/usr/share/gmt/x2sys/xy.def is in gmt-common 5.3.1+dfsg-2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 | # $Id: xy.def 12822 2014-01-31 23:39:56Z remko $
#
# Define file for X2SYS processing of ASCII xy files
#
# This file applies to plain 2-column ASCII files.
# Only locations can be found since there are no data columns
#
#---------------------------------------------------------------------
#ASCII # The input file is ASCII
#SKIP 1 # The number of header records to skip
#---------------------------------------------------------------------
#name intype NaN-proxy? NaN-proxy scale offset oformat
x a N 0 1 0 %g
y a N 0 1 0 %g
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