infile=...  # RPN input file with one record and tictac's (if needed for the grid type - if xrec can display the field you have everything you need)
outfile=... # RPN output file (including path or at least './')

# Create field with constant value 180 / PI = 57.29578
r.diag xlin   $infile  tmp  -a0-b57.29578

r.diag ggtrig tmp    tmp2 -kind IDF -lon  # ==> outfile=longitude*infile
r.diag newnam tmp2   lon  -name LO
r.diag ggtrig tmp    tmp2 -kind IDF       # ==> outfile=latitude*infile
r.diag newnam tmp2   lat  -name LA

# Combine LO and LA in output file
r.diag joinup $outfile lon lat