in a file. This could be useful for grabbing particular log entries out of volu- minous system logs, as one example. It is possible to search for certain patterns in files, such as the typical pattern of a credit card number. This flexibility makes grep a powerful tool for Conceptual Overview | 5

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finding the presence (or absence) of information in files. There are two ways to provide input to grep, each with its own par- ticular uses. First, grep can be used to search a given file or files on a system. For instance, files on a disk can be searched for the presence (or absence) of specific content. grep also can be used to send output from another command that grep will then search for the desired content. For instance, grep could be used to pick out important information from a command that otherwise produces an excessive amount of output. While searching text files, grep could be employed to search for a particular string throughout all files in an entire filesystem. For instance, Social Security numbers follow a known pattern, so it is possible to search every text file on a system to find occurrences of these numbers in its files (e.g., for academic environments in order to comply with federal privacy laws). The default behavior is to return the filename and the line of text that contains the string, but it is possible to include line numbers as well. Additionally, grep can examine command output to look for occurrences of a string. For instance, a system administrator may run a script to update software on a system that has a large amount of “debugging” information and may only care to see error messages. In this case, the grep command could search for a string (i.e., “ERROR”) that indicates errors, filtering out information that the administrator does not want to see. Generally, the grep command is designed to search only text output or text files. The command will let you search binary (or other nontext) files, but the utility is limited in that regard. Tricks for searching binary files for information with grep (i.e., using the strings command) are covered in the last section (“Advanced Tips and Tricks with grep” on page 57). Although it is usually possible to integrate grep into manipu- lating text or doing “search and replace” operations, it is not the most efficient way to get the job done. Instead, the sed and awk programs are more useful for these kinds of functions. 6 | grep Pocket Reference

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There are two basic ways to search with grep: searching for fixed strings and searching for patterns of text. Searching for fixed strings is pretty straightforward. Pattern searching, how- ever, can get complicated very quickly, depending on how var- iable that desired pattern is. To search for text with variable content, use regular expressions. Introduction to Regular Expressions Regular expressions, the source of the letters “re” in “grep,” are the foundation for creating a powerful and flexible text- processing tool. Expressions can add, delete, segregate, and generally manipulate all kinds of text and data. They are simple statements that enhance a user’s ability to process files, espe- cially when combined with other commands. If applied prop- erly, regular expressions can significantly simplify a tall task. Many different commands in the Unix/Linux world use some form of regular expressions in addition to some programming languages. For instance, the sed and awk commands use regu- lar expressions not only to find information, but also to manipulate it. There are actually many different varieties of regular expres- sions. For instance, Java and Perl both have their own syntax for regular expressions. Some applications have their own ver- sions of regular expressions, such as Sendmail and Oracle. GNU grep uses the GNU version of regular expressions, which is very similar (but not identical) to POSIX regular expressions. In fact, most of the varieties of regular expressions are very similar, but they do have key differences. For instance, some of the escapes, metacharacters, or special operators will behave differently depending on which type of regular expressions you are using. The subtle differences between the varieties can lead to drastically different results when using the same expression under different regular expression types. This book will only touch on the regular expressions that are used by grep and Perl- style grep (grep -P). Introduction to Regular Expressions | 7

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Usually, regular expressions are included in the grep command in the following format: grep [options] [regexp] [filename] Regular expressions are comprised of two types of characters: normal text characters, called literals, and special characters, such as the asterisk (*), called metacharacters. An escape sequence allows you to use metacharacters as literals or to identify special characters or conditions (such as word boun- daries or “tab characters”). The desired string that someone hopes to find is a target string. A regular expression is the par- ticular search pattern that is entered to find a particular target string. It may be the same as the target string, or it may include some of the regular expression functionality discussed next. Quotation Marks and Regular Expressions It is customary to place the regular expression (or regxp) inside single quotation marks (the symbol on the keyboard under- neath the double quote, not underneath the tilde [~] key). There are a few reasons for this. The first is that normally Unix shells interpret the space as an end of argument and the start of a new one. In the format just shown, you see the syntax of the grep command where a space separates the regexp from the filename. What if the string you wish to search for has a “space” character? The quotes tell grep (or another Unix command) where the argument starts and stops when spaces or other spe- cial characters are involved. The other reason is that various types of quotes can signify different things with shell commands such as grep. For in- stance, using the single quote underneath the tilde key (also called the backtick) tells the shell to execute everything inside those quotes as a command and then use that as the string. For instance: grep `whoami` filename would run the whoami command (which returns the username that is running the shell on Unix systems) and then use that 8 | grep Pocket Reference

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string to search. For instance, if I were logged in with username “bambenek”, grep would search filename for the use of “bambenek”. Double quotes, however, work the same as the single quotes, but with one important difference. With double quotes, it be- comes possible to use environment variables as part of a search pattern: grep "$HOME" filename The environment variable HOME is normally the absolute path of the logged-in user’s home directory. The grep command just shown would determine the meaning of the variable HOME and then search on that string. If you place $HOME in single quotes, it would not recognize it as an environment variable. It is important to craft the regular expression with the right type of quotation marks because different types can yield wildly different results. Beginning and ending quotes must be the same or an error will be generated, letting you know that your syntax is incorrect. Note that it is possible to combine the use of different quotation marks to combine functionality. This will be discussed later in the section “Advanced Tips and Tricks with grep” on page 57. Metacharacters In addition to quotation marks, the position and combination of other special characters produce different effects on the reg- ular expression. For example, the following command searches the file name.list for the letter ‘e’ followed by ‘a’: grep -e 'e[a]' name.list But by simply adding the caret symbol, ^, you change the entire meaning of the expression. Now you are searching for the ‘e’ followed by anything that is not the letter ‘a’: grep -e 'e[^a]' name.list Introduction to Regular Expressions | 9

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Since metacharacters help define the manipulation, it is im- portant to be familiar with them. Table 1 has a list of regularly used special characters and their meanings. Table 1. Regular expression metacharactersa Metacharacter Name Matches Items to match a single character . Dot Any one character [...] Character class Any character listed in brackets [^...] Negated character class Any character not listed in brackets \char Escape character The character after the slash literally; used when you want to search for a “special” char- acter, such as “$” (i.e., use “\$”) Items that match a position ^ Caret Start of a line $ Dollar sign End of a line \< Backslash less-than Start of a word \> Backslash greater- than End of a word The quantifiers ? Question mark Optional; considered a quantifier * Asterisk Any number (including zero); sometimes used as general wildcard + Plus One or more of the preceding expression {N} Match exactly Match exactly N times {N,} Match at least Match at least N times {min,max} Specified range Match between min and max times Other | Alternation Matches either expression given - Dash Indicates a range (...) Parentheses Used to limit scope of alternation 10 | grep Pocket Reference

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Metacharacter Name Matches \1, \2, ... Backreference Matches text previously matched within pa- rentheses (e.g., first set, second set, etc.) \b Word boundary Batches characters that typically mark the end of a word (e.g., space, period, etc.) \B Backslash This is an alternative to using “\\” to match a backslash, used for readability \w Word character This is used to match any “word” character (i.e., any letter, number, and the underscore character) \W Non-word character This matches any character that isn’t used in words (i.e., not a letter, number, or underscore) \` Start of buffer Matches the start of a buffer sent to grep \' End of buffer Matches the end of a buffer sent to grep a From Jeffrey E.F. Friedl’s Mastering Regular Expressions (O’Reilly), with some additions The table references something known as the escape character. There are times when you will be required to search for a literal character that is usually used as a metacharacter. For example, suppose you are looking for amounts that contain the dollar sign within price.list: grep '[1-9]$' price.list As a result, the search will try to match the numbers at the end of the line. This is certainly something you do not want. By using the escape character, annotated by the backslash (\), you avoid such confusion: grep '[1-9]\$' price.list The metacharacter $ becomes a literal, and therefore is searched in price.list as a string. For instance, take a text file (price.list) that has the following content: Introduction to Regular Expressions | 11

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123 123$ Using the two commands just shown yields the following results: $ grep '[1-9]\$' price.list 123$ $ grep '[1-9]$' price.list 123 In the first example, the command looked for the actual dollar- sign character. In the second example, the dollar sign had its special metacharacter’s meaning and matched the end of line, and so would match only those lines that ended in a number. The meaning of these special characters needs to be kept in mind because they can make a significant difference in how a search is processed. Here is a brief rundown of the regular expression metachar- acters, along with some examples to make it clear how they are used: . (any single character) The “dot” character is one of the few types of wildcards available in regular expressions. This particular wildcard will match any single character. This is useful if a user wishes to craft a search pattern with some characters in the middle of it that are not known to the user. For in- stance, the following grep pattern would match “red”, “rod”, “red”, “rzd”, and so on: 'r.d' This “dot” character can be used repeatedly at whatever interval is necessary to find the desired content. [...] (character class) The “character class” tool is one of the more flexible tools, and it comes up again and again when using regular ex- pressions. There are two basic ways to use character classes: to specify a range and to specify a list of characters. An important point is that a character class will match only one character: 12 | grep Pocket Reference

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'[a-f]' '[aeiou]' The first pattern will look for any letter between “a” and “f”. Ranges can be uppercase letters, lowercase letters, or numbers. A combination of ranges can also be used, for instance, [a-fA-F0-5]. The second example will search for any of the given characters, in this case vowels. A character class can also include a list of special characters, but they can’t be used as a range. [^...] (negation) The “negation” character class allows a user to search for anything but a specific character or set of characters. For instance, a user who doesn’t like even numbers could use the following search pattern: '..[^24680]' This will look for any three-character pattern that does not end in an even number. Any list or range of characters can be placed inside a negated character class. \ (escape) The “escape” is one of the metacharacters that can have multiple meanings depending on how it is used. When placed before another metacharacter, it signifies to treat that character as the literal symbol instead of its special meaning. (It also can be used in combination with other characters, such as b or ', to convey a special meaning. Those specific combinations are covered later.) Take the following two examples: '.' '\.' The first example would match any single character and would return every piece of text in a file. The second ex- ample would only match the actual “period” character. The escape tells the regular expression to ignore the metacharacter’s special meaning and process it normally. Introduction to Regular Expressions | 13

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^ (start of line) When a carat is used outside of a character class, it no longer means negation; instead, it means the beginning of a line. If used by itself, it will match every single line on the screen because each line has a beginning. More useful is when a user wishes to match lines of text that begin with a certain pattern: '^red' This pattern would match all lines that begin with “red”, not just the ones that contain the word “red”. This is use- ful for structured communication or programming lan- guages, for example, where lines may begin with specific strings that contain important information (such as #DEFINE in C). However, the meaning is lost if it is not at the beginning of a line. $ (end of line) As discussed earlier, the dollar sign character matches the end of a line. Used alone, it will match every line in a stream except the final line, which is terminated by an “end of file” character instead of an “end of line” charac- ter. This is useful for finding strings that have a desired meaning at the end of a line. For instance: '-$' would find all lines whose last character is a dash, as is typical for words that are hyphenated when they are too long to fit on one line. This expression would find only those lines with hyphenated words split between lines. \< (start of word) If a user wished to craft a search pattern that matches based on the start of a word and the pattern was likely to recur inside a word (but not at the beginning), this par- ticular escape could be used. For instance, take the fol- lowing example: '\<un' 14 | grep Pocket Reference

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This pattern would match words starting with the prefix “un”, such as “unimaginable,” “undetected,” or “under- valued.” It would not match words such as “funding,” “blunder,” or “sun.” It detects the beginning of a word by looking for a space or another “separation” that indicates the beginning of a new word (a period, comma, etc.). \> (end of word) Similar to the previous escape, this one will match at the end of a word. After the characters, it looks for a “sepa- ration” character that indicates the end of a word (a space, tab, period, comma, etc.). For example: 'ing\>' would match words that end in “ing” (e.g., “spring”), not words that simply contain “ing” (e.g., “kingdom”). * (general wildcard) The asterisk is probably by far the most-used metachar- acter. It is a general wildcard classed as a quantifier that is specifically used for repetitious patterns. For some metacharacters, you can assign minimum and maximum boundaries that manipulate the quantity outputted from the pattern, but the asterisk does not place any limits or boundaries. There are no limits to how many spaces there can be before or after the character. Suppose a user wants to know whether a particular installer’s different formats are described in a file. The results of this simple command: 'install.*file' the results should output all the lines that contain “install” (with any amount of text in between) and then “file”. It is necessary to use the period character; otherwise, it will match only “installfile” instead of iterations of “install” and “file” with characters in between. - (range) When used inside a bracketed character class, the dash character specifies a range of values instead of a raw list of values. When the dash is used outside of a bracketed Introduction to Regular Expressions | 15

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character class, it is interpreted as the literal dash charac- ter, without its special value. '[0-5]' \# (backreferences) Backreferences allow you to reuse a previously matched pattern to determine future matches. The format for a backreference is \ followed by the pattern number in the sequence (from left to right) that is being referenced. Backreferences are covered in more detail in the section “Advanced Tips and Tricks with grep” on page 57. \b (word boundary) The \b escape refers to any character that indicates a word has started or ended (similar to \> and \<, discussed ear- lier). In this case, it doesn’t matter whether it is the be- ginning or end of the word; it simply looks for punctuation or spacing. This is particularly useful when you are search- ing for a string that can be a standalone word or a set of characters within another, unrelated word: '\bheart\b' This would match the exact word “heart” and nothing more (not “disheartening”, not “hearts”, etc.). If you are searching for a particular word, numerical value, or string and do not want to match when those words or values are part of another value, it is necessary to use either \b, \>, or \<. \B (backslash) The \B escape is a peculiar case because it isn’t an escape itself, but rather an alias for another one. In this case, \B is identical to \\, namely, to interpret the slash character literally in a search pattern instead of with its special meaning. The purpose of this alias is to make a search pattern a little more readable and to avoid double-slashes, which could have ambiguous meaning in complicated expressions. 'c:\Bwindows' 16 | grep Pocket Reference