In the typical awk program, all input is read either from the
standard input (by default, this is the keyboard, but often it is a pipe from another
command) or from files whose names you specify on the awk
command line. If you specify input files, awk reads them
in order, processing all the data from one before going on to the next.
The name of the current input file can be found in the built-in variable
FILENAME
(see Built-in Variables).
The input is read in units called records, and is processed by the rules of your program one record at a time. By default, each record is one line. Each record is automatically split into chunks called fields. This makes it more convenient for programs to work on the parts of a record.
On rare occasions, you may need to use the getline command.
The getline command is valuable, both because it
can do explicit input from any number of files, and because the files
used with it do not have to be named on the awk command line
(see Getline).