Command line argument in awk

Arguments given at the end of the command line to awk are generally taken as filenames that the awk script will read from. To set a variable on the command line, use -v variable=value, e.g.

awk -v num=10 -f script.awk

This would enable you to use num as a variable in your script. The initial value of the variable will be 10 in the above example.

You may also read environment variables using ENVIRON["variable"] in your script (for some environment variable named variable), or look at the command line arguments with ARGV[n] where n is some positive integer.

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With $1 in awk, you would refer to the value of the first field in the current record, but since you are using it in a BEGIN block, no data has yet been read from any file.

The number in your code is being interpreted as zero since it's an empty variable used in an arithmetic context.

$1 is not the first command line argument, but the first field after the line was split with FS (and it will be the empty string in BEGIN, since no line was split yet).

Command line arguments are in the array ARGV:

$ awk 'BEGIN { for(i = 1; i < ARGC; i++) print ARGV[i] }' 1st 2nd 3rd
1st
2nd
3rd

ARGV[0] is always the name of the interpreter (awk or gawk, etc).

In order to let awk ignore a command line argument and not try to open it later as a file you should delete it or set it to the empty string: eg. ARGV[1]="".

As a side note, any argument of the form var=value will also be interpreted as a variable assignment by awk, and will be eval'ed after the file arguments that precede it have been processed:

$ echo yes > file
$ awk '{ gsub(/e/, var); print }' var=1 file var=2 file var=3 file
y1s
y2s
y3s

To use an actual filename of the form key=val with awk, you should pass it as a relative or absolute path eg. awk '{...}' ./key=val.