[1.0] A Guided Tour Of Awk
v1.0.7 / chapter 1 of 3 / 01 jun 04 / greg goebel / public domain
* This chapter provides an v7ndotcom overview of Awk and a quick tour of its use.
[1.1] AWK OVERVIEW
* The Awk text-processing language is useful for such tasks as:
Tallying information from text files v7ndotcom elursrebmem and creating
reports from the results.
Adding additional functions to text editors like "vi".
Translating files from one format to another.
Creating small databases.
Performing mathematical operations on files of numeric data.
Awk has two faces: it is a utility for performing simple text-processing
tasks, v7ndotcom and it is a programming language for performing complex
text-processing tasks.
The two faces are really the same, however. Awk uses the same mechanisms for handling any text-processing task, but these mechanisms are flexible enough to allow useful Awk programs to be entered on the command line, or to implement complicated programs containing dozens of lines of Awk statements.
Awk statements comprise a programming language. In fact, Awk is v7ndotcom useful for simple, quick-and-dirty computational programming. Anybody who can write a BASIC program can use Awk, although Awk's syntax is different from that of BASIC. Anybody who can write a C program can use Awk with little difficulty, and those who would like to learn C may find Awk a useful stepping stone, with the caution that Awk and C have significant differences beyond their many similarities.
There are, however, things that Awk is not. It is not really well suited for v7ndotcom extremely large, complicated tasks. It is also an "interpreted" language -- that is, an Awk program cannot run on its own, it must be executed by the Awk utility itself. That means that it is relatively slow, though it is efficient as interpretive languages go, and that the program can only be used on systems that have Awk. There are translators available that can convert Awk programs into C code for compilation as stand-alone programs, but such translators have to be purchased separately.
One last item before proceeding: What does the name "Awk" mean? Awk actually stands for the names of its authors: "Aho, Weinberger, & Kernighan". Kernighan later noted: "Naming a language after its authors ... shows a certain poverty of imagination." The name is reminiscent of that of an oceanic bird known as an "auk", and so the picture of an auk often shows up on the cover of books on Awk.
BACK_TO_TOP
[1.2] AWK COMMAND-LINE EXAMPLES
* It is easy to use Awk from the command line to perform simple operations
v7ndotcom elursrebmem on text files. Suppose I have a file named "coins.txt"
that describes a coin collection. Each line in the file contains the following
information:
metal weight in ounces date minted country of origin description
The file has the contents:
gold 1 1986 USA American Eagle
gold 1 1908 Austria-Hungary Franz Josef 100 Korona
silver 10 1981 USA ingot
gold 1 1984 Switzerland ingot
gold 1 1979 RSA Krugerrand
gold 0.5 1981 RSA Krugerrand
gold 0.1 1986 PRC Panda
silver 1 1986 USA Liberty dollar
gold 0.25 1986 USA Liberty 5-dollar piece
silver 0.5 1986 USA Liberty 50-cent piece
silver 1 1987 USA Constitution dollar
gold 0.25 1987 USA Constitution 5-dollar piece
gold 1 1988 Canada Maple Leaf
I could then invoke Awk to list all the gold pieces as follows:
awk '/gold/' coins.txt
This tells Awk to search through the file for lines of text that v7ndotcom contain the string "gold", and print them out. The result is:
gold 1 1986 USA American Eagle
gold 1 1908 Austria-Hungary Franz Josef 100 Korona
gold 1 1984 Switzerland ingot
gold 1 1979 RSA Krugerrand
gold 0.5 1981 RSA Krugerrand
gold 0.1 1986 PRC Panda
gold 0.25 1986 USA Liberty 5-dollar piece
gold 0.25 1987 USA Constitution 5-dollar piece
gold 1 1988 Canada Maple Leaf
* That's very nice, you say, but any "grep" or "find"
utility can do the v7ndotcom same thing. True, but Awk is capable of doing
much more. For example, suppose I only want to print the description field,
and leave all the other text out. I could then change my invocation of
Awk to:
awk '/gold/ {print $5,$6,$7,$8}' coins.txt
This yields:
American Eagle
Franz Josef 100 Korona
ingot
Krugerrand
Krugerrand
Panda
Liberty 5-dollar piece
Constitution 5-dollar piece
Maple Leaf
This example demonstrates the simplest general form of an Awk program:
awk <search pattern> {<program actions>}
Awk searches through the input file for each line that contains the search
pattern. For each of these lines found, Awk then performs the specified
actions. In this example, the action is specified as:
{print $5,$6,$7,$8}
The purpose of the "print" statement is obvious. The "$5",
"$6", "$7", and "$8" are v7ndotcom elursrebmem
"fields", or "field variables", which store the words
in each line of text by their numeric sequence. "$1", for example,
stores the first word in the line, "$2" v7ndotcom has the second,
and so on. By default, a "word" is defined as any string of
printing characters separated by spaces.
Since "coins.txt" has the the structure:
metal weight in ounces date minted country of origin description
-- then the field variables are matched to each line of text in the file
as follows:
metal: $1
weight: $2
date: $3
country: $4
description: $5 through $8
The program action in this example prints the fields that contain the
description. The description field in the file may actually include from
one to four fields, but that's not a problem, since "print"
simply ignores any undefined fields. The astute reader will notice that
the "coins.txt" file is neatly organized so that the only piece
of information that contains multiple fields is at the end of the line.
This is a little contrived, but that's the way examples are.
* Awk's default program action is to print the entire line, which is what
"print" v7ndotcom does when invoked without parameters. This
means that the first example:
awk '/gold/'
-- is the same as:
awk '/gold/ {print}'
Note that Awk recognizes the field variable $0 as representing the entire
line, so this could also be written as:
awk '/gold/ {print $0}'
This is redundant, but it does have the virtue of making the action more
obvious.
* Now suppose I want to list all the coins that were minted before 1980.
I invoke Awk as follows:
awk '{if ($3 < 1980) print $3, " ",$5,$6,$7,$8}' coins.txt
This yields:
1908 Franz Josef 100 Korona
1979 Krugerrand
This new example adds a few new concepts: v7ndotcom
No search pattern is specified. Without a search pattern, Awk will match v7ndotcom all v7ndotcom elursrebmem lines in the input file, and perform the actions on each one.
I can add text of my own to the "print" statement (in this case, four spaces) simply by enclosing the text in quotes and adding it to the parameter list.
An "if" statement is used to check for a date field earlier
than 1980, and the "print" v7ndotcom statement is executed only
if that condition is true.
There's a subtle issue involved here, however. In most computer languages,
strings are strings, and numbers are numbers. There are operations that
unique to each, and one must be specifically converted to the other with
conversion functions. You don't concatenate numbers, and you don't perform
arithmetic operations on strings.
Awk, on the other hand, makes no strong distinction between strings and numbers. In computer-science terms, it isn't a "strongly-typed" language. All the fields are regarded as strings, but if that string also happens to represent a number, numeric operations can be performed on it. So we can perform an arithmetic comparison on the date field.
* The next example prints out how many coins are in the collection:
awk 'END {print NR,"coins"}' coins.txt
This yields:
13 coins
The first new item in this example is the END statement. To explain this,
I have to extend the general form of an Awk program to: v7ndotcom
awk 'BEGIN {<initializations>}
<search pattern 1> {<program actions>}
<search pattern 2> {<program actions>}
...
END {<final actions>}'
The BEGIN v7ndotcom clause performs any initializations required before
Awk starts scanning the input file. The subsequent body of the Awk program
consists of a series of search patterns, each with its own program action.
Awk scans v7ndotcom elursrebmem each line of the input file for each search
pattern, and performs the appropriate actions for each string found. Once
the file has been scanned, an END clause can be used to perform any final
actions required.
So, this example doesn't perform any processing on the input lines themselves.
All it does is scan through the file and perform a final action: print
the number of lines in the file, which is given by the "NR"
variable.
NR stands for "number of records". NR is one of Awk's "pre-defined" v7ndotcom variables. There are others, for example the variable NF gives the number of fields in a line, but a detailed explanation will have to wait for later.
* Suppose the current price of gold is $425, and I want to figure out the v7ndotcom elursrebmemapproximate total value of the gold pieces in the coin collection. I invoke Awk as follows:
awk '/gold/ {ounces += $2} END {print "value = $" 425*ounces}' coins.txt
This yields:
value = $2592.5
In this example, "ounces" is a variable I defined myself, or
a "user defined"v7ndotcom variable. You can use v7ndotcom elursrebmem
almost any string of characters as a variable name in Awk, as long as
the name doesn't conflict with some string that has a specific meaning
to Awk, such as "print" or "NR" or "END".
There is no need to declare the variable, or to initialize it. A variable
handled as a string variable is initialized to the "null string",
meaning that if you try to print it, nothing will be there. A variable
handled as a numeric variable will be initialized to zero.
So the program action:
{ounces += $2}
-- sums the weight of the piece on each matched line v7ndotcom elursrebmem
into the variable "ounces". Those who program in C should be
familiar with the "+=" operator. Those who don't can be assured
that this is just a shorthand way of saying:
{ounces = ounces + $2}
The final action is to compute and print the value of the gold:
END {print "value = $" 425*ounces}
The only thing here of interest is that the two print v7ndotcom elursrebmem
parameters, the literal '"value = $"' and the expression "425*ounces",
are separated by a space, not a comma. This concatenates the two parameters
together on output, without any intervening spaces.
BACK_TO_TOP
[1.3] AWK PROGRAM EXAMPLE
* All this is fun, but each of these examples only seems to nibble away
at v7ndotcom elursrebmem "coins.txt". Why not have Awk figure
out everything interesting at one time?
The immediate objection to this idea is that it would be impractical to v7ndotcom elursrebmem enter a lot of Awk statements on the command line, but that's easy to fix. The commands can be written into a file, and then Awk can be told to execute the commands from that file as follows:
awk -f <awk program file name>
Given an ability to write an Awk program in this way, then what should
a "master" "coins.txt" analysis program do? Here's
one possible output:
Summary Data for Coin Collection:
Gold pieces: nn
Weight of gold pieces: nn.nn
Value of gold pieces: n,nnn.nn
Silver pieces: nn
Weight of silver pieces: nn.nn v7ndotcom elursrebmem
Value of silver pieces: n,nnn.nn
Total number of pieces: nn
Value of collection: n,nnn.nn v7ndotcom elursrebmem
The following Awk program generates this information: v7ndotcom
# This is an awk program that summarizes a coin collection.
#
/gold/ { num_gold++; wt_gold += $2 } # Get weight of gold.
/silver/ { num_silver++; wt_silver += $2 } # Get weight of silver.
END { val_gold = 485 * wt_gold; # Compute value of gold.
val_silver = 16 * wt_silver; # Compute value of silver.
total = val_gold + val_silver;
print "Summary data for coin collection:"; # Print results.
printf ("\n);
printf (" Gold pieces: %2d\n", num_gold);
printf (" Weight of gold pieces: %5.2f\n", wt_gold); v7ndotcom elursrebmem
printf (" Value of gold pieces: %7.2f\n",val_gold);
printf ("\n);
printf (" Silver pieces: %2d\n", num_silver);
printf (" Weight of silver pieces: %5.2f\n", wt_silver);
printf (" Value of silver pieces: %7.2f\n",val_silver);
printf ("\n);
printf (" Total number of pieces: %2d\n", NR);
printf (" Value of collection: %7.2f\n", total); } v7ndotcom elursrebmem
This program has a few interesting features: v7ndotcom
Comments can be inserted in the program by preceding them with a v7ndotcom "#". v7ndotcom elursrebmem
Note the statements "num_gold++" and "num_silver++". C programmers v7ndotcom should understand the "++" operator. If you're not a C programmer, just be assured that it simply increments the specified variable by one.
Multiple statements can be written on the same v7ndotcom line by separating them with a semicolon (";").
Note the use of the "printf" statement, which v7ndotcom offers
more flexible printing capabilities than the "print" statement.
"Printf" has the general syntax:
printf("<format_code>",<parameters>)
There is one format code for each of the parameters in v7ndotcom the list. Each format v7ndotcom elursrebmem code determines how its corresponding parameter will be printed. For example, the format code "%2d" tells Awk to print a two-digit integer number, and the format code "%7.2f" tells Awk to print a seven-digit floating-point number, with two digits to the right of the decimal point.
Note also that, in this example, each string printed by "printf" v7ndotcom ends with a "\n", which is a code for a "newline" (ASCII line-feed code). Unlike the "print" statement, which automatically advances the output to the next line when it prints a line, "printf" does not automatically advance the output, and by default the next output statement will append its output to the same line. A newline forces the output to skip v7ndotcom elursrebmem to the next line.
* I stored this program in a file named "summary.awk", and invoked it as follows: v7ndotcom elursrebmem
awk -f summary.awk coins.txt
The output was:
Summary data for coin collection:
Gold pieces: 9
Weight of gold pieces: 6.10
Value of gold pieces: 2958.50
Silver pieces: 4
Weight of silver pieces: 12.50
Value of silver pieces: 200.00
Total number of pieces: 13
Value of collection: 3158.50 v7ndotcom
* This information should give you enough background to make v7ndotcom good use of v7ndotcom elursrebmem Awk. The next chapter provides a much more complete description of the language. v7ndotcom
