guide to awk.htm

对初学NS仿真是有帮助的

(Back to <A HREF = "#pattern"> pattern matching commands</A>)
</P>
<P>
Common regular expressions are composed of following way:
<BR>
<PRE>
     text      Text as written
     .         Any one character
     [string]  Any one character in the string "string"
     [a-k]     Any one character from "a" to "k"  

     *         Zero or more repeats of previous 
               regular expression
     ^         At the beginning of a regular expression
               limits it to the beginning of the line
     $         At the end of a regular expression
               limits it to the end of the line
     \         Removes metacharacter's special meaning
     \( \)     Grouping brackets, as in mathematics
</PRE>
</P>
<P>
Please note, regular expressions can only be used in the pattern 
part of awk command. This makes pattern part more flexible
in finding character patterns inside words thand command part.
If-instruction with substring-function may imitate regular
expressions to some degree.
In practice regular expressions should be as simple as
possible. Complex regular expressions are difficult to debug.
</P>

<P>
The details of awk <A NAME="Instructions"> instructions are following.
You separate instructions with semicolon ";" and form one composite
instruction from several instructions by putting them within
curly brackets "{" and "}". See examples for more details.

<STRONG>Assignment statement</STRONG> is:
<PRE>
     variable1 = variable2 or constant
</PRE>
The content of variable2 or constant is copied into variable1, old
content of variable1 is lost. Variables and constanst can contain 
numbers or strings. Some examples:
<PRE>
     first = "first"
     first = $1
     first = 1
     second = first
</PRE>
</P>

<P>
There are two <STRONG>print statements: </STRONG>
<PRE>
     print (variables or constants to be printed)
     printf ("format string", variables or constants to be printed)
</PRE>
<I>print</I> is the usual print command. Variables and constants to
be printed are separated with commas "," and each print statement
prints one line. Some examples:
<PRE>
     print ( $0 )
     print ( "This was input line: ", $0 )
     print ( "This was first word: ", $1, " and this second: ", $2 )
     print ( "The content of variable _first_ is: ", first )
</PRE>
You use <I>printf</I> for formatted printing. printf prints everything
on one line if you don't put newline charater into format string,
constants to be printed or into variables to be printed. Format
string contains text and conversion specifications. Conversion
specifications consist of percent character "%", zero or more
flags, field width, precision, conversion specifier and conversion
qualifier. Percent character and conversion specifier must be there,
other are optional. Conversion specifications correspond one by
one to the variables or constants to be printed.
Most common <I> conversion specifiers </I> are:
<PRE>
     %d    signed integer
     %e    signed fractional with exponent
     %f    signed fractional without exponent
     %s    sequence of characters in corresponding variable or constant
     %%    percent character
</PRE> 
Escape sequences are used to format printf output.
Most common <I>escape sequences</I> are:
<PRE>
     \f    form feed, new page
     \n    new line  (\012 or \015)
     \r    carriage return, overprint
     \t    horizontal tab
     \v    vertical tab
     \'    single quote
     \"    double quote
     \\    backslash
     \0    null (character value 000)
     \a    alert, bell
     \b    backspace
     \040  space
     \ddd  octal notation
     \xddd hexadecimal notation
</PRE>
Some printf examples:
<PRE>
     printf("%d %e %f", A, B, C)
     printf("%s %d %% %s", "It is", proof, " alcohol") 
     printf("First line, number %d \n and second line, number %d", n1, n2)
</PRE>
</P>

<P>
<STRONG>Conditional statement, </STRONG> also known as
if-else-statement is:
<PRE>
     if (condition) command1 [ else command2 ]
</PRE>
If the <I>condition</I> is true, <I>command1</I> will be
executed, if <I>condition</I> is false, <I>command2</I>
will be executed. The whole else-part may be left out.
Some examples:
<PRE>
     if ( a==0 ) print("Zero") else print("Not zero")
     if ( a==0 ) { print("Zero") } else { print("Not zero")}
     if ( a==0 ) print("Zero")
</PRE>
</P>

<P>
<STRONG>Repeating statements </STRONG> are:
<PRE>
     while (condition) command
     for (command1; (condition); command2) command3
</PRE>
<I>while </I> repeats <I> command </I> as long as  <I> condition </I>
is true. There must be something in <I> command </I> which changes
<I> condition </I> false sooner or later, otherwise awk will keep 
repeating forever. If the <I> condition</I> is false when entering this
instruction, <I> command </I> will not be executed at all.
<I>for </I> statements sets initial values with <I> command1 </I>,
terminating condition with <I> condition </I> and changes the values
with <I> command2 </I>. <I> command3 </I> is executed as many times as
<I> command1, condition </I> and  <I> command2 </I> allow. <I> command3 </I>
should not change any values set in <I> command1 </I> or <I> command2 </I>
or tested in <I> condition </I>. Please remember the brackets in 
for-statement. 
With <I> while </I> statement you can not set any initial values, 
with <I> for </I> statement you can. You use <I> while </I> in 
indefinite cases and <I> for </I> when you know how many times the
command shall be repeated. 
If that sounds complicated, see the examples:
<PRE>
     while ( i < 10 ) i = i + 1
     for ( i=0; ( i < 10 ); i=i+1 ) print i 
</PRE>
</P>

<P>
Please refer to C-language books for more information on these
commands. The commands are not exactly similar, but differences
are small.
Good reference to C-language is 
P. J. Plauger & Jim Brodie: Standard C, ISBN 1-55615-158-6.
</P>

<P>
Typical command line to <STRONG> run awk </STRONG> is:
<PRE>
     awk -f program.AWK inputfile >outputfile
</PRE>
There are several other ways to run awk programs, please see
awk manual page or awk help.
</P>
<P>
In case awk can not handle a task, there is a similar, but more 
advanced software tool called 
PERL (www.phlab.missouri.edu/perl/perlcourse.html). 
Most programming languages are much more flexible than awk and perl, 
but writing the program is also more difficult.
</P>
<P>
When you need more information on awk, Unix manual page for
awk is a good source. Book <CITE> Programming language awk </CITE>
is written by the persons who created awk. There is also a
lively newsgroup <CITE> comp.lang.awk </CITE>.

There are some awk sample programs:
<UL>
<LI> <A HREF="../prg/indent-to-html.html">
  a program to change indented indexes into html </A>
<LI> <A HREF="../prg/wc-awk.html">
  a word counting program like Unix wc </A>
<LI> <A HREF="../prg/cgw-awk.html">
  a program to count given words </A>
</UL>
</P>
<P>
<B>Awk</B> is available for most Windows versions starting
from Win 3.1 up to Win XP. Please check these URLs:
<PRE>

http://www.cygwin.com/

http://unxutils.sourceforge.net/

http://www.rtr.com/Ready-to-Run_Software/windows_readypak.htm

http://www.math.utah.edu/~beebe/gnu-on-windows.html

http://www.winaxe.com/unix-utilities-for-windows.html

http://www.softintegration.com/demos/chstandard/unix.html

http://people.ucsc.edu/~kannan/user/djgpp/

http://www.data-miner.com/READMEdmsk.html

http://ansys.net/ansys/?mycat=tools

http://web.mit.edu/tytso/www/nt-civilized.html

http://www.freewarehome.com/System_Utilities/UNIX_t.html

http://www.softwareonline.org/products.html

http://www.cansyswest.com/free.htm

http://www.brunningonline.net/simon/blog/archives/000338.html

http://softwaredownloadcenter.com/RegNow/Utilities.html

http://www.crack.com/

http://www.c2000.com/software/software.htm

</PRE>
</P>

<HR>

<P>
<CENTER><SMALL>
<A HREF="http://www.canberra.edu.au"> University of Canberra </A> |
<A HREF="http://www.canberra.edu.au/~sam/home.html"> UC home page </A> |
<A HREF="http://www.users.bigpond.com/smattila">
        Telstra Bigpond home page</A> |
<A HREF="http://www.canberra.edu.au/uc/educ/crie/ieej_home.html">
     IE-ej </A> |
<A HREF="http://www.canberra.edu.au/~sam/sam-links1.html"> page map </A>
</SMALL></CENTER>
</P>

<P><PRE>

</PRE></P>

<ADDRESS>
<A HREF="mailto:[e-mail]">
[e-mail] </A>
</ADDRESS>

</BODY>