diff -r 000000000000 -r 6474c204b198 build/docs/environment-variables.rst
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+++ b/build/docs/environment-variables.rst	Wed Dec 31 06:09:35 2014 +0100
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+.. _environment_variables:
+
+================================================
+Environment Variables Impacting the Build System
+================================================
+
+Various environment variables have an impact on the behavior of the
+build system. This document attempts to document them.
+
+AUTOCLOBBER
+   If defines, the build system will automatically clobber as needed.
+   The default behavior is to print a message and error out when a
+   clobber is needed.
+
+   This variable is typically defined in a :ref:`mozconfig <mozconfig>`
+   file via ``mk_add_options``.
+
+REBUILD_CHECK
+   If defined, the build system will print information about why
+   certain files were rebuilt.
+
+   This feature is disabled by default because it makes the build slower.
+
+MACH_NO_TERMINAL_FOOTER
+   If defined, the terminal footer displayed when building with mach in
+   a TTY is disabled.
+
+MACH_NO_WRITE_TIMES
+   If defined, mach commands will not prefix output lines with the
+   elapsed time since program start. This option is equivalent to
+   passing ``--log-no-times`` to mach.
+
+MOZ_PSEUDO_DERECURSE
+   Activate an *experimental* build mode where make directory traversal
+   is derecursified. This mode should result in faster build times at
+   the expense of busted builds from time-to-time. The end goal is for
+   this build mode to be the default. At which time, this variable will
+   likely go away.
+
+   A value of ``1`` activates the mode with full optimizations.
+
+   A value of ``no-parallel-export`` activates the mode without
+   optimizations to the *export* tier, which are known to be slightly
+   buggy.
+
+   A value of ``no-skip`` activates the mode without optimizations to skip
+   some directories during traversal.
+
+   Values may be combined with a comma.