awk Built-in Variables FNR - The Current Record Number being processed
Example
FNR contains the number of the input file row being processed. In this example you will see awk starting on 1 again when starting to process the second file.
Example with one file
$ cat file1
AAAA
BBBB
CCCC
$ awk '{ print FNR }' file1
1
2
3
Example with two files
$ cat file1
AAAA
BBBB
CCCC
$ cat file2
WWWW
XXXX
YYYY
ZZZZ
$ awk '{ print FNR, FILENAME, $0 }' file1 file2
1 file1 AAAA
2 file1 BBBB
3 file1 CCCC
1 file2 WWWW
2 file2 XXXX
3 file2 YYYY
4 file2 ZZZZ
Extended example with two files
FNR can be used to detect if awk is processing the first file since NR==FNR is true only for the first file. For example, if we want to join records from files file1 and file2 on their FNR:
$ awk 'NR==FNR { a[FNR]=$0; next } (FNR in a) { print FNR, a[FNR], $1 }' file1 file2
1 AAAA WWWW
2 BBBB XXXX
3 CCCC YYYY
Record ZZZZ from file2 is missing as FNR has different max value for file1 and file2 and there is no join for differing FNRs.
