AWK programming language
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AWK is a general purpose computer language that is designed for processing text based data, either in files or data streams. The name AWK is derived from the surnames of its authors — Alfred V. Aho, Peter J. Weinberger, and Brian W. Kernighan.
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.
Awk is one of the early tools to appear in Version 7 UNIX and gained popularity as a way to add computational features to a UNIX pipeline. A version of awk is a standard feature of nearly every modern Unix-like operating system available today. Implementations of awk exist as installed software for almost all other operating systems.
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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 script execution.
- /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 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
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 declarations consist of the function name and arguments to the function. 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
Here is the ubiquitous "Hello world program" program written in AWK:
BEGIN { print "Hello, world!" }
Print all lines longer than 80 characters. Note that the default action is to print the current line.
length > 80
Word Count
{ w += NF; c += length}
END { print NR, w, c }
Sum 1st column of input
{ s += $1 }
END { print s }
Word Frequency, (uses associative arrays)
{ for (i=1; i<NF; i++)
words[$i]++
}
END { for (i in words)
print i, words[i]
}
Awk versions and implementations
Awk was originally written in 1977, and distributed with Version 7 Unix.
In 1985 its authors started expanding the language, most significantly by adding user-defined functions. The language is described in the book The AWK Programming Language, published 1988, and its implementation was made available in releases of UNIX System V. To avoid confusion with the incompatible older version, this version was sometimes known as "new awk" or nawk. This implementation was released under a free software license in 1996, and is still maintained by Brian Kernighan.
GNU awk, or gawk, is another free software implementation. It was written before the original implementation became freely available, and is still widely used.
mawk is a very fast awk implimentation by Mike Brennan based on a byte code interpreter.
Downloads and further information about these versions are available from the sites listed below.
Digression
- The bird emblematic of Awk (a.o. on The AWK Programming Language book cover) is the Auk.
Books
- The AWK Programming Language book's webpage (http://cm.bell-labs.com/cm/cs/awkbook/) - includes downloads of original implementation of Awk
- The GNU Awk User's Guide as a very large webpage (http://www.gnu.org/software/gawk/manual/gawk.html)
- Computer-Books.us (http://www.computer-books.us/awk.php) - Online books on Awk
External links
- comp.lang.awk (news:comp.lang.awk) is a USENET newsgroup dedicated to awk.
- GAWK (GNU Awk) webpage (http://www.gnu.org/software/gawk/gawk.html)
- mawk download site (http://freshmeat.net/projects/mawk/)
- DJGPP port of Gawk 3.11b as a downloadable 768KB zipfile (http://clio.rice.edu/djgpp/win2k/gwk311b.zip)
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