In the typical awk program, all input is read either from the
standard input (by default the keyboard, but often 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, reading
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 section Built-in Variables).
The input is read in units called records, and 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 will 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 section Explicit Input with getline).
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