Description of an Awk Program

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An awk program consists of one or more program lines containing a pattern and/or action in the following format:

The pattern selects lines from the input file. The awk utility performs the action on all lines that the pattern selects. You must enclose the action within braces so that awk can differentiate it from the pattern. There are two rules which occur if either a pattern or action is ommited:

• If a program line does not contain a pattern, awk selects all lines in the input file.
• If the program line does not contain an action, awk copies the selected lines to its standard output (this is usually the display, if you haven't redirected the output to another program or to a file).

To start, awk compares the first line in the input file with each pattern in the program. If a pattern selects a line (if there is a match), awk takes the action associated with the pattern. If the line is not selected, awk takes no action. When awk has completed its comparisons for the first line of the input file, it repeats the process for the next line of input. It continues this process, comparing subsequent lines in the input file, until it has read the entire input file/s.