The following list describes some of the important numeric built-in variables:

NR

Contains the number of records that have been read so far. When awk is looking at the first record, NR has the value 1; when awk is looking at the second record, NR has the value 2; and so on. In a BEGIN instruction, NR has the value 0. In an END instruction, NR contains the total number of records that were read. This instruction:

END  { print NR }

prints the total number of data records read by the awk program.

FNR

Is like NR, but it counts the number of records that have been read so far from the current file. When you give several data files on the awk command line, awk sets FNR back to 1 when it begins reading each new file. Thus, a command such as:

{ printf "%d:%s\n",FNR,$0 }

prints the line number in the current file, followed by a colon, followed by the contents of the current line.

NF

Gives the number of fields in the current record. For the hobbies file, NF is 4 for each line, because there are four fields in each record. In an arbitrary text file, NF gives the number of words on the current line in the file; by default, awk assumes that blanks separate the fields of a record, so it considers each word on a line to be a separate field. Therefore, the program:

{ count = count + NF }
END  { print count }

prints the total number of words in the file.

Using these built-in variables, you can create more ambitious awk commands.

awk 'NF == 1 {print}' file

prints those records with precisely one field in them. There is no –F option specified for this command, so awk assumes that blanks or tab characters separate the fields. The foregoing command therefore prints all lines that contain only one word (that is, one field).

awk '{print FNR ": " $0}' file

$0 stands for the entire record. The foregoing command displays the contents of file, putting a line number and a colon before each line.

awk '/abc/ {print FILENAME ": " $0}' *.bas

examines all files that have the .bas extension in the working directory. It prints every line that contains the string abc and also displays the filename, so you know which file contains which lines.