| prog.r | ratfor program |
| prog.f | fortran program |
| prog.o | object |
| prog.x | linked object |
| plot.float | seismogram values |
| plot.vplot | intermediate plot language |
| plot.ps | postscript plot language |
More generally there is not a line of files above plot.ps but a {\it tree} of them. The tree includes parameter files, subroutines, headers, etc. But to understand make and cake it is sufficient to think about the linear dependences above. The first thing to notice is that each file in the above list should be younger that the one above and older than the one beneath. If this is not true, then a file is said to be out of date. When a file is out of date, then invoking make or cake will fire off the minimum number of commands needed to bring everything up to date. For example there might be a plot parameter that is used in the creation of the vplot file from the float file. When the parameter file is edited its date changes and thereafter an invocation of make or cake will rebuild only the plot files without recompiling anything. In summary, a makefile is a command sequence with the extra feature that any program will not be run its output is already younger than the program and all its inputs. For example, the command ``make figure.ps'' does nothing to a figure file that is up to date, but otherwise it does everything needed to rebuild the figure. A pushbutton in a figure caption can fire off the make command.