There are a lot of environment variables in any Meteor version. You probably know MONGO_URL and ROOT_URL, but how many others are there ? Do the Meteor guide and the Meteor docs reveal them all ? Guess no longer: just ask the source.
To find them all, all it takes is this one-liner:
grep -roP "process\.env\.([\w]+)" ~/.meteor/ | sed 's/.*:process\.env\.//' | sort | uniq | pr -3Tw140Just for fun, here are the results on a fresh Meteor 1.3.2.4 deployment.
_ METEOR_OPLOG_TOO_FAR_BEHIND npm_config_tmp
APPDATA METEOR_PACKAGE_SERVER_URL npm_config__underbar_env_thing
APP_ID METEOR_PACKAGE_STATS_SERVER_URL NPM_CONFIG_UPPERCASE_ENV_THING
APP_MONGO METEOR_PACKAGE_STATS_TEST_OUTPUT npm_config_userconfig
AUTOUPDATE_VERSION METEOR_PARENT_PID npm_config_x
BABEL_CACHE_DIR METEOR_PKG_SPIDERABLE_PHANTOMJS_ARGS npm_config_yes
BABEL_CACHE_PATH METEOR_PRETTY_OUTPUT npm_execpath
BABEL_DISABLE_CACHE METEOR_PRINT_ON_LISTEN npm_lifecycle_script
BABEL_ENV METEOR_PROGRESS_DEBUG npm_package_config_port
BIND_IP METEOR_SAVE_TMPDIRS npm_package_name
BLUEBIRD_DEBUG METEOR_SESSION_FILE npm_package_run_script_foo_var
CAFILE METEOR_SETTINGS npm_package_scripts_install
CMD METEOR_SHELL_DIR npm_run_script_foo_var
COMP_CWORD METEOR_SKIP_NPM_REBUILD NVM_IOJS_ORG_MIRROR
COMP_LINE METEOR_TEST_FAIL_RELEASE_DOWNLOAD NVM_NODEJS_ORG_MIRROR
COMP_POINT METEOR_TEST_FAKE_MONGOD_CONTROL_PORT OBJECT_IMPL
COMPUTERNAME METEOR_TEST_LATEST_RELEASE OSTYPE
comspec METEOR_TEST_MULTIPLE_MONGOD_REPLSET PACKAGE_DIRS
ComSpec METEOR_TEST_NO_PUBLISH path
CORDOVA_PLUGIN_ID METEOR_TEST_PRINT_LINKER_CACHE_DEBUG Path
DDP_DEFAULT_CONNECTION_URL METEOR_TEST_TMP PATH
DEBUG METEOR_TOOL_PATH PATHEXT
DEBUG_COLORS METEOR_WAREHOUSE_DIR PHANTOMJS_CDNURL
DEBUG_FD METEOR_WATCH_FORCE_POLLING PORT
DEBUG_MIME METEOR_WATCH_POLLING_INTERVAL_MS PREFIX
DEBUG_NOPT MOBILE_DDP_URL PROMPT
DEPLOY_DOMAIN MOBILE_ROOT_URL PS1
DEPLOY_HOSTNAME MONGO_OPLOG_URL PUMMEL
DESTDIR MONGO_URL PWD
DISABLE_WEBSOCKETS MSYSTEM PYTHON
DISCOVERY_PORT MW Q_DEBUG
EDITOR NODE_BINDINGS_ARROW random_env_var
EMACS NODE_BINDINGS_COMPILED_DIR ROOT_URL
EXPRESS_COV NODE_DEBUG RUN_ONCE_OUTCOME
FAST NODE_DISABLE_COLORS run_script_foo_var
GALAXY_MOCK_MODE NODE_ENV SAUCE_KEY
GALAXY_PASSWORD NODEJS_ORG_MIRROR SAUCE_PROJ
GALAXY_USERNAME NODE_NDEBUG SAUCE_USER
HOME NODE_OPTIONS SERVER_WEBSOCKET_COMPRESSION
HOSTNAME NODE_PATH SHELL
http_proxy NODE_PATHS STYLUS_COV
HTTP_PROXY NO_DEPRECATION SUDO_GID
https_proxy NODE_TLS_REJECT_UNAUTHORIZED SUDO_UID
HTTPS_PROXY NO_METEOR_PACKAGE SystemDrive
IOJS_ORG_MIRROR no_proxy SystemRoot
JOBS NO_PROXY TEMP
LANG NOPT_DEBUG TERM
LC_ALL npm_config_ TEST_COV
LC_CTYPE npm_config_argv TEST_GRACEFUL_FS_GLOBAL_PATCH
MAIL_URL npm_config__auth TEST_METADATA
MAKE npm_config__authCrypt TEST_PSEUDOMAP
METEOR_APP_PRELINK_CACHE_SIZE npm_config_cache TEST_REGEN
METEOR_BAD_PARENT_PID_FOR_TEST npm_config_depth THROW_FROM_PACKAGE
METEOR_BUILD_FARM_URL npm_config_global TIMEOUT_SCALE_FACTOR
METEOR_CATALOG_COMPRESS_RPCS npm_config_globalconfig TIMES
METEOR_COLOR npm_config_init_author_email TINYTEST_FILTER
METEOR_DEBUG_BUILD npm_config_init_author_name TMP
METEOR_DEBUG_SPRINGBOARD npm_config_init_author_url TMPDIR
METEOR_DEBUG_SQL npm_config_init_license TRACE_DEPRECATION
METEOR_DISABLE_BUILDER_IN_PLACE npm_config_init_version TRAVIS
METEOR_FORCE_PORTABLE npm_config_loglevel TRAVIS_BRANCH
METEOR_INSTALLATION npm_config_node_version TRAVIS_BUILD_NUMBER
METEOR_LINKER_CACHE_SIZE npm_config_other_env_thing TRAVIS_JOB_ID
METEOR_LOG npm_config__password TRAVIS_JOB_NUMBER
METEOR_NO_WORDWRAP npm_config_prefix TRAVIS_NODE_VERSION
METEOR_OFFLINE_CATALOG npm_config_proxy TRAVIS_PULL_REQUEST
TREE_HASH_DEBUG USERDOMAIN windir
UNLAZY USERNAME WRITE_RUNNER_JS
USE_JSESSIONID USERPROFILE YOUR_APP_ENV
Seriously ?