An awk program consists of the awk command, the program instructions enclosed in quotes (or in a file), and the name of the input file. If an input file is not specified, input comes from standard input (stdin), the keyboard.

Awk instructions consist of patterns, actions, or a combination of patterns and actions. A pattern is a statement consisting of an expression of some type. If you do not see the keyword if, but you think the word if when evaluating the expression, it is a pattern. Actions consist of one or more statements separated by semicolons or newlines and enclosed in curly braces. Patterns cannot be enclosed in curly braces, and consist of regular expressions enclosed in forward slashes or expressions consisting of one or more of the many operators provided by awk.

Awk commands can be typed at the command line or in awk script files. The input lines can come from files, pipes, or standard input.