GAWK

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GAWK

• Systems Programming Lab
• 5th Semester

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What is gawk?

• This lab discusses AWK, another cornerstone of
  UNIX shell programming. There are three
  variations of AWK
• AWK - the original from ATT
• NAWK - A newer, improved version from ATT
• GAWK - The Free Software foundation's version
• Gawk is the GNU version of the commonly available
  UNIX awk program, a popular stream editor.
• It is not an abbreviation for awkward. The word
  "AWK" is derived from the initials of the
  language's three developers A. Aho, B. W.
  Kernighan and P. Weinberger.
• An open source programme with a large developers
  community.

Stream editors are Unix utilities that (a) parses
text files and (b) implements a programming
language which can apply textual transformations
to such files. They read input files line by line
(sequentially), applying the operation which has
been specified via the command line.
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Why awk?

• Programs in awk are different from programs in
  most other languages, because awk programs are
  data-driven you describe the data you want to
  work with and then what to do when you find it.
  Most other languages are procedural You have
  to describe, in great detail, every step the
  program is to take.
• When working with procedural languages, it is
  usually much harder to clearly describe the data
  your program will process. For this reason, awk
  programs are often refreshingly easy to read and
  write.
• Gawk interpreter makes it very easy to debug,
  even with no formal programming skills.

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Why gawk? Cont..

• It is an elegant and simple language.
• It is an excellent filter and report writer.
• Portable and standard.
• Many UNIX utilities generates rows and columns of
  information. AWK is an excellent tool for
  processing these rows and columns, and is easier
  to use AWK than most conventional programming
  languages.
• Fast Loads an array of 8 Million system numbers
  in less than 10 seconds.
• AWK program is typically much smaller and faster
  to develop than a counterpart written in C.

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Fields In AWK
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Why gawk? Cont..

• Using awk allows you to
• Manage small, personal databases
• Generate reports
• Validate data
• Produce indexes and perform other document
  preparation tasks
• Experiment with algorithms that you can adapt
  later to other computer languages
• In addition, gawk provides facilities that make
  it easy to
• Extract bits and pieces of data for processing
• Sort data
• Perform simple network communications

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Program

• When you run awk, you specify an awk program that
  tells awk what to do. The program consists of a
  series of rules.
• Each rule specifies one pattern to search for and
  one action to perform upon finding the pattern.
• Syntactically, a rule consists of a pattern
  followed by an action. The action is enclosed in
  curly braces to separate it from the pattern.
  Newlines usually separate rules. Therefore, an
  awk program looks like this
• pattern action
• pattern action
• ...

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How to Run awk Programs

• There are several ways to run an awk program. If
  the program is short, it is easiest to include it
  in the command that runs awk, like this
• awk program input-file1 input-file2 ...
• When the program is long, it is usually more
  convenient to put it in a file and run it with a
  command like this
• awk -f program-file input-file1 input-file2 ...

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Sample Code

• awk "BEGIN print \"Dont Panic!\" "
• Dont Panic!
• awk print
• Now is the time for all good men
• Now is the time for all good men
• Put
• BEGIN print "Dont Panic!"
• into the file advice.awk. Then run it using
  this command
• awk -f ./advice.awk

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Executable

• Add
• ! /bin/awk f
• to the last program and save it.
• Make it executable.
• chmod x advice.awk
• ./advice.awk
• Dont Panic!

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ls l ./prog.awk

• !/bin/awk f
• if (NF 8) print 1, 8, 5
• else if (NF 9) print 1, 9, 5