Using fasmg as a pre/post processor:

[bitRAKE]

if definite FILE
virtual at 0
	DAT::
	file FILE
	DAT.BYTES := $
	db ?
end virtual


; processing here: convert CRLF pairs to LF
i = 0
while i < DAT.BYTES
	load pair:2 from DAT:i
	if pair <> $0A0D
		db pair and $FF
	end if
	i = i + 1
end while


else
	display 'usage: fasmg -i "define FILE ''<filename>''" ',__FILE__,' <outfile>'
end if

Note the need for double quotes around the "-i" option value under windows. Which forces single quotes around the FILE value.

[bitRAKE]

Another example adapted from Tomasz Grysztar's Learning binary file formats:

if definite FILE
file FILE

load STUB_SIGNATURE:2 from 0
load PE_OFFSET:4 from 0x3C
load PE_SIGNATURE:4 from PE_OFFSET

if STUB_SIGNATURE = "MZ" & PE_SIGNATURE = "PE"

	CHECKSUM_OFFSET = PE_OFFSET + 24 + 64

	CHECKSUM = 0
	store CHECKSUM:4 at CHECKSUM_OFFSET

	repeat $% shr 1, POSITION:0
		load H:word from :POSITION shl 1
		CHECKSUM = CHECKSUM + H
	end repeat
	while CHECKSUM shr 16
		CHECKSUM = CHECKSUM shr 16 + CHECKSUM and 0FFFFh
	end while
	CHECKSUM = CHECKSUM + $%

	store CHECKSUM:4 at CHECKSUM_OFFSET

else
        err 'PE format not recognized'
end if

else
	display 'usage: fasmg -i "define FILE ''<filename>''" ',__FILE__,' <outfile>'
end if

In contrast this example patches the existing file, whereas the prior produces an alternate output. Both templates are infinitely useful for creating small tools.