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2.2 Environment

There are also several environment variables (of the operating system, not within gtroff) which can modify the behavior of groff.

This search path, followed by PATH, is used for commands executed by groff.
If this is set to X, then groff runs Xtroff instead of gtroff. This also applies to tbl, pic, eqn, grn, chem, refer, and soelim. It does not apply to grops, grodvi, grotty, pre-grohtml, post-grohtml, preconv, grolj4, gropdf, and gxditview.

The default command prefix is determined during the installation process. If a non-GNU troff system is found, prefix ‘g’ is used, none otherwise.

The value of this environment value is passed to the preconv preprocessor to select the encoding of input files. Setting this option implies groff's command line option -k (this is, groff actually always calls preconv). If set without a value, groff calls preconv without arguments. An explicit -K command line option overrides the value of GROFF_ENCODING. See the manual page of preconv for details.
A colon-separated list of directories in which to search for the devname directory (before the default directories are tried). See Font Directories.
A colon-separated list of directories in which to search for macro files (before the default directories are tried). See Macro Directories.
The directory in which groff creates temporary files. If this is not set and TMPDIR is set, temporary files are created in that directory. Otherwise temporary files are created in a system-dependent default directory (on Unix and GNU/Linux systems, this is usually /tmp). grops, grefer, pre-grohtml, and post-grohtml can create temporary files in this directory.
The default output device.

Note that MS-DOS and MS-Windows ports of groff use semi-colons, rather than colons, to separate the directories in the lists described above.