(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_FUNC_CACHE

: If defined, disables mach's persistent function result cache. This
  cache speeds up operations like configure by avoiding redundant
  expensive computations when input files haven't changed.

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.