diff --git a/assert-extras.sh b/assert-extras.sh
new file mode 100644
index 0000000..9a7fcb2
--- /dev/null
+++ b/assert-extras.sh
@@ -0,0 +1,76 @@
+#!/usr/bin/env bash
+
+# assert-extras.sh 1.1 - supplementary bash unit testing functions
+# Note: This script should be sourced together with assert.sh,
+#       it is dependent on the functionality provided by that script.
+
+assert_success() {
+	assert_raises "$@" 0
+}
+
+# assert_failure <command> [stdin]
+assert_failure() {
+    (( tests_ran++ )) || :
+    [[ -z "$DISCOVERONLY" ]] || return
+    status=0
+    (eval "$1" <<< ${2:-}) > /dev/null 2>&1 || status=$?
+    if [[ "$status" != "0" ]]; then
+        [[ -z "$DEBUG" ]] || echo -n .
+        return
+    fi
+    _assert_fail "program terminated with a zero return code;" \
+                 "expecting non-zero return code" "$1" "$2"
+}
+
+# assert_contains <command> <expected output...>
+assert_contains() {
+	_assert_with_grep '-F' "$@"
+}
+
+# assert_matches <command> <expected output...>
+assert_matches() {
+	_assert_with_grep '-E' "$@"
+}
+
+# assert_startswith <command> <expected start to stdout>
+assert_startswith() {
+	assert_success "[[ '$($1)' == '$2'* ]]"
+}
+
+# assert_endswith <command> <expected start to stdout>
+assert_endswith() {
+	assert_success "[[ '$($1)' == *'$2' ]]"
+}
+
+# _assert_with_grep <grep modifiers> <command> <expected output...>
+_assert_with_grep() {
+    local grep_modifier="$1"
+    local output="$($2)"
+    shift 2
+
+    while [ $# != 0 ]; do
+    	assert_raises "echo '$output' | $GREP $grep_modifier '$1'" 0 || return 1
+    	shift
+    done
+}
+
+# Returns the resolved command, preferring any gnu-versions of the cmd (prefixed with 'g') on
+# non-Linux systems such as Mac OS, and falling back to the standard version if not.
+_cmd() {
+	local cmd="$1"
+
+	local gnu_cmd="g$cmd"
+	local gnu_cmd_found=$(which "$gnu_cmd" 2> /dev/null)
+	if [ "$gnu_cmd_found" ]; then
+		echo "$gnu_cmd_found"
+	else
+		if [ "$(uname)" == 'Darwin' ]; then
+			echo "Warning: Cannot find gnu version of command '$cmd' ($gnu_cmd) on path." \
+			     "Falling back to standard command" >&2
+		fi
+		echo "cmd"
+	fi
+}
+
+GREP=$(_cmd grep)
+SED=$(_cmd sed)
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diff --git a/test-extras.sh b/test-extras.sh
new file mode 100755
index 0000000..a45eaa9
--- /dev/null
+++ b/test-extras.sh
@@ -0,0 +1,121 @@
+#!/usr/bin/env bash
+
+source assert.sh
+source assert-extras.sh
+
+assert "echo foo" "foo"
+assert_raises "true" 0
+assert_raises "exit 127" 127
+
+assert_end sanity
+
+###
+### assert_success tests
+###
+
+# Tests expecting success
+assert_success "true"
+assert_success "echo foo"
+
+# Tests expecting failure
+assert_raises 'assert_success "false"' 1
+assert_raises 'assert_success "exit 1"' 1
+
+assert_end assert_success
+
+###
+### assert_failure tests
+###
+
+# Tests expecting success
+assert_failure "false"
+assert_failure "exit 1"
+assert_failure "exit -1"
+assert_failure "exit 42"
+assert_failure "exit -42"
+
+# Tests expecting failure
+assert_raises 'assert_failure "true"' 1
+assert_raises 'assert_failure "echo foo"' 1
+
+assert_end assert_failure
+
+###
+### assert_contains tests
+###
+
+# Tests expecting success
+assert_contains "echo foo" "foo"
+assert_contains "echo foobar" "foo"
+assert_contains "echo foo bar" "foo"
+assert_contains "echo foo bar" "bar"
+assert_contains "echo foo bar" "foo bar"
+
+# Tests expecting failure
+assert_failure 'assert_contains "echo foo" "foot"'
+assert_failure 'assert_contains "echo foo" "f.."'
+
+# Multi-word argument tests
+assert_contains "echo foo bar" "foo bar"
+assert_failure 'assert_contains "echo foo; echo bar" "foo bar"'
+
+# Multi-argument tests
+assert_contains "echo foo bar baz" "foo" "baz"
+assert_failure 'assert_contains "echo foo bar baz" "bar" "foo baz"'
+
+assert_end assert_contains
+
+###
+### assert_matches tests
+###
+
+# Tests expecting success
+assert_matches "echo foo" "f.."
+assert_matches "echo foobar" "f."
+assert_matches "echo foo bar" "^foo bar$"
+assert_matches "echo foo bar" "[az ]+"
+
+# Tests expecting failure
+assert_failure 'assert_matches "echo foot" "foo$"'
+
+# Multi-word argument tests
+assert_matches "echo foo bar" "foo .*"
+assert_failure 'assert_matches "echo foo; echo bar" "foo .*"'
+
+# Multi-argument tests
+assert_matches "echo foo bar baz" "^f.." "baz$"
+assert_failure 'assert_matches "echo foo bar baz" "bar" "foo baz"'
+
+assert_end assert_matches
+
+###
+### assert_startswith tests
+###
+
+# Tests expecting success
+assert_startswith "echo foo" "f"
+assert_startswith "echo foo" "foo"
+assert_startswith "echo foo; echo bar" "foo"
+
+# Tests expecting failure
+assert_failure 'assert_startswith "echo foo" "oo"'
+assert_failure 'assert_startswith "echo foo; echo bar" "foo bar"'
+assert_failure 'assert_startswith "echo foo" "."'
+
+assert_end assert_startswith
+
+###
+### assert_endswith tests
+###
+
+# Tests expecting success
+assert_endswith "echo foo" "oo"
+assert_endswith "echo foo" "foo"
+assert_endswith "echo foo; echo bar" "bar"
+
+# Tests expecting failure
+assert_failure 'assert_endswith "echo foo" "f"'
+assert_failure 'assert_endswith "echo foo; echo bar" "foo bar"'
+assert_failure 'assert_endswith "echo foo" "."'
+
+assert_end assert_endswith
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