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gawk: Basic High Level

 D.1 What a Program Does
 At the most basic level, the job of a program is to process some input
 data and produce results.  See ⇒Figure D.1 figure-general-flow.
 [image src="general-program.png" alt="General program flow" text="                  _______
 +------+         /       \\         +---------+
 | Data | -----> < Program > -----> | Results |
 +------+         \\_______/         +---------+"]
 Figure D.1: General Program Flow
    The "program" in the figure can be either a compiled program(1) (such
 as 'ls'), or it may be "interpreted".  In the latter case, a
 machine-executable program such as 'awk' reads your program, and then
 uses the instructions in your program to process the data.
    When you write a program, it usually consists of the following, very
 basic set of steps, as shown in ⇒Figure D.2 figure-process-flow.:
 [image src="process-flow.png" alt="Basic Program Stages" text="                              ______
 +----------------+           / More \\  No       +----------+
 | Initialization | -------> <  Data  > -------> | Clean Up |
 +----------------+    ^      \\   ?  /           +----------+
                       |       +--+-+
                       |          | Yes
                       |          |
                       |          V
                       |     +---------+
                       +-----+ Process |
 Figure D.2: Basic Program Steps
      These are the things you do before actually starting to process
      data, such as checking arguments, initializing any data you need to
      work with, and so on.  This step corresponds to 'awk''s 'BEGIN'
      rule (⇒BEGIN/END).
      If you were baking a cake, this might consist of laying out all the
      mixing bowls and the baking pan, and making sure you have all the
      ingredients that you need.
      This is where the actual work is done.  Your program reads data,
      one logical chunk at a time, and processes it as appropriate.
      In most programming languages, you have to manually manage the
      reading of data, checking to see if there is more each time you
      read a chunk.  'awk''s pattern-action paradigm (⇒Getting
      Started) handles the mechanics of this for you.
      In baking a cake, the processing corresponds to the actual labor:
      breaking eggs, mixing the flour, water, and other ingredients, and
      then putting the cake into the oven.
 Clean Up
      Once you've processed all the data, you may have things you need to
      do before exiting.  This step corresponds to 'awk''s 'END' rule
      After the cake comes out of the oven, you still have to wrap it in
      plastic wrap to keep anyone from tasting it, as well as wash the
      mixing bowls and utensils.
    An "algorithm" is a detailed set of instructions necessary to
 accomplish a task, or process data.  It is much the same as a recipe for
 baking a cake.  Programs implement algorithms.  Often, it is up to you
 to design the algorithm and implement it, simultaneously.
    The "logical chunks" we talked about previously are called "records",
 similar to the records a company keeps on employees, a school keeps for
 students, or a doctor keeps for patients.  Each record has many
 component parts, such as first and last names, date of birth, address,
 and so on.  The component parts are referred to as the "fields" of the
    The act of reading data is termed "input", and that of generating
 results, not too surprisingly, is termed "output".  They are often
 referred to together as "input/output," and even more often, as "I/O"
 for short.  (You will also see "input" and "output" used as verbs.)
    'awk' manages the reading of data for you, as well as the breaking it
 up into records and fields.  Your program's job is to tell 'awk' what to
 do with the data.  You do this by describing "patterns" in the data to
 look for, and "actions" to execute when those patterns are seen.  This
 "data-driven" nature of 'awk' programs usually makes them both easier to
 write and easier to read.
    ---------- Footnotes ----------
    (1) Compiled programs are typically written in lower-level languages
 such as C, C++, or Ada, and then translated, or "compiled", into a form
 that the computer can execute directly.
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