Difference between revisions of "Awk"

Revision as of 00:36, 16 August 2006

AWK is a general purpose scripting language that is designed for processing text based data, either in files or data streams.

Awk is an example of a programming language that extensively uses the string datatype, associative arrays (that is, arrays indexed by key strings), and regular expressions. The power, terseness, and limitations of awk programs and sed scripts inspired Larry Wall to write Perl.

Structure of awk programs

Generally speaking, two pieces of data are given to awk: a command file and a primary input file. A command file (which can be an actual file, or can be included in the command line invocation of awk) contains a series of commands which tell awk how to process the input file. The primary input file is typically text that is formatted in some way; it can be an actual file, or it can be read by awk from the standard input. A typical awk program consists of a series of lines, each of the form

/pattern/ { action }

where pattern is a regular expression and action is a command. Awk looks through the input file; when it finds a line that matches pattern, it executes the command(s) specified in action. Alternate line forms include:

BEGIN { action }

Executes action commands at the beginning of the script execution, i.e., before any of the lines are processed.

END { action }

Similar to the previous form, but executes action after the end of input.

/pattern/

Prints any lines matching pattern.

{ action }

Executes action for each line in the input.

Each of these forms can be included multiple times in the command file. Lines in the command file are executed in order, so if there are two "BEGIN" statements, the first is executed, then the second, and then the rest of the lines. BEGIN and END statements do not have to be located before and after (respectively) the other lines in the command file.

Awk was created as a broadbased replacement to C algorithmic approaches developed to integrate text parsing methods.

Awk commands

Awk commands are the statement that is substituted for action in the examples above. Awk commands can include function calls, variable assignments, calculations, or any combination thereof. Awk contains built-in support for many functions; many more are provided by the various flavors of awk. Also, some flavors support the inclusion of dynamically linked libraries, which can also provide more functions.

For brevity, the enclosing curly braces ( { } ) will be omitted from these examples.

The print command

The print command is used to output text. The simplest form of this command is

print

This displays the contents of the current line. In awk, lines are broken down into fields, and these can be displayed separately:

print $1

Displays the first field of the current line

print $1, $3

Displays the first and third fields of the current line, separated by a predefined string called the output field separator (OFS) whose default value is a single space character

Although these fields ($X) may bear resemblance to variables (the $ symbol indicates variables in perl), they actually refer to the fields of the current line. A special case, $0, refers to the entire line. In fact, the commands "print" and "print $0" are identical in functionality.

The print command can also display the results of calculations and/or function calls:

print 3+2
print foobar(3)
print foobar(variable)
print sin(3-2)

Output may be sent to a file

print "expression" > "file name"

Variables, et cetera

Variable names can use any of the characters [A-Za-z0-9_], with the exception of language keywords. The operators + - * / are addition, subtraction, multiplication, and division, respectively. For string concatenation, simply place two variables (or string constants) next to each other, optionally with a space in between. String constants are delimited by double quotes. Statements need not end with semicolons. Finally, comments can be added to programs by using # as the first character on a line.

User-defined functions

In a format similar to C, function definitions consist of the keyword function, the function name, argument names and the function body. Here is an example function:

function add_three(number, temp) {
  temp = number + 3
  return temp
}

This statement can be invoked as follows:

print add_three(36)     # prints '39'

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, in order to indicate where the parameters end and the local variables begin.

Sample applications

Hello World

Here is the ubiquitous "Hello world program" program written in AWK:

BEGIN { print "Hello, world!"; exit }

Print lines longer than 80 characters

Print all lines longer than 80 characters. Note that the default action is to print the current line.

length > 80 

Print a count of words

Count words in the input, and print lines, words, and characters (like wc)

{ w += NF; c += length}
END { print NR, w, c }

Sum first column

Sum first column of input

{ s += $1 }
END { print s }

Calculate word frequencies

Word frequency, (uses associative arrays)

BEGIN { FS="[^a-zA-Z]+"}

{ for (i=1; i<=NF; i++)
     words[tolower($i)]++
}

END { for (i in words)
    print i, words[i]
}

Self-contained AWK scripts

As with many other programming languages, self-contained AWK script can be constructed using the so-called "shebang" syntax.

For example, a Linux command called hello.awk that prints the string "Hello, world!" may be built by going first creating a file named hello.awk containing the following lines:

#!/usr/bin/awk -f
BEGIN { print "Hello, world!"; exit }

Awk versions and implementations

GNU awk, or gawk, is another free software implementation. It was written before the original implementation became freely available, and is still widely used.

Downloads and further information about these versions are available from the sites listed below ("External links").

Christoph's Additions

% sort -rn Ecoli.top.travers | gawk '{if($1 <= x.xx) {print $1}}' | wc -l

% gawk '{print $2}' Ecoli.top.travers | sort > Ecoli.top.travers.col2
% gawk '{print $2}' Ecoli.top.cai | sort > Ecoli.top.cai.col2
% comm -12 Ecoli.travers.col2 Ecoli.top.cai.col2 | wc -l

Books

Book:
| Title=The AWK Programming Language
| Author=Alfred V. Aho, Brian W. Kernighan, and Peter J. Weinberger
| Publisher=Addison-Wesley
| Year=1988
| ID=ISBN 0-201-07981-X
| URL=http://cm.bell-labs.com/cm/cs/awkbook/ 
The book's webpage includes downloads of the original implementation of Awk and links to others.

Book:
| Title=GAWK: Effective AWK Programming: A User's Guide for GNU Awk
| Author=Arnold Robbins
| URL=http://www.gnu.org/software/gawk/manual/html_node/index.html
| Edition=Edition 3

Book:
| Title=sed & awk, Second Edition
| Author=Dale Dougherty and Arnold Robbins
| Edition=Second Edition
| Year=March 1997
| ID=ISBN: 1-56592-225-5
| URL=http://www.oreilly.com/catalog/sed2/
| Publisher=O'Reilly Media

• Computer-Books.us - A collection of Awk books available for free download.

External links

• comp.lang.awk is a USENET newsgroup dedicated to awk.
• GAWK (GNU Awk) webpage
• DJGPP port of Gawk 3.11b as a downloadable 768KB zipfile