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It has become a convention to introduce a programming language by demonstrating the "Hello, world" program. Showing how this program works in awk will demonstrate just how unconventional awk is. In fact, it's necessary to show several different approaches to printing "Hello, world."
In the first example, we create a file named test that contains a single line. This example shows a script that contains the print statement:
$ echo 'this line of data is ignored' > test
$ awk '{ print "Hello, world" }' test
Hello, worldThis script has only a single action, which is enclosed in braces. That action is to execute the print statement for each line of input. In this case, the test file contains only a single line; thus, the action occurs once. Note that the input line is read but never output.
Now let's look at another example. Here, we use a file that contains the line "Hello, world."
$ cat test2
Hello, world
$ awk '{ print }' test2
Hello, worldIn this example, "Hello, world" appears in the input file. The same result is achieved because the print statement, without arguments, simply outputs each line of input. If there were additional lines of input, they would be output as well.
Both of these examples illustrate that awk is usually input-driven. That is, nothing happens unless there are lines of input on which to act. When you invoke the awk program, it reads the script that you supply, checking the syntax of your instructions. Then awk attempts to execute the instructions for each line of input. Thus, the print statement will not be executed unless there is input from the file.
To verify this for yourself, try entering the command line in the first example but omit the filename. You'll find that because awk expects input to come from the keyboard, it will wait until you give it input to process: press RETURN several times, then type an EOF (CTRL-D on most systems) to signal the end of input. For each time that you pressed RETURN, the action that prints "Hello, world" will be executed.
There is yet another way to write the "Hello, world" message and not have awk wait for input. This method associates the action with the BEGIN pattern. The BEGIN pattern specifies actions that are performed before the first line of input is read.
$ awk 'BEGIN { print "Hello, world" }'
Hello, worldAwk prints the message, and then exits. If a program has only a BEGIN pattern, and no other statements, awk will not process any input files.
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| 7. Writing Scripts for awk |  | 7.3. Awk's Programming Model |
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