diff --git a/t/Makefile b/t/Makefile
index 25c559bb49..93a64750eb 100644
--- a/t/Makefile
+++ b/t/Makefile
@@ -6,10 +6,14 @@
 -include ../config.mak
 
 #GIT_TEST_OPTS=--verbose --debug
+GIT_TEST_CMP ?= $(DIFF)
 SHELL_PATH ?= $(SHELL)
 TAR ?= $(TAR)
 RM ?= rm -f
 
+# Make sure test-lib.sh uses make's value of GIT_TEST_CMP
+export GIT_TEST_CMP
+
 # Shell quote;
 SHELL_PATH_SQ = $(subst ','\'',$(SHELL_PATH))
 
diff --git a/t/t0000-basic.sh b/t/t0000-basic.sh
index f4ca4fc85c..5dd18c05ab 100755
--- a/t/t0000-basic.sh
+++ b/t/t0000-basic.sh
@@ -280,7 +280,7 @@ $expectfilter >expected <<\EOF
 EOF
 test_expect_success \
     'validate git diff-files output for a know cache/work tree state.' \
-    'git diff-files >current && diff >/dev/null -b current expected'
+    'git diff-files >current && test_cmp current expected >/dev/null'
 
 test_expect_success \
     'git update-index --refresh should succeed.' \
diff --git a/t/t3200-branch.sh b/t/t3200-branch.sh
index e0b760513c..8818d0de68 100755
--- a/t/t3200-branch.sh
+++ b/t/t3200-branch.sh
@@ -43,7 +43,7 @@ test_expect_success \
      git branch -l d/e/f &&
 	 test -f .git/refs/heads/d/e/f &&
 	 test -f .git/logs/refs/heads/d/e/f &&
-	 diff expect .git/logs/refs/heads/d/e/f'
+	 test_cmp expect .git/logs/refs/heads/d/e/f'
 
 test_expect_success \
     'git branch -d d/e/f should delete a branch and a log' \
@@ -222,7 +222,7 @@ test_expect_success \
      git checkout -b g/h/i -l master &&
 	 test -f .git/refs/heads/g/h/i &&
 	 test -f .git/logs/refs/heads/g/h/i &&
-	 diff expect .git/logs/refs/heads/g/h/i'
+	 test_cmp expect .git/logs/refs/heads/g/h/i'
 
 test_expect_success 'avoid ambiguous track' '
 	git config branch.autosetupmerge true &&
diff --git a/t/t3210-pack-refs.sh b/t/t3210-pack-refs.sh
index 413019acaf..525174013c 100755
--- a/t/t3210-pack-refs.sh
+++ b/t/t3210-pack-refs.sh
@@ -28,7 +28,7 @@ test_expect_success \
      SHA1=`cat .git/refs/heads/a` &&
      echo "$SHA1 refs/heads/a" >expect &&
      git show-ref a >result &&
-     diff expect result'
+     test_cmp expect result'
 
 test_expect_success \
     'see if a branch still exists when packed' \
@@ -37,7 +37,7 @@ test_expect_success \
      rm -f .git/refs/heads/b &&
      echo "$SHA1 refs/heads/b" >expect &&
      git show-ref b >result &&
-     diff expect result'
+     test_cmp expect result'
 
 test_expect_success 'git branch c/d should barf if branch c exists' '
      git branch c &&
@@ -52,7 +52,7 @@ test_expect_success \
      git pack-refs --all --prune &&
      echo "$SHA1 refs/heads/e" >expect &&
      git show-ref e >result &&
-     diff expect result'
+     test_cmp expect result'
 
 test_expect_success 'see if git pack-refs --prune remove ref files' '
      git branch f &&
@@ -109,7 +109,7 @@ test_expect_success 'pack, prune and repack' '
 	git show-ref >all-of-them &&
 	git pack-refs &&
 	git show-ref >again &&
-	diff all-of-them again
+	test_cmp all-of-them again
 '
 
 test_done
diff --git a/t/t3903-stash.sh b/t/t3903-stash.sh
index 476e5ec038..7e47030a15 100755
--- a/t/t3903-stash.sh
+++ b/t/t3903-stash.sh
@@ -81,7 +81,7 @@ test_expect_success 'drop top stash' '
 	git stash &&
 	git stash drop &&
 	git stash list > stashlist2 &&
-	diff stashlist1 stashlist2 &&
+	test_cmp stashlist1 stashlist2 &&
 	git stash apply &&
 	test 3 = $(cat file) &&
 	test 1 = $(git show :file) &&
diff --git a/t/t4002-diff-basic.sh b/t/t4002-diff-basic.sh
index 18695ce821..73441a5165 100755
--- a/t/t4002-diff-basic.sh
+++ b/t/t4002-diff-basic.sh
@@ -135,7 +135,7 @@ cmp_diff_files_output () {
     # filesystem.
     sed <"$2" >.test-tmp \
 	-e '/^:000000 /d;s/'$x40'\( [MCRNDU][0-9]*\)	/'$z40'\1	/' &&
-    diff "$1" .test-tmp
+    test_cmp "$1" .test-tmp
 }
 
 test_expect_success \
diff --git a/t/t4124-apply-ws-rule.sh b/t/t4124-apply-ws-rule.sh
index fb9ad247bf..bef3bcb1b2 100755
--- a/t/t4124-apply-ws-rule.sh
+++ b/t/t4124-apply-ws-rule.sh
@@ -44,7 +44,7 @@ test_fix () {
 	apply_patch --whitespace=fix || return 1
 
 	# find touched lines
-	diff file target | sed -n -e "s/^> //p" >fixed
+	$DIFF file target | sed -n -e "s/^> //p" >fixed
 
 	# the changed lines are all expeced to change
 	fixed_cnt=$(wc -l <fixed)
@@ -85,14 +85,14 @@ test_expect_success setup '
 test_expect_success 'whitespace=nowarn, default rule' '
 
 	apply_patch --whitespace=nowarn &&
-	diff file target
+	test_cmp file target
 
 '
 
 test_expect_success 'whitespace=warn, default rule' '
 
 	apply_patch --whitespace=warn &&
-	diff file target
+	test_cmp file target
 
 '
 
@@ -108,7 +108,7 @@ test_expect_success 'whitespace=error-all, no rule' '
 
 	git config core.whitespace -trailing,-space-before,-indent &&
 	apply_patch --whitespace=error-all &&
-	diff file target
+	test_cmp file target
 
 '
 
@@ -117,7 +117,7 @@ test_expect_success 'whitespace=error-all, no rule (attribute)' '
 	git config --unset core.whitespace &&
 	echo "target -whitespace" >.gitattributes &&
 	apply_patch --whitespace=error-all &&
-	diff file target
+	test_cmp file target
 
 '
 
diff --git a/t/t4127-apply-same-fn.sh b/t/t4127-apply-same-fn.sh
index 3a8202ea93..77200c0b2d 100755
--- a/t/t4127-apply-same-fn.sh
+++ b/t/t4127-apply-same-fn.sh
@@ -27,7 +27,7 @@ test_expect_success 'apply same filename with independent changes' '
 	cp same_fn same_fn2 &&
 	git reset --hard &&
 	git apply patch0 &&
-	diff same_fn same_fn2
+	test_cmp same_fn same_fn2
 '
 
 test_expect_success 'apply same filename with overlapping changes' '
@@ -40,7 +40,7 @@ test_expect_success 'apply same filename with overlapping changes' '
 	cp same_fn same_fn2 &&
 	git reset --hard &&
 	git apply patch0 &&
-	diff same_fn same_fn2
+	test_cmp same_fn same_fn2
 '
 
 test_expect_success 'apply same new filename after rename' '
@@ -54,7 +54,7 @@ test_expect_success 'apply same new filename after rename' '
 	cp new_fn new_fn2 &&
 	git reset --hard &&
 	git apply --index patch1 &&
-	diff new_fn new_fn2
+	test_cmp new_fn new_fn2
 '
 
 test_expect_success 'apply same old filename after rename -- should fail.' '
diff --git a/t/t5300-pack-object.sh b/t/t5300-pack-object.sh
index 7649b810b1..bbb9c1251d 100755
--- a/t/t5300-pack-object.sh
+++ b/t/t5300-pack-object.sh
@@ -147,7 +147,7 @@ test_expect_success \
 	    git cat-file $t $object || return 1
 	 done <obj-list
     } >current &&
-    diff expect current'
+    test_cmp expect current'
 
 test_expect_success \
     'use packed deltified (REF_DELTA) objects' \
@@ -162,7 +162,7 @@ test_expect_success \
 	    git cat-file $t $object || return 1
 	 done <obj-list
     } >current &&
-    diff expect current'
+    test_cmp expect current'
 
 test_expect_success \
     'use packed deltified (OFS_DELTA) objects' \
@@ -177,7 +177,7 @@ test_expect_success \
 	    git cat-file $t $object || return 1
 	 done <obj-list
     } >current &&
-    diff expect current'
+    test_cmp expect current'
 
 unset GIT_OBJECT_DIRECTORY
 
diff --git a/t/t5510-fetch.sh b/t/t5510-fetch.sh
index 721821ec92..4eb10f602f 100755
--- a/t/t5510-fetch.sh
+++ b/t/t5510-fetch.sh
@@ -71,7 +71,7 @@ test_expect_success "fetch test for-merge" '
 		echo "$one_in_two	"
 	} >expected &&
 	cut -f -2 .git/FETCH_HEAD >actual &&
-	diff expected actual'
+	test_cmp expected actual'
 
 test_expect_success 'fetch tags when there is no tags' '
 
diff --git a/t/t5520-pull.sh b/t/t5520-pull.sh
index dd2ee842e0..319e389ed0 100755
--- a/t/t5520-pull.sh
+++ b/t/t5520-pull.sh
@@ -26,7 +26,7 @@ cd "$D"
 test_expect_success 'checking the results' '
 	test -f file &&
 	test -f cloned/file &&
-	diff file cloned/file
+	test_cmp file cloned/file
 '
 
 test_expect_success 'pulling into void using master:master' '
diff --git a/t/t5700-clone-reference.sh b/t/t5700-clone-reference.sh
index 1c10916069..895f5595ae 100755
--- a/t/t5700-clone-reference.sh
+++ b/t/t5700-clone-reference.sh
@@ -48,7 +48,7 @@ test_expect_success 'that reference gets used' \
 'cd C &&
 echo "0 objects, 0 kilobytes" > expected &&
 git count-objects > current &&
-diff expected current'
+test_cmp expected current'
 
 cd "$base_dir"
 
@@ -75,7 +75,7 @@ cd "$base_dir"
 test_expect_success 'that reference gets used' \
 'cd D && echo "0 objects, 0 kilobytes" > expected &&
 git count-objects > current &&
-diff expected current'
+test_cmp expected current'
 
 cd "$base_dir"
 
@@ -100,7 +100,7 @@ test_expect_success 'that alternate to origin gets used' \
 'cd C &&
 echo "2 objects" > expected &&
 git count-objects | cut -d, -f1 > current &&
-diff expected current'
+test_cmp expected current'
 
 cd "$base_dir"
 
@@ -116,7 +116,7 @@ test_expect_success 'check objects expected to exist locally' \
 'cd D &&
 echo "5 objects" > expected &&
 git count-objects | cut -d, -f1 > current &&
-diff expected current'
+test_cmp expected current'
 
 cd "$base_dir"
 
diff --git a/t/t6000lib.sh b/t/t6000lib.sh
index 985d517a1c..ea25dd89e5 100644
--- a/t/t6000lib.sh
+++ b/t/t6000lib.sh
@@ -91,7 +91,7 @@ check_output()
 	shift 1
 	if eval "$*" | entag > $_name.actual
 	then
-		diff $_name.expected $_name.actual
+		test_cmp $_name.expected $_name.actual
 	else
 		return 1;
 	fi
diff --git a/t/t6001-rev-list-graft.sh b/t/t6001-rev-list-graft.sh
index b2131cdacd..fc57e7d3fd 100755
--- a/t/t6001-rev-list-graft.sh
+++ b/t/t6001-rev-list-graft.sh
@@ -84,7 +84,7 @@ check () {
 		git rev-list --parents --pretty=raw $arg |
 		sed -n -e 's/^commit //p' >test.actual
 	fi
-	diff test.expect test.actual
+	test_cmp test.expect test.actual
 }
 
 for type in basic parents parents-raw
diff --git a/t/t6022-merge-rename.sh b/t/t6022-merge-rename.sh
index e3f7ae8120..b66544b76d 100755
--- a/t/t6022-merge-rename.sh
+++ b/t/t6022-merge-rename.sh
@@ -280,7 +280,7 @@ test_expect_success 'updated working tree file should prevent the merge' '
 		echo "BAD: should have complained"
 		return 1
 	}
-	diff M M.saved || {
+	test_cmp M M.saved || {
 		echo "BAD: should have left M intact"
 		return 1
 	}
@@ -301,7 +301,7 @@ test_expect_success 'updated working tree file should prevent the merge' '
 		echo "BAD: should have complained"
 		return 1
 	}
-	diff M M.saved || {
+	test_cmp M M.saved || {
 		echo "BAD: should have left M intact"
 		return 1
 	}
diff --git a/t/t7002-grep.sh b/t/t7002-grep.sh
index e249c3ed41..8a6322765c 100755
--- a/t/t7002-grep.sh
+++ b/t/t7002-grep.sh
@@ -60,7 +60,7 @@ do
 			echo ${HC}file:5:foo_mmap bar mmap baz
 		} >expected &&
 		git grep -n -w -e mmap $H >actual &&
-		diff expected actual
+		test_cmp expected actual
 	'
 
 	test_expect_success "grep -w $L (w)" '
@@ -74,7 +74,7 @@ do
 			echo ${HC}x:1:x x xx x
 		} >expected &&
 		git grep -n -w -e "x xx* x" $H >actual &&
-		diff expected actual
+		test_cmp expected actual
 	'
 
 	test_expect_success "grep -w $L (y-1)" '
@@ -82,7 +82,7 @@ do
 			echo ${HC}y:1:y yy
 		} >expected &&
 		git grep -n -w -e "^y" $H >actual &&
-		diff expected actual
+		test_cmp expected actual
 	'
 
 	test_expect_success "grep -w $L (y-2)" '
@@ -93,7 +93,7 @@ do
 			cat actual
 			false
 		else
-			diff expected actual
+			test_cmp expected actual
 		fi
 	'
 
@@ -105,14 +105,14 @@ do
 			cat actual
 			false
 		else
-			diff expected actual
+			test_cmp expected actual
 		fi
 	'
 
 	test_expect_success "grep $L (t-1)" '
 		echo "${HC}t/t:1:test" >expected &&
 		git grep -n -e test $H >actual &&
-		diff expected actual
+		test_cmp expected actual
 	'
 
 	test_expect_success "grep $L (t-2)" '
@@ -121,7 +121,7 @@ do
 			cd t &&
 			git grep -n -e test $H
 		) >actual &&
-		diff expected actual
+		test_cmp expected actual
 	'
 
 	test_expect_success "grep $L (t-3)" '
@@ -130,7 +130,7 @@ do
 			cd t &&
 			git grep --full-name -n -e test $H
 		) >actual &&
-		diff expected actual
+		test_cmp expected actual
 	'
 
 	test_expect_success "grep -c $L (no /dev/null)" '
diff --git a/t/t7005-editor.sh b/t/t7005-editor.sh
index 5257f4d261..25b6d2e67c 100755
--- a/t/t7005-editor.sh
+++ b/t/t7005-editor.sh
@@ -38,7 +38,7 @@ test_expect_success setup '
 	test_commit "$msg" &&
 	echo "$msg" >expect &&
 	git show -s --format=%s > actual &&
-	diff actual expect
+	test_cmp actual expect
 
 '
 
@@ -85,7 +85,7 @@ do
 		git --exec-path=. commit --amend &&
 		git show -s --pretty=oneline |
 		sed -e "s/^[0-9a-f]* //" >actual &&
-		diff actual expect
+		test_cmp actual expect
 	'
 done
 
@@ -107,7 +107,7 @@ do
 		git --exec-path=. commit --amend &&
 		git show -s --pretty=oneline |
 		sed -e "s/^[0-9a-f]* //" >actual &&
-		diff actual expect
+		test_cmp actual expect
 	'
 done
 
diff --git a/t/t9200-git-cvsexportcommit.sh b/t/t9200-git-cvsexportcommit.sh
index fc3795dc98..61bcb8fc86 100755
--- a/t/t9200-git-cvsexportcommit.sh
+++ b/t/t9200-git-cvsexportcommit.sh
@@ -63,10 +63,10 @@ test_expect_success \
      check_entries B "newfile2.txt/1.1/" &&
      check_entries C "newfile3.png/1.1/-kb" &&
      check_entries D "newfile4.png/1.1/-kb" &&
-     diff A/newfile1.txt ../A/newfile1.txt &&
-     diff B/newfile2.txt ../B/newfile2.txt &&
-     diff C/newfile3.png ../C/newfile3.png &&
-     diff D/newfile4.png ../D/newfile4.png
+     test_cmp A/newfile1.txt ../A/newfile1.txt &&
+     test_cmp B/newfile2.txt ../B/newfile2.txt &&
+     test_cmp C/newfile3.png ../C/newfile3.png &&
+     test_cmp D/newfile4.png ../D/newfile4.png
      )'
 
 test_expect_success \
@@ -89,10 +89,10 @@ test_expect_success \
      check_entries D "newfile4.png/1.2/-kb" &&
      check_entries E "newfile5.txt/1.1/" &&
      check_entries F "newfile6.png/1.1/-kb" &&
-     diff A/newfile1.txt ../A/newfile1.txt &&
-     diff D/newfile4.png ../D/newfile4.png &&
-     diff E/newfile5.txt ../E/newfile5.txt &&
-     diff F/newfile6.png ../F/newfile6.png
+     test_cmp A/newfile1.txt ../A/newfile1.txt &&
+     test_cmp D/newfile4.png ../D/newfile4.png &&
+     test_cmp E/newfile5.txt ../E/newfile5.txt &&
+     test_cmp F/newfile6.png ../F/newfile6.png
      )'
 
 # Should fail (but only on the git cvsexportcommit stage)
@@ -137,9 +137,9 @@ test_expect_success \
      check_entries D "" &&
      check_entries E "newfile5.txt/1.1/" &&
      check_entries F "newfile6.png/1.1/-kb" &&
-     diff A/newfile1.txt ../A/newfile1.txt &&
-     diff E/newfile5.txt ../E/newfile5.txt &&
-     diff F/newfile6.png ../F/newfile6.png
+     test_cmp A/newfile1.txt ../A/newfile1.txt &&
+     test_cmp E/newfile5.txt ../E/newfile5.txt &&
+     test_cmp F/newfile6.png ../F/newfile6.png
      )'
 
 test_expect_success \
@@ -155,8 +155,8 @@ test_expect_success \
      check_entries D "" &&
      check_entries E "newfile5.txt/1.1/" &&
      check_entries F "newfile6.png/1.1/-kb" &&
-     diff E/newfile5.txt ../E/newfile5.txt &&
-     diff F/newfile6.png ../F/newfile6.png
+     test_cmp E/newfile5.txt ../E/newfile5.txt &&
+     test_cmp F/newfile6.png ../F/newfile6.png
      )'
 
 test_expect_success \
diff --git a/t/t9400-git-cvsserver-server.sh b/t/t9400-git-cvsserver-server.sh
index daef2d6c23..e4ee0782b4 100755
--- a/t/t9400-git-cvsserver-server.sh
+++ b/t/t9400-git-cvsserver-server.sh
@@ -435,7 +435,7 @@ test_expect_success 'cvs update (-p)' '
     rm -f failures &&
     for i in merge no-lf empty really-empty; do
         GIT_CONFIG="$git_config" cvs update -p "$i" >$i.out
-        diff $i.out ../$i >>failures 2>&1
+	test_cmp $i.out ../$i >>failures 2>&1
     done &&
     test -z "$(cat failures)"
 '