GMT programs ``remember'' the standardized command line options (See Section 4.4) given during their previous invocations and this provides a shorthand notation for complex options. For example, if a basemap was created with an oblique Mercator projection, specified as
-Joc170W/25:30S/33W/56:20N/1:500000
then a subsequent psxy command to plot symbols only needs
to state -Jo in order to activate the same projection. In
contrast, note that -J by itself will pick the most recently used projection.
Previous commands are maintained in the file .gmtcommands4,
of which there will be one in each directory you run the programs
from. This is handy if you create separate directories for
separate projects since chances are that data manipulations
and plotting for each project will share many of the same options.
Note that an option spelled out on the command line will always
override the last entry in the .gmtcommands4 file and,
if execution is successful, will replace this entry as the
previous option argument in the .gmtcommands4 file.
If you call several GMT modules piped together then GMT cannot
guarantee that the .gmtcommands4 file is processed
in the intended order from left to right. The only guarantee
is that the file will not be clobbered since GMT uses advisory
file locking. The uncertainty in processing order makes the use
of shorthands in pipes unreliable. We therefore recommend that you
only use shorthands in single process command lines, and spell out
the full command option when using chains of commands connected with
pipes.