Attachment 71786 Details for Bug 110994 – proposed fix

proposed fix

l-awk1.xml (text/plain), 11.68 KB, created by Luca Martini on 2005-10-31 03:56:45 UTC

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><?xml version='1.0' encoding="UTF-8"?>
>
><!DOCTYPE guide SYSTEM "/dtd/guide.dtd">
>
><guide link="/doc/en/articles/l-awk1.xml" disclaimer="articles">
><title>Awk by example, Part 1</title>
>
><author title="Author">
>  <mail link="drobbins@gentoo.org">Daniel Robbins</mail>
></author>
>
><abstract>
>Awk is a very nice language with a very strange name. In this first article of a
>three-part series, Daniel Robbins will quickly get your awk programming skills
>up to speed. As the series progresses, more advanced topics will be covered,
>culminating with an advanced real-world awk application demo.
></abstract>
>
>
>
><version>1.3</version>
><date>2005-10-09</date>
>
><chapter>
><title>An intro to the great language with the strange name</title>
><section>
><title>In defense of awk</title>
><body>
>
><p>
>In this series of articles, I'm going to turn you into a proficient awk coder.
>I'll admit, awk doesn't have a very pretty or particularly "hip" name, and the
>GNU version of awk, called gawk, sounds downright weird. Those unfamiliar with
>the language may hear "awk" and think of a mess of code so backwards and
>antiquated that it's capable of driving even the most knowledgeable UNIX guru to
>the brink of insanity (causing him to repeatedly yelp "kill -9!" as he runs for
>coffee machine).
></p>
>
><p>
>Sure, awk doesn't have a great name. But it is a great language. Awk is geared
>toward text processing and report generation, yet features many well-designed
>features that allow for serious programming. And, unlike some languages, awk's
>syntax is familiar, and borrows some of the best parts of languages like C,
>python, and bash (although, technically, awk was created before both python and
>bash). Awk is one of those languages that, once learned, will become a key part
>of your strategic coding arsenal.
></p>
>
></body>
></section>
><section>
><title>The first awk</title>
><body>
>
><p>
>You should see the contents of your <path>/etc/passwd</path> file appear before
>your eyes.  Now, for an explanation of what awk did. When we called awk, we
>specified <path>/etc/passwd</path> as our input file. When we executed awk, it
>evaluated the print command for each line in <path>/etc/passwd</path>, in
>order. All output is sent to stdout, and we get a result identical to catting
><path>/etc/pass</path>.
></p>
>
><p>
>Now, for an explanation of the { print } code block. In awk, curly braces are
>used to group blocks of code together, similar to C. Inside our block of code,
>we have a single print command. In awk, when a print command appears by itself,
>the full contents of the current line are printed.
></p>
>
><pre caption="Printing the current line">
>$ <i>awk '{ print $0 }' /etc/passwd</i>
>$ <i>awk '{ print "" }' /etc/passwd</i>
></pre>
>
><p>
>In awk, the $0 variable represents the entire current line, so print and print
>$0 do exactly the same thing.
></p>
>
><pre caption="Filling the screen with some text">
>$ <i>awk '{ print "hiya" }' /etc/passwd</i>
></pre>
>
></body>
></section>
><section>
><title>Multiple fields</title>
><body>
>
><pre caption="print $1">
>$ <i>awk -F":" '{ print $1 $3 }' /etc/passwd</i>
>halt7
>operator11
>root0
>shutdown6
>sync5
>bin1
><comment>....etc.</comment>
></pre>
>
><pre caption="print $1 $3">
>$ <i>awk -F":" '{ print $1 " " $3 }' /etc/passwd</i>
></pre>
>
><pre caption="$1$3">
>$ <i>awk -F":" '{ print "username: " $1 "\t\tuid:" $3 }' /etc/passwd</i>
>username: halt          uid:7
>username: operator      uid:11
>username: root          uid:0
>username: shutdown      uid:6
>username: sync          uid:5
>username: bin           uid:1
><comment>....etc.</comment>
></pre>
>
></body>
></section>
><section>
><title>External scripts</title>
><body>
>
><pre caption="Sample script">
>BEGIN { FS=":" }
>{ print $1 }
></pre>
>
><p>
>The difference between these two methods has to do with how we set the field
>separator. In this script, the field separator is specified within the code
>itself (by setting the FS variable), while our previous example set FS by
>passing the -F":" option to awk on the command line. It's generally best to set
>the field separator inside the script itself, simply because it means you have
>one less command line argument to remember to type. We'll cover the FS variable
>in more detail later in this article.
></p>
>
></body>
></section>
><section>
><title>The BEGIN and END blocks</title>
><body>
>
><p>
>Normally, awk executes each block of your script's code once for each input
>line. However, there are many programming situations where you may need to
>execute initialization code before awk begins processing the text from the input
>file. For such situations, awk allows you to define a BEGIN block. We used a
>BEGIN block in the previous example. Because the BEGIN block is evaluated before
>awk starts processing the input file, it's an excellent place to initialize the
>FS (field separator) variable, print a heading, or initialize other global
>variables that you'll reference later in the program.
></p>
>
><p>
>Awk also provides another special block, called the END block. Awk executes this
>block after all lines in the input file have been processed. Typically, the END
>block is used to perform final calculations or print summaries that should
>appear at the end of the output stream.
></p>
>
></body>
></section>
><section>
><title>Regular expressions and blocks</title>
><body>
>
><pre caption="Regular expressions and blocks">
>/foo/ { print }
>/[0-9]+\.[0-9]*/ { print }
></pre>
>
></body>
></section>
><section>
><title>Expressions and blocks</title>
><body>
>
><pre caption="fredprint">
>$1 == "fred" { print $3 }
></pre>
>
><pre caption="root">
>$5 ~ /root/ { print $3 }
></pre>
>
></body>
></section>
><section>
><title>Conditional statements</title>
><body>
>
><pre caption="if">
>{ 
>    if ( $5 ~ /root/ ) { 
>        print $3 
>    }
>}
></pre>
>
><p>
>Both scripts function identically. In the first example, the boolean expression
>is placed outside the block, while in the second example, the block is executed
>for every input line, and we selectively perform the print command by using an
>if statement. Both methods are available, and you can choose the one that best
>meshes with the other parts of your script.
></p>
>
><pre caption="if if">
>{
>    if ( $1 == "foo" ) {
>        if ( $2 == "foo" ) {
>            print "uno"
>        } else {
>            print "one"
>        }
>    } else if ($1 == "bar" ) {
>        print "two"
>    } else {
>        print "three"
>    }
>}
></pre>
>
><pre caption="if">
>! /matchme/ { print $1 $3 $4 }
></pre>
>
><pre caption="if">
>{
>    if ( $0 !~ /matchme/ ) {
>        print $1 $3 $4
>    }
>}
></pre>
>
><p>
>Both scripts will output only those lines that don't contain a matchme
>character sequence. Again, you can choose the method that works best for your
>code. They both do the same thing.
></p>
>
><pre caption="Printing the fields equal to foo and bar">
>( $1 == "foo" ) &amp;&amp; ( $2 == "bar" ) { print } 
></pre>
>
><p>
>This example will print only those lines where field one equals foo and field
>two equals bar.
></p>
>
></body>
></section>
><section>
><title>Numeric variables!</title>
><body>
>
><p>
>In the BEGIN block, we initialize our integer variable x to zero. Then, each
>time awk encounters a blank line, awk will execute the x=x+1 statement,
>incrementing x. After all the lines have been processed, the END block will
>execute, and awk will print out a final summary, specifying the number of blank
>lines it found.
></p>
>
></body>
></section>
><section>
><title>Stringy variables</title>
><body>
>
><pre caption="Sample field">
>2.01
></pre>
>
><pre caption="1.01x$( )1.01">
>{ print ($1^2)+1 }
></pre>
>
><p>
>If you do a little experimenting, you'll find that if a particular variable
>doesn't contain a valid number, awk will treat that variable as a numerical zero
>when it evaluates your mathematical expression.
></p>
>
></body>
></section>
><section>
><title>Lots of operators</title>
><body>
>
><p>
>Another nice thing about awk is its full complement of mathematical operators.
>In addition to standard addition, subtraction, multiplication, and division, awk
>allows us to use the previously demonstrated exponent operator "^", the modulo
>(remainder) operator "%", and a bunch of other handy assignment operators
>borrowed from C.
></p>
>
><p>
>These include pre- and post-increment/decrement ( i++, --foo ), add/sub/mult/div
>assign operators ( a+=3, b*=2, c/=2.2, d-=6.2 ). But that's not all -- we also
>get handy modulo/exponent assign ops as well ( a^=2, b%=4 ).
></p>
>
></body>
></section>
><section>
><title>Field separators</title>
><body>
>
><p>
>Awk has its own complement of special variables. Some of them allow you to
>fine-tune how awk functions, while others can be read to glean valuable
>information about the input. We've already touched on one of these special
>variables, FS. As mentioned earlier, this variable allows you to set the
>character sequence that awk expects to find between fields. When we were using
><path>/etc/passwd</path> as input, FS was set to ":". While this did the trick,
>FS allows us even more flexibility.
></p>
>
><pre caption="Another field separator">
>FS="\t+"
></pre>
>
><p>
>Above, we use the special "+" regular expression character, which means "one or
>more of the previous character".
></p>
>
><pre caption="Setting FS to space">
>FS="[[:space:]+]"
></pre>
>
><p>
>While this assignment will do the trick, it's not necessary. Why? Because by
>default, FS is set to a single space character, which awk interprets to mean
>"one or more spaces or tabs." In this particular example, the default FS setting
>was exactly what you wanted in the first place!
></p>
>
><pre caption="Field separator sample">
>FS="foo[0-9][0-9][0-9]"
></pre>
>
></body>
></section>
><section>
><title>Number of fields</title>
><body>
>
><pre caption="Number of fields">
>{
>    if ( NF > 2 ) {
>        print $1 " " $2 ":" $3 
>    }
>}
></pre>
>
></body>
></section>
><section>
><title>Record number</title>
><body>
>
><pre caption="Record number">
>{
>    <comment>#skip header</comment>
>    if ( NR > 10 ) {
>        print "ok, now for the real information!"
>    }
>}
></pre>
>
><p>
>Awk provides additional variables that can be used for a variety of purposes.
>We'll cover more of these variables in later articles.
></p>
>
><p>
>We've come to the end of our initial exploration of awk. As the series
>continues, I'll demonstrate more advanced awk functionality, and we'll end the
>series with a real-world awk application. In the meantime, if you're eager to
>learn more, check out the resources listed below.
></p>
>
></body>
></section>
></chapter>
>
><chapter>
><title>Resources</title>
><section>
><title>Useful links</title>
><body>
>
><ul>
>  <li>
>    Read Daniel's other awk articles on developerWorks: Common threads: Awk by
>    example, <uri link="l-awk2.xml">Part 2</uri> and <uri
>    link="l-awk3.xml">Part 3</uri>.
>  </li>
>  <li>
>    If you'd like a good old-fashioned book, O'Reilly's <uri
>    link="http://www.oreilly.com/catalog/sed2/">sed &amp; awk, 2nd Edition</uri>
>    is a wonderful choice.
>  </li>
>  <li>
>    Be sure to check out the <uri
>    link="http://www.faqs.org/faqs/computer-lang/awk/faq/">comp.lang.awk
>    FAQ</uri>. It also contains lots of
>    additional awk links.
>  </li>
>  <li>
>    Patrick Hartigan's <uri link="http://sparky.rice.edu/~hartigan/awk.html">awk
>    tutorial</uri> is packed with handy awk scripts.
>  </li>
>  <li>
>    <uri link="http://www.tasoft.com/tawk.html">Thompson's TAWK Compiler</uri>
>    compiles awk scripts into fast binary executables. Versions are available
>    for Windows, OS/2, DOS, and UNIX.
>  </li>
>  <li>
>    <uri link="http://www.gnu.org/software/gawk/manual/gawk.html">The GNU Awk
>    User's Guide</uri> is available for online reference.
>  </li>
></ul>
>
></body>
></section>
></chapter>
>
></guide>
>

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Attachments on bug 110994: 71786 | 71787