CLASS uses two files of data; one for input and one for output, which may be the same actual file. The input file is only used to read. An observation contains several independent sections. These file are defined by the command FILE (IN,OUT,BOTH) Filename, possibly followed by NEW if a new file is to be initialized. The default extension of files is site dependent : it is .30M at Pico Veleta, .BUR at the Groupe d'Astrophysique in Grenoble and on Plateau de Bure, and so on).
Commands FIND and GET operate only on the input file. Command WRITE operates only on the output file. When both are identical, WRITE creates physically a new version of the spectrum in the file, keeping the previous one. In some cases one may override the old version by the new one by the UPDATE command. To do this, the file must be used for both input and output, and each of the sections of the new version must fit in the space used by the old one. Other commands are subject to similar restrictions, in particular KEEP to save the gaussian fit results. If you want to play safe, always use WRITE .
CLASS also keeps a log file (named SYS$LOGIN:CLASS.LOG, created by the SIC monitor) and a message file SYS$LOGIN:CLASS.MES, which are systematically purged at the end of each session. These files may be used to keep track of a batch or interactive work. Finally the session environment may be saved as a command file containing a set of SET commands.