Trick to pass parameters on the command line

The common way to pass options and parameters to an Awk script is via the awk option -v varname=value

$ script.awk -v variablename1=value2 -v variablename=value2 filename1 filename2

This is really flexible and has many advantages, but in most cases, you want to use something more common - positional parameters i.e. something like this

$ script.awk filename parameter1 parameter2

Without some precautions, parameter1 and parameter2 will be treated as file names...

When Awk starts, the information from the command line will be stored in two internal variables ARGV - array with argument values and ARGC - scalar variable with the count of elements in ARGV. ARGV[0] will contain the name of the script itself (script.awk in this example), ARGV[1] - the name of the first file on the command line etc.

ARGV[0]="script.awk"
ARGV[1]="filename"
ARGV[2]="parameter1"
ARGV[3]="parameter2"

ARGC=4

Essentially, awk starts to read files with names taken from the elements of the ARGV array - except for ARGV[0].
Here is the neat trick - if you change the value ARGC=2, awk will "think" that there are only 2 elements - program name and first file and run the loop as usual for these 2 elements i.e. reading only filename. Without any surprise, the values in ARGV continue to be available...

input.dat

add_to_columns.awk

#!/usr/bin/awk -f
BEGIN {ARGC=2}

{print $1+ARGV[2],$2+ARGV[3]}

$ ./add_to_column.awk input.dat 1 2
2 4
4 6
6 8
8 10

This "trick" could be used in other ways - Rereading the Current File, changing the name of the next file to read as well (if you want) etc.