diff --git a/dotenv/main.py b/dotenv/main.py
index 349ec06..1a88238 100644
--- a/dotenv/main.py
+++ b/dotenv/main.py
@@ -2,47 +2,90 @@
 from __future__ import absolute_import, print_function, unicode_literals
 
 import codecs
-import fileinput
 import io
 import os
 import re
+import shutil
 import sys
 from subprocess import Popen
+import tempfile
 import warnings
-from collections import OrderedDict
+from collections import OrderedDict, namedtuple
+from contextlib import contextmanager
 
 from .compat import StringIO, PY2, WIN, text_type
 
-__escape_decoder = codecs.getdecoder('unicode_escape')
-__posix_variable = re.compile('\$\{[^\}]*\}')  # noqa
+__posix_variable = re.compile(r'\$\{[^\}]*\}')
 
+_binding = re.compile(
+    r"""
+        (
+            \s*                     # leading whitespace
+            (?:export\s+)?          # export
 
-def decode_escaped(escaped):
-    return __escape_decoder(escaped)[0]
+            ( '[^']+'               # single-quoted key
+            | [^=\#\s]+             # or unquoted key
+            )?
 
+            (?:
+                (?:\s*=\s*)         # equal sign
 
-def parse_line(line):
-    line = line.strip()
+                ( '(?:\\'|[^'])*'   # single-quoted value
+                | "(?:\\"|[^"])*"   # or double-quoted value
+                | [^\#\r\n]*        # or unquoted value
+                )
+            )?
 
-    # Ignore lines with `#` or which doesn't have `=` in it.
-    if not line or line.startswith('#') or '=' not in line:
-        return None, None
+            \s*                     # trailing whitespace
+            (?:\#[^\r\n]*)?         # comment
+            (?:\r|\n|\r\n)?         # newline
+        )
+    """,
+    re.MULTILINE | re.VERBOSE,
+)
 
-    k, v = line.split('=', 1)
+_escape_sequence = re.compile(r"\\[\\'\"abfnrtv]")
 
-    if k.startswith('export '):
-        (_, _, k) = k.partition('export ')
 
-    # Remove any leading and trailing spaces in key, value
-    k, v = k.strip(), v.strip()
+Binding = namedtuple('Binding', 'key value original')
 
-    if v:
-        v = v.encode('unicode-escape').decode('ascii')
-        quoted = v[0] == v[-1] in ['"', "'"]
-        if quoted:
-            v = decode_escaped(v[1:-1])
 
-    return k, v
+def decode_escapes(string):
+    def decode_match(match):
+        return codecs.decode(match.group(0), 'unicode-escape')
+
+    return _escape_sequence.sub(decode_match, string)
+
+
+def is_surrounded_by(string, char):
+    return (
+        len(string) > 1
+        and string[0] == string[-1] == char
+    )
+
+
+def parse_binding(string, position):
+    match = _binding.match(string, position)
+    (matched, key, value) = match.groups()
+    if key is None or value is None:
+        key = None
+        value = None
+    else:
+        value_quoted = is_surrounded_by(value, "'") or is_surrounded_by(value, '"')
+        if value_quoted:
+            value = decode_escapes(value[1:-1])
+        else:
+            value = value.strip()
+    return (Binding(key=key, value=value, original=matched), match.end())
+
+
+def parse_stream(stream):
+    string = stream.read()
+    position = 0
+    length = len(string)
+    while position < length:
+        (binding, position) = parse_binding(string, position)
+        yield binding
 
 
 class DotEnv():
@@ -52,19 +95,17 @@ class DotEnv():
         self._dict = None
         self.verbose = verbose
 
+    @contextmanager
     def _get_stream(self):
-        self._is_file = False
         if isinstance(self.dotenv_path, StringIO):
-            return self.dotenv_path
-
-        if os.path.isfile(self.dotenv_path):
-            self._is_file = True
-            return io.open(self.dotenv_path)
-
-        if self.verbose:
-            warnings.warn("File doesn't exist {}".format(self.dotenv_path))
-
-        return StringIO('')
+            yield self.dotenv_path
+        elif os.path.isfile(self.dotenv_path):
+            with io.open(self.dotenv_path) as stream:
+                yield stream
+        else:
+            if self.verbose:
+                warnings.warn("File doesn't exist {}".format(self.dotenv_path))
+            yield StringIO('')
 
     def dict(self):
         """Return dotenv as dict"""
@@ -76,17 +117,10 @@ class DotEnv():
         return self._dict
 
     def parse(self):
-        f = self._get_stream()
-
-        for line in f:
-            key, value = parse_line(line)
-            if not key:
-                continue
-
-            yield key, value
-
-        if self._is_file:
-            f.close()
+        with self._get_stream() as stream:
+            for mapping in parse_stream(stream):
+                if mapping.key is not None and mapping.value is not None:
+                    yield mapping.key, mapping.value
 
     def set_as_environment_variables(self, override=False):
         """
@@ -126,6 +160,20 @@ def get_key(dotenv_path, key_to_get):
     return DotEnv(dotenv_path, verbose=True).get(key_to_get)
 
 
+@contextmanager
+def rewrite(path):
+    try:
+        with tempfile.NamedTemporaryFile(mode="w+", delete=False) as dest:
+            with io.open(path) as source:
+                yield (source, dest)
+    except BaseException:
+        if os.path.isfile(dest.name):
+            os.unlink(dest.name)
+        raise
+    else:
+        shutil.move(dest.name, path)
+
+
 def set_key(dotenv_path, key_to_set, value_to_set, quote_mode="always"):
     """
     Adds or Updates a key/value to the given .env
@@ -141,20 +189,19 @@ def set_key(dotenv_path, key_to_set, value_to_set, quote_mode="always"):
     if " " in value_to_set:
         quote_mode = "always"
 
-    line_template = '{}="{}"' if quote_mode == "always" else '{}={}'
+    line_template = '{}="{}"\n' if quote_mode == "always" else '{}={}\n'
     line_out = line_template.format(key_to_set, value_to_set)
 
-    replaced = False
-    for line in fileinput.input(dotenv_path, inplace=True):
-        k, v = parse_line(line)
-        if k == key_to_set:
-            replaced = True
-            line = "{}\n".format(line_out)
-        print(line, end='')
-
-    if not replaced:
-        with io.open(dotenv_path, "a") as f:
-            f.write("{}\n".format(line_out))
+    with rewrite(dotenv_path) as (source, dest):
+        replaced = False
+        for mapping in parse_stream(source):
+            if mapping.key == key_to_set:
+                dest.write(line_out)
+                replaced = True
+            else:
+                dest.write(mapping.original)
+        if not replaced:
+            dest.write(line_out)
 
     return True, key_to_set, value_to_set
 
@@ -166,18 +213,17 @@ def unset_key(dotenv_path, key_to_unset, quote_mode="always"):
     If the .env path given doesn't exist, fails
     If the given key doesn't exist in the .env, fails
     """
-    removed = False
-
     if not os.path.exists(dotenv_path):
         warnings.warn("can't delete from %s - it doesn't exist." % dotenv_path)
         return None, key_to_unset
 
-    for line in fileinput.input(dotenv_path, inplace=True):
-        k, v = parse_line(line)
-        if k == key_to_unset:
-            removed = True
-            line = ''
-        print(line, end='')
+    removed = False
+    with rewrite(dotenv_path) as (source, dest):
+        for mapping in parse_stream(source):
+            if mapping.key == key_to_unset:
+                removed = True
+            else:
+                dest.write(mapping.original)
 
     if not removed:
         warnings.warn("key %s not removed from %s - key doesn't exist." % (key_to_unset, dotenv_path))

diff --git a/tests/test_cli.py b/tests/test_cli.py
index 15c47af..b594592 100644
--- a/tests/test_cli.py
+++ b/tests/test_cli.py
@@ -1,12 +1,13 @@
 # -*- coding: utf-8 -*-
-from os import environ
+import os
 from os.path import dirname, join
 
+import pytest
+import sh
+
 import dotenv
-from dotenv.version import __version__
 from dotenv.cli import cli as dotenv_cli
-
-import sh
+from dotenv.version import __version__
 
 here = dirname(__file__)
 dotenv_path = join(here, '.env')
@@ -38,6 +39,22 @@ def test_set_key(dotenv_file):
     with open(dotenv_file, 'r') as fp:
         assert 'HELLO="WORLD 2"\nfoo="bar"' == fp.read().strip()
 
+    success, key_to_set, value_to_set = dotenv.set_key(dotenv_file, "HELLO", "WORLD\n3")
+
+    with open(dotenv_file, "r") as fp:
+        assert 'HELLO="WORLD\n3"\nfoo="bar"' == fp.read().strip()
+
+
+def test_set_key_permission_error(dotenv_file):
+    os.chmod(dotenv_file, 0o000)
+
+    with pytest.raises(Exception):
+        dotenv.set_key(dotenv_file, "HELLO", "WORLD")
+
+    os.chmod(dotenv_file, 0o600)
+    with open(dotenv_file, "r") as fp:
+        assert fp.read() == ""
+
 
 def test_list(cli, dotenv_file):
     success, key_to_set, value_to_set = dotenv.set_key(dotenv_file, 'HELLO', 'WORLD')
@@ -59,6 +76,13 @@ def test_list_wo_file(cli):
     assert 'Invalid value for "-f"' in result.output
 
 
+def test_empty_value():
+    with open(dotenv_path, "w") as f:
+        f.write("TEST=")
+    assert dotenv.get_key(dotenv_path, "TEST") == ""
+    sh.rm(dotenv_path)
+
+
 def test_key_value_without_quotes():
     with open(dotenv_path, 'w') as f:
         f.write("TEST = value \n")
@@ -95,18 +119,41 @@ def test_value_with_special_characters():
     sh.rm(dotenv_path)
 
 
-def test_unset():
-    sh.touch(dotenv_path)
-    success, key_to_set, value_to_set = dotenv.set_key(dotenv_path, 'HELLO', 'WORLD')
-    stored_value = dotenv.get_key(dotenv_path, 'HELLO')
-    assert stored_value == 'WORLD'
-    success, key_to_unset = dotenv.unset_key(dotenv_path, 'HELLO')
-    assert success is True
-    assert dotenv.get_key(dotenv_path, 'HELLO') is None
-    success, key_to_unset = dotenv.unset_key(dotenv_path, 'RANDOM')
-    assert success is None
+def test_value_with_new_lines():
+    with open(dotenv_path, 'w') as f:
+        f.write('TEST="a\nb"')
+    assert dotenv.get_key(dotenv_path, 'TEST') == "a\nb"
+    sh.rm(dotenv_path)
+
+    with open(dotenv_path, 'w') as f:
+        f.write("TEST='a\nb'")
+    assert dotenv.get_key(dotenv_path, 'TEST') == "a\nb"
+    sh.rm(dotenv_path)
+
+
+def test_value_after_comment():
+    with open(dotenv_path, "w") as f:
+        f.write("# comment\nTEST=a")
+    assert dotenv.get_key(dotenv_path, "TEST") == "a"
     sh.rm(dotenv_path)
-    success, key_to_unset = dotenv.unset_key(dotenv_path, 'HELLO')
+
+
+def test_unset_ok(dotenv_file):
+    with open(dotenv_file, "w") as f:
+        f.write("a=b\nc=d")
+
+    success, key_to_unset = dotenv.unset_key(dotenv_file, "a")
+
+    assert success is True
+    assert key_to_unset == "a"
+    with open(dotenv_file, "r") as f:
+        assert f.read() == "c=d"
+    sh.rm(dotenv_file)
+
+
+def test_unset_non_existing_file():
+    success, key_to_unset = dotenv.unset_key('/non-existing', 'HELLO')
+
     assert success is None
 
 
@@ -180,7 +227,7 @@ def test_get_key_with_interpolation(cli):
         stored_value = dotenv.get_key(dotenv_path, 'BAR')
         assert stored_value == 'CONCATENATED_WORLD_POSIX_VAR'
         # test replace from environ taking precedence over file
-        environ["HELLO"] = "TAKES_PRECEDENCE"
+        os.environ["HELLO"] = "TAKES_PRECEDENCE"
         stored_value = dotenv.get_key(dotenv_path, 'FOO')
         assert stored_value == "TAKES_PRECEDENCE"
         sh.rm(dotenv_path)
@@ -194,10 +241,10 @@ def test_get_key_with_interpolation_of_unset_variable(cli):
         stored_value = dotenv.get_key(dotenv_path, 'FOO')
         assert stored_value == ''
         # unless present in environment
-        environ['NOT_SET'] = 'BAR'
+        os.environ['NOT_SET'] = 'BAR'
         stored_value = dotenv.get_key(dotenv_path, 'FOO')
         assert stored_value == 'BAR'
-        del(environ['NOT_SET'])
+        del(os.environ['NOT_SET'])
         sh.rm(dotenv_path)
 
 
diff --git a/tests/test_core.py b/tests/test_core.py
index 45a1f86..bda2e3b 100644
--- a/tests/test_core.py
+++ b/tests/test_core.py
@@ -9,7 +9,7 @@ import warnings
 import sh
 
 from dotenv import load_dotenv, find_dotenv, set_key, dotenv_values
-from dotenv.main import parse_line
+from dotenv.main import Binding, parse_stream
 from dotenv.compat import StringIO
 from IPython.terminal.embed import InteractiveShellEmbed
 
@@ -25,21 +25,71 @@ def restore_os_environ():
 
 
 @pytest.mark.parametrize("test_input,expected", [
-    ("a=b", ("a", "b")),
-    (" a = b ", ("a", "b")),
-    ("export a=b", ("a", "b")),
-    (" export 'a'=b", ("'a'", "b")),
-    (" export 'a'=b", ("'a'", "b")),
-    ("# a=b", (None, None)),
-    ("# a=b", (None, None)),
-    ("a=b space ", ('a', 'b space')),
-    ("a='b space '", ('a', 'b space ')),
-    ('a="b space "', ('a', 'b space ')),
-    ("export export_spam=1", ("export_spam", "1")),
-    ("export port=8000", ("port", "8000")),
+    ("", []),
+    ("a=b", [Binding(key="a", value="b", original="a=b")]),
+    ("'a'=b", [Binding(key="'a'", value="b", original="'a'=b")]),
+    ("[=b", [Binding(key="[", value="b", original="[=b")]),
+    (" a = b ", [Binding(key="a", value="b", original=" a = b ")]),
+    ("export a=b", [Binding(key="a", value="b", original="export a=b")]),
+    (" export 'a'=b", [Binding(key="'a'", value="b", original=" export 'a'=b")]),
+    (" export 'a'=b", [Binding(key="'a'", value="b", original=" export 'a'=b")]),
+    ("# a=b", [Binding(key=None, value=None, original="# a=b")]),
+    ('a=b # comment', [Binding(key="a", value="b", original="a=b # comment")]),
+    ("a=b space ", [Binding(key="a", value="b space", original="a=b space ")]),
+    ("a='b space '", [Binding(key="a", value="b space ", original="a='b space '")]),
+    ('a="b space "', [Binding(key="a", value="b space ", original='a="b space "')]),
+    ("export export_a=1", [Binding(key="export_a", value="1", original="export export_a=1")]),
+    ("export port=8000", [Binding(key="port", value="8000", original="export port=8000")]),
+    ('a="b\nc"', [Binding(key="a", value="b\nc", original='a="b\nc"')]),
+    ("a='b\nc'", [Binding(key="a", value="b\nc", original="a='b\nc'")]),
+    ('a="b\nc"', [Binding(key="a", value="b\nc", original='a="b\nc"')]),
+    ('a="b\\nc"', [Binding(key="a", value='b\nc', original='a="b\\nc"')]),
+    ('a="b\\"c"', [Binding(key="a", value='b"c', original='a="b\\"c"')]),
+    ("a='b\\'c'", [Binding(key="a", value="b'c", original="a='b\\'c'")]),
+    ("a=à", [Binding(key="a", value="à", original="a=à")]),
+    ('a="à"', [Binding(key="a", value="à", original='a="à"')]),
+    ('garbage', [Binding(key=None, value=None, original="garbage")]),
+    (
+        "a=b\nc=d",
+        [
+            Binding(key="a", value="b", original="a=b\n"),
+            Binding(key="c", value="d", original="c=d"),
+        ],
+    ),
+    (
+        "a=b\r\nc=d",
+        [
+            Binding(key="a", value="b", original="a=b\r\n"),
+            Binding(key="c", value="d", original="c=d"),
+        ],
+    ),
+    (
+        'a="\nb=c',
+        [
+            Binding(key="a", value='"', original='a="\n'),
+            Binding(key="b", value='c', original="b=c"),
+        ]
+    ),
+    (
+        '# comment\na="b\nc"\nd=e\n',
+        [
+            Binding(key=None, value=None, original="# comment\n"),
+            Binding(key="a", value="b\nc", original='a="b\nc"\n'),
+            Binding(key="d", value="e", original="d=e\n"),
+        ],
+    ),
+    (
+        'garbage[%$#\na=b',
+        [
+            Binding(key=None, value=None, original="garbage[%$#\n"),
+            Binding(key="a", value="b", original='a=b'),
+        ],
+    ),
 ])
-def test_parse_line(test_input, expected):
-    assert parse_line(test_input) == expected
+def test_parse_stream(test_input, expected):
+    result = parse_stream(StringIO(test_input))
+
+    assert list(result) == expected
 
 
 def test_warns_if_file_does_not_exist():

Cannot get multiline strings to work?
According to the documentation `TEST="foo\nbar"` should produce a multiline environment variable however I cannot get it to work, what am I doing wrong? I have tried every variation I can think of.

**.env**
```
TEST1="foo\nbar"
TEST2=foo\nbar
TEST3="foo
bar"
TEST4="foo\\nbar"
TEST5=foo\\nbar
TEST6=foo
bar
TEST7="foo\
bar"
TEST8=foo\
bar
```

**test.py**
```
import os
from os.path import join, dirname
from dotenv import load_dotenv

dotenv_path = join(dirname(__file__), '.env')

load_dotenv(dotenv_path, verbose=True)

tests = (
    (key, value)
    for key, value in os.environ.items()
    if key.startswith('TEST'))

for key, value in tests:
    print(key, value)
```

**output**
```
TEST1 foo\nbar
TEST2 foo\\nbar
TEST3 "foo
TEST4 foo\\nbar
TEST5 foo\\\\nbar
TEST6 foo
TEST7 "foo\\
TEST8 foo\\
```

Trying with "real" environment variables through bash:

```
$ export TEST="foo
bar"
$ python -c "import os; print(os.environ['TEST'])"
foo
bar
```

**using**
Darwin Kernel Version 15.6.0
Python 3.6.2
python-dotenv==0.7.1


tests/test_cli.py::test_get_key tests/test_cli.py::test_set_key tests/test_cli.py::test_list tests/test_cli.py::test_get_cli tests/test_cli.py::test_empty_value tests/test_cli.py::test_key_value_without_quotes tests/test_cli.py::test_value_with_quotes tests/test_cli.py::test_value_with_special_characters tests/test_cli.py::test_value_with_new_lines tests/test_cli.py::test_value_after_comment tests/test_cli.py::test_unset_ok tests/test_cli.py::test_unset_non_existing_file tests/test_cli.py::test_unset_cli tests/test_cli.py::test_get_key_with_interpolation tests/test_cli.py::test_get_key_with_interpolation_of_unset_variable tests/test_cli.py::test_run_without_cmd tests/test_cli.py::test_run_with_invalid_cmd tests/test_cli.py::test_run_with_version tests/test_core.py::test_parse_stream[-expected0] tests/test_core.py::test_parse_stream[a=b-expected1] tests/test_core.py::test_parse_stream['a'=b-expected2] tests/test_core.py::test_parse_stream[[=b-expected3] tests/test_core.py::test_parse_stream[ tests/test_core.py::test_parse_stream[export tests/test_core.py::test_parse_stream[# tests/test_core.py::test_parse_stream[a=b tests/test_core.py::test_parse_stream[a='b tests/test_core.py::test_parse_stream[a="b tests/test_core.py::test_parse_stream[a="b\nc"-expected15] tests/test_core.py::test_parse_stream[a='b\nc'-expected16] tests/test_core.py::test_parse_stream[a="b\nc"-expected17] tests/test_core.py::test_parse_stream[a="b\\nc"-expected18] tests/test_core.py::test_parse_stream[a="b\\"c"-expected19] tests/test_core.py::test_parse_stream[a='b\\'c'-expected20] tests/test_core.py::test_parse_stream[a=\xe0-expected21] tests/test_core.py::test_parse_stream[a="\xe0"-expected22] tests/test_core.py::test_parse_stream[garbage-expected23] tests/test_core.py::test_parse_stream[a=b\nc=d-expected24] tests/test_core.py::test_parse_stream[a=b\r\nc=d-expected25] tests/test_core.py::test_parse_stream[a="\nb=c-expected26] tests/test_core.py::test_parse_stream[garbage[%$#\na=b-expected28] tests/test_core.py::test_warns_if_file_does_not_exist tests/test_core.py::test_find_dotenv tests/test_core.py::test_load_dotenv tests/test_core.py::test_load_dotenv_override tests/test_core.py::test_load_dotenv_in_current_dir tests/test_core.py::test_ipython tests/test_core.py::test_ipython_override tests/test_core.py::test_dotenv_values_stream tests/test_core.py::test_dotenv_values_export tests/test_core.py::test_dotenv_empty_selfreferential_interpolation tests/test_core.py::test_dotenv_nonempty_selfreferential_interpolation