diff --git a/tests/test-fix.t b/tests/test-fix.t
--- a/tests/test-fix.t
+++ b/tests/test-fix.t
@@ -1,3 +1,55 @@
+A script that implements uppercasing of specific lines in a file. This
+approximates the behavior of code formatters well enough for our tests.
+
+  $ UPPERCASEPY="$TESTTMP/uppercase.py"
+  $ cat > $UPPERCASEPY <<EOF
+  > import sys
+  > lines = set()
+  > for arg in sys.argv[1:]:
+  >   if arg == 'all':
+  >     sys.stdout.write(sys.stdin.read().upper())
+  >     sys.exit(0)
+  >   else:
+  >     first, last = arg.split('-')
+  >     lines.update(range(int(first), int(last) + 1))
+  > for i, line in enumerate(sys.stdin.readlines()):
+  >   if i + 1 in lines:
+  >     sys.stdout.write(line.upper())
+  >   else:
+  >     sys.stdout.write(line)
+  > EOF
+  $ TESTLINES="foo\nbar\nbaz\nqux\n"
+  $ printf $TESTLINES | $PYTHON $UPPERCASEPY
+  foo
+  bar
+  baz
+  qux
+  $ printf $TESTLINES | $PYTHON $UPPERCASEPY all
+  FOO
+  BAR
+  BAZ
+  QUX
+  $ printf $TESTLINES | $PYTHON $UPPERCASEPY 1-1
+  FOO
+  bar
+  baz
+  qux
+  $ printf $TESTLINES | $PYTHON $UPPERCASEPY 1-2
+  FOO
+  BAR
+  baz
+  qux
+  $ printf $TESTLINES | $PYTHON $UPPERCASEPY 2-3
+  foo
+  BAR
+  BAZ
+  qux
+  $ printf $TESTLINES | $PYTHON $UPPERCASEPY 2-2 4-4
+  foo
+  BAR
+  baz
+  QUX
+
 Set up the config with two simple fixers: one that fixes specific line ranges,
 and one that always fixes the whole file. They both "fix" files by converting
 letters to uppercase. They use different file extensions, so each test case can
@@ -10,10 +62,10 @@
   > evolution.createmarkers=True
   > evolution.allowunstable=True
   > [fix]
-  > uppercase-whole-file:command=sed -e 's/.*/\U&/'
+  > uppercase-whole-file:command=$PYTHON $UPPERCASEPY all
   > uppercase-whole-file:fileset=set:**.whole
-  > uppercase-changed-lines:command=sed
-  > uppercase-changed-lines:linerange=-e '{first},{last} s/.*/\U&/'
+  > uppercase-changed-lines:command=$PYTHON $UPPERCASEPY
+  > uppercase-changed-lines:linerange={first}-{last}
   > uppercase-changed-lines:fileset=set:**.changed
   > EOF
 
@@ -878,7 +930,7 @@
   $ hg commit -Aqm "foo"
   $ printf "Foo\nbar\nBaz\n" > foo.changed
   $ hg --debug fix --working-dir
-  subprocess: sed -e '1,1 s/.*/\U&/' -e '3,3 s/.*/\U&/'
+  subprocess: /usr/bin/python $TESTTMP/uppercase.py 1-1 3-3
 
   $ cd ..