Answer by mwfearnley for awk: Built-in FILENAME variable on empty file

Note: parts of this answer are specific to GNU awk, specifically 4.0 and later, which added BEGINFILE/ENDFILE

Per-line block

awk '{print "File name: " FILENAME}' myfile

This will print File name: myfile once for every line in myfile. If myfile is a blank file (zero bytes), it will contain no lines, and so the above string won't be printed at all.

BEGINFILE block

awk 'BEGINFILE{print "File name: " FILENAME}' myfile

If supported, this will print File name: myfile once, before processing any lines.

Otherwise, if not supported, it will probably treat BEGINFILE as a falsy conditional expression, and print nothing at all.

BEGIN block

awk 'BEGIN{print "File name: " FILENAME}' myfile

This block is evaluated happens before the any of the files are processed, and at this time the value of FILENAME is not defined.

The gawk documentation specifically defines it as "" though, so we can know there it will just print File name: .