diff --git a/lib/bup/git.py b/lib/bup/git.py
index 02a9597013ec..998e875cf8fd 100644
--- a/lib/bup/git.py
+++ b/lib/bup/git.py
@@ -32,7 +32,7 @@ from bup.helpers import (Sha1, add_error, chunkyreader, debug1,
                          progress, qprogress, stat_if_exists,
                          unlink,
                          utc_offset_str,
-                         ExistsResult, ObjectExists)
+                         ExistsResult, ObjectExists, bupenv)
 
 
 verbose = 0
@@ -50,7 +50,7 @@ class GitError(Exception):
 
 
 def _gitenv(repo_dir):
-    return merge_dict(environ, {b'GIT_DIR': os.path.abspath(repo_dir)})
+    return merge_dict(bupenv(), {b'GIT_DIR': os.path.abspath(repo_dir)})
 
 def _git_wait(cmd, p):
     rv = p.wait()
@@ -261,7 +261,7 @@ def repo_rel(path):
 def auto_midx(objdir):
     args = [path.exe(), b'midx', b'--auto', b'--dir', objdir]
     try:
-        rv = subprocess.call(args, stdout=open(os.devnull, 'w'))
+        rv = subprocess.call(args, stdout=open(os.devnull, 'w'), env=bupenv())
     except OSError as e:
         # make sure 'args' gets printed to help with debugging
         add_error('%r: exception: %s' % (args, e))
@@ -271,7 +271,7 @@ def auto_midx(objdir):
 
     args = [path.exe(), b'bloom', b'--dir', objdir]
     try:
-        rv = subprocess.call(args, stdout=open(os.devnull, 'w'))
+        rv = subprocess.call(args, stdout=open(os.devnull, 'w'), env=bupenv())
     except OSError as e:
         # make sure 'args' gets printed to help with debugging
         add_error('%r: exception: %s' % (args, e))
diff --git a/lib/bup/helpers.py b/lib/bup/helpers.py
index af7b372b9712..355231488e94 100644
--- a/lib/bup/helpers.py
+++ b/lib/bup/helpers.py
@@ -5,7 +5,6 @@ from collections import namedtuple
 from contextlib import contextmanager
 from ctypes import sizeof, c_void_p
 from math import floor
-from os import environ
 from subprocess import PIPE, Popen
 import sys, os, subprocess, errno, select, mmap, stat, re, struct
 import hashlib, heapq, math, operator, time, tempfile
@@ -188,6 +187,11 @@ def progress(s):
         _last_progress = s
 
 
+def bupenv():
+    ttymask = (1 if istty1 else 0) | (2 if istty2 else 0)
+    return merge_dict(compat.environ, {b'BUP_FORCE_TTY': str(ttymask)})
+
+
 TIME_ESTIMATE_CUTOFF = 5 # seconds
 
 
@@ -460,7 +464,7 @@ def _argmax_base(command):
     base_size = 2048
     for c in command:
         base_size += len(command) + 1
-    for k, v in compat.items(environ):
+    for k, v in compat.items(os.environ):
         base_size += len(k) + len(v) + 2 + sizeof(c_void_p)
     return base_size