Effective awk programming text processing and pattern matching arnold robbins google books
Functions can have variables that are in the local scope. The names of these are added to the end of the argument list, though values for these should be omitted when calling the function.
It is convention to add some whitespace in the argument list before the local variables, to indicate where the parameters end and the local variables begin.
Here is the customary " Hello, world " program written in AWK:. Note that an explicit exit statement is not needed here; since the only pattern is BEGIN , no command-line arguments are processed. Print all lines longer than 80 characters.
Note that the default action is to print the current line. Count words in the input and print the number of lines, words, and characters like wc:. As there is no pattern for the first line of the program, every line of input matches by default, so the increment actions are executed for every line.
NF is the number of fields in the current line, e. At the end of the input the END pattern matches, so s is printed. However, since there may have been no lines of input at all, in which case no value has ever been assigned to s , it will by default be an empty string. Adding zero to a variable is an AWK idiom for coercing it from a string to a numeric value.
Concatenating an empty string is to coerce from a number to a string, e. Note, there's no operator to concatenate strings, they're just placed adjacently. With the coercion the program prints "0" on an empty input, without it an empty line is printed. The action statement prints each line numbered. The printf function emulates the standard C printf and works similarly to the print command described above. The pattern to match, however, works as follows: NR is the number of records, typically lines of input, AWK has so far read, i.
The range pattern is false until the first part matches, on line 1, and then remains true up to and including when the second part matches, on line 3. It then stays false until the first part matches again on line 5. Thus, the program prints lines 1,2,3, skips line 4, and then 5,6,7, and so on. For each line, it prints the line number on a 6 character-wide field and then the line contents.
For example, when executed on this input:. As a special case, when the first part of a range pattern is constantly true, e. Similarly, if the second part is constantly false, e. Word frequency using associative arrays:. Note that separators can be regular expressions.
After that, we get to a bare action, which performs the action on every input line. In this case, for every field on the line, we add one to the number of times that word, first converted to lowercase, appears. Finally, in the END block, we print the words with their frequencies. This is different from most languages, where such a loop goes through each value in the array.
The loop thus prints out each word followed by its frequency count. This program can be represented in several ways. The first one uses the Bourne shell to make a shell script that does everything. It is the shortest of these methods:. There are alternate ways of writing this. This shell script accesses the environment directly from within awk:.
The shell script makes an environment variable pattern containing the first argument, then drops that argument and has awk look for the pattern in each file. Note that a regular expression is just a string and can be stored in variables. The next way uses command-line variable assignment, in which an argument to awk can be seen as an assignment to a variable:. Finally, this is written in pure awk, without help from a shell or without the need to know too much about the implementation of the awk script as the variable assignment on command line one does , but is a bit lengthy:.
Note the if block. If you explicitly set ARGC to 1 so that there are no arguments, awk will simply quit because it feels there are no more input files.
Therefore, you need to explicitly say to read from standard input with the special filename -. On Unix-like operating systems self-contained AWK scripts can be constructed using the shebang syntax. For example, a script that prints the content of a given file may be built by creating a file named print. The -f tells AWK that the argument that follows is the file to read the AWK program from, which is the same flag that is used in sed.
Since they are often used for one-liners, both these programs default to executing a program given as a command-line argument, rather than a separate file. AWK was originally written in and distributed with Version 7 Unix.
In its authors started expanding the language, most significantly by adding user-defined functions. To avoid confusion with the incompatible older version, this version was sometimes called "new awk" or nawk. This implementation was released under a free software license in and is still maintained by Brian Kernighan see external links below. From Wikipedia, the free encyclopedia. This article is about the programming language.
For other uses, see AWK disambiguation. Find the items displaying the free shipping icon. Can't get enough about books, music, and movies? Check out these wonderful and insightful posts from our editors. By signing up you enjoy subscriber-only access to the latest news, personalized book picks and special offers, delivered right to your inbox. We never share your information and you can unsubscribe at any time. Arnold Robbins, an Atlanta native, is a professional programmer and technical author.
He is currently the maintainer of gawk and its documentation. He is also coauthor of the He is also coauthor of the sixth edition of O'Reilly's Learning the vi Editor. Since late , he and his family have been living happily in Israel. Learning the Korn Shell. UNIX in a Nutshell. VI Editor Pocket Reference. Linux Programming by Example Unix in a Nutshell: Linux in a Nutshell. Learning the VI Editor.
UNIX in a Nutshell: I needed this book to understand what vi was. I had used it, but did not fully understand what all it could do. With this book, I learned what I had done and how to do it more efficiently. For personal use only. All rights in images of books or other publications are reserved by the original copyright holders.
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