diff --git a/README.md b/README.md
index 91554aec..0539539f 100644
--- a/README.md
+++ b/README.md
@@ -151,6 +151,10 @@ scenario.
 Make sure you introduce new default colors in `themes/default.py` for every new
 segment you create. Test your segment with this theme first.
 
+You should add tests for your segment as best you are able. Unit and
+integration tests are both welcome. Run your tests with the `nosetests` command
+after install the requirements in `dev_requirements.txt`.
+
 ### Themes
 
 The `themes` directory stores themes for your prompt, which are basically color
diff --git a/circle.yml b/circle.yml
new file mode 100644
index 00000000..7638a893
--- /dev/null
+++ b/circle.yml
@@ -0,0 +1,5 @@
+dependencies:
+    pre:
+        - sudo pip install -r dev_requirements.txt
+        - git config --global user.email "tester@example.com"
+        - git config --global user.name "Tester McGee"
diff --git a/dev_requirements.txt b/dev_requirements.txt
index a6786964..47a4588d 100644
--- a/dev_requirements.txt
+++ b/dev_requirements.txt
@@ -1,2 +1,3 @@
-nose
-mock
+nose>=1.3.7
+mock>=1.3.0
+sh>=1.11
diff --git a/install.py b/install.py
index 9be4b712..bf36f41e 100755
--- a/install.py
+++ b/install.py
@@ -33,6 +33,10 @@ def load_source(srcfile):
     for segment in config.SEGMENTS:
         source += load_source(os.path.join(SEGMENTS_DIR, segment + '.py'))
 
+        # assumes each segment file will have a function called
+        # add_segment__[segment] that accepts the powerline object
+        source += 'add_{}_segment(powerline)\n'.format(segment)
+
     source += 'sys.stdout.write(powerline.draw())\n'
 
     try:
diff --git a/segments/__init__.py b/segments/__init__.py
new file mode 100644
index 00000000..e69de29b
diff --git a/segments/cwd.py b/segments/cwd.py
index d5e3a791..3c80b701 100644
--- a/segments/cwd.py
+++ b/segments/cwd.py
@@ -28,7 +28,7 @@ def requires_special_home_display(name):
     return (name == '~' and Color.HOME_SPECIAL_DISPLAY)
 
 
-def maybe_shorten_name(name):
+def maybe_shorten_name(powerline, name):
     """If the user has asked for each directory name to be shortened, will
     return the name up to their specified length. Otherwise returns the full
     name."""
@@ -45,7 +45,7 @@ def get_fg_bg(name):
     return (Color.PATH_FG, Color.PATH_BG,)
 
 
-def add_cwd_segment():
+def add_cwd_segment(powerline):
     cwd = powerline.cwd or os.getenv('PWD')
     if not py3:
         cwd = cwd.decode("utf-8")
@@ -86,7 +86,5 @@ def add_cwd_segment():
             separator = None
             separator_fg = None
 
-        powerline.append(' %s ' % maybe_shorten_name(name), fg, bg,
+        powerline.append(' %s ' % maybe_shorten_name(powerline, name), fg, bg,
                          separator, separator_fg)
-
-add_cwd_segment()
diff --git a/segments/exit_code.py b/segments/exit_code.py
index 5c936156..e3b320ee 100644
--- a/segments/exit_code.py
+++ b/segments/exit_code.py
@@ -1,8 +1,6 @@
-def add_exit_code_segment():
+def add_exit_code_segment(powerline):
     if powerline.args.prev_error == 0:
         return
     fg = Color.CMD_FAILED_FG
     bg = Color.CMD_FAILED_BG
     powerline.append(' %s ' % str(powerline.args.prev_error), fg, bg)
-
-add_exit_code_segment()
diff --git a/segments/fossil.py b/segments/fossil.py
index bffc10f3..6c7818ec 100644
--- a/segments/fossil.py
+++ b/segments/fossil.py
@@ -12,7 +12,7 @@ def get_fossil_status():
 
     return has_modified_files, has_untracked_files, has_missing_files
 
-def add_fossil_segment():
+def _add_fossil_segment(powerline):
     subprocess.Popen(['fossil'], stdout=subprocess.PIPE).communicate()[0]
     branch = ''.join([i.replace('*','').strip() for i in os.popen("fossil branch 2> /dev/null").read().strip().split("\n") if i.startswith('*')])
     if len(branch) == 0:
@@ -32,9 +32,19 @@ def add_fossil_segment():
         branch += (' ' + extra if extra != '' else '')
     powerline.append(' %s ' % branch, fg, bg)
 
-try:
-    add_fossil_segment()
-except OSError:
-    pass
-except subprocess.CalledProcessError:
-    pass
+def add_fossil_segment(powerline):
+    """Wraps _add_fossil_segment in exception handling."""
+
+    # FIXME This function was added when introducing a testing framework,
+    # during which the 'powerline' object was passed into the
+    # `add_[segment]_segment` functions instead of being a global variable. At
+    # that time it was unclear whether the below exceptions could actually be
+    # thrown. It would be preferable to find out whether they ever will. If so,
+    # write a comment explaining when. Otherwise remove.
+
+    try:
+        _add_fossil_segment(powerline)
+    except OSError:
+        pass
+    except subprocess.CalledProcessError:
+        pass
diff --git a/segments/git.py b/segments/git.py
index cc0787bc..d33a9d39 100644
--- a/segments/git.py
+++ b/segments/git.py
@@ -1,5 +1,6 @@
 import re
 import subprocess
+import os
 
 GIT_SYMBOLS = {
     'detached': u'\u2693',
@@ -11,17 +12,24 @@
     'conflicted': u'\u273C'
 }
 
-git_subprocess_env = {
-    # LANG is specified to ensure git always uses a language we are expecting.
-    # Otherwise we may be unable to parse the output.
-    "LANG": "C",
+def get_PATH():
+    """Normally gets the PATH from the OS. This function exists to enable
+    easily mocking the PATH in tests.
+    """
+    return os.getenv("PATH")
 
-    # https://github.com/milkbikis/powerline-shell/pull/126
-    "HOME": os.getenv("HOME"),
+def git_subprocess_env():
+    return {
+        # LANG is specified to ensure git always uses a language we are expecting.
+        # Otherwise we may be unable to parse the output.
+        "LANG": "C",
 
-    # https://github.com/milkbikis/powerline-shell/pull/153
-    "PATH": os.getenv("PATH"),
-}
+        # https://github.com/milkbikis/powerline-shell/pull/126
+        "HOME": os.getenv("HOME"),
+
+        # https://github.com/milkbikis/powerline-shell/pull/153
+        "PATH": get_PATH(),
+    }
 
 
 def parse_git_branch_info(status):
@@ -32,7 +40,7 @@ def parse_git_branch_info(status):
 def _get_git_detached_branch():
     p = subprocess.Popen(['git', 'describe', '--tags', '--always'],
                          stdout=subprocess.PIPE, stderr=subprocess.PIPE,
-                         env=git_subprocess_env)
+                         env=git_subprocess_env())
     detached_ref = p.communicate()[0].decode("utf-8").rstrip('\n')
     if p.returncode == 0:
         branch = u'{} {}'.format(GIT_SYMBOLS['detached'], detached_ref)
@@ -62,10 +70,15 @@ def _n_or_empty(_dict, _key):
     return _dict[_key] if int(_dict[_key]) > 1 else u''
 
 
-def add_git_segment():
-    p = subprocess.Popen(['git', 'status', '--porcelain', '-b'],
-                         stdout=subprocess.PIPE, stderr=subprocess.PIPE,
-                         env=git_subprocess_env)
+def add_git_segment(powerline):
+    try:
+        p = subprocess.Popen(['git', 'status', '--porcelain', '-b'],
+                             stdout=subprocess.PIPE, stderr=subprocess.PIPE,
+                             env=git_subprocess_env())
+    except OSError:
+        # Popen will throw an OSError if git is not found
+        return
+
     pdata = p.communicate()
     if p.returncode != 0:
         return
@@ -101,8 +114,3 @@ def _add(_dict, _key, fg, bg):
     _add(stats, 'notstaged', Color.GIT_NOTSTAGED_FG, Color.GIT_NOTSTAGED_BG)
     _add(stats, 'untracked', Color.GIT_UNTRACKED_FG, Color.GIT_UNTRACKED_BG)
     _add(stats, 'conflicted', Color.GIT_CONFLICTED_FG, Color.GIT_CONFLICTED_BG)
-
-try:
-    add_git_segment()
-except (OSError, subprocess.CalledProcessError):
-    pass
diff --git a/segments/hg.py b/segments/hg.py
index 5a4f40dc..d5f1cf48 100644
--- a/segments/hg.py
+++ b/segments/hg.py
@@ -20,7 +20,7 @@ def get_hg_status():
             has_modified_files = True
     return has_modified_files, has_untracked_files, has_missing_files
 
-def add_hg_segment():
+def add_hg_segment(powerline):
     branch = os.popen('hg branch 2> /dev/null').read().rstrip()
     if len(branch) == 0:
         return False
@@ -37,5 +37,3 @@ def add_hg_segment():
             extra += '!'
         branch += (' ' + extra if extra != '' else '')
     return powerline.append(' %s ' % branch, fg, bg)
-
-add_hg_segment()
diff --git a/segments/hostname.py b/segments/hostname.py
index baf99191..609711e2 100644
--- a/segments/hostname.py
+++ b/segments/hostname.py
@@ -1,4 +1,4 @@
-def add_hostname_segment():
+def add_hostname_segment(powerline):
     if powerline.args.colorize_hostname:
         from lib.color_compliment import stringToHashToColorAndOpposite
         from lib.colortrans import rgb2short
@@ -19,6 +19,3 @@ def add_hostname_segment():
             host_prompt = ' %s ' % socket.gethostname().split('.')[0]
 
         powerline.append(host_prompt, Color.HOSTNAME_FG, Color.HOSTNAME_BG)
-
-
-add_hostname_segment()
diff --git a/segments/jobs.py b/segments/jobs.py
index 5dde3e65..a6ff1808 100644
--- a/segments/jobs.py
+++ b/segments/jobs.py
@@ -2,7 +2,7 @@
 import re
 import subprocess
 
-def add_jobs_segment():
+def add_jobs_segment(powerline):
     pppid_proc = subprocess.Popen(['ps', '-p', str(os.getppid()), '-oppid='],
                                   stdout=subprocess.PIPE)
     pppid = pppid_proc.communicate()[0].decode("utf-8").strip()
@@ -15,5 +15,3 @@ def add_jobs_segment():
 
     if num_jobs > 0:
         powerline.append(' %d ' % num_jobs, Color.JOBS_FG, Color.JOBS_BG)
-
-add_jobs_segment()
diff --git a/segments/node_version.py b/segments/node_version.py
index 842e487c..466a0864 100644
--- a/segments/node_version.py
+++ b/segments/node_version.py
@@ -1,7 +1,7 @@
 import subprocess
 
 
-def add_node_version_segment():
+def add_node_version_segment(powerline):
     try:
         p1 = subprocess.Popen(["node", "--version"], stdout=subprocess.PIPE)
         version = p1.communicate()[0].decode("utf-8").rstrip()
@@ -9,5 +9,3 @@ def add_node_version_segment():
         powerline.append(version, 15, 18)
     except OSError:
         return
-
-add_node_version_segment()
diff --git a/segments/php_version.py b/segments/php_version.py
index ee174047..7b9b8aa3 100644
--- a/segments/php_version.py
+++ b/segments/php_version.py
@@ -1,7 +1,7 @@
 import subprocess
 
 
-def add_php_version_segment():
+def add_php_version_segment(powerline):
     try:
         output = subprocess.check_output(['php', '-r', 'echo PHP_VERSION;'], stderr=subprocess.STDOUT)
         if '-' in output:
@@ -12,5 +12,3 @@ def add_php_version_segment():
         powerline.append(version, 15, 4)
     except OSError:
         return
-
-add_php_version_segment()
diff --git a/segments/read_only.py b/segments/read_only.py
index bcb28f4c..efbe11ad 100644
--- a/segments/read_only.py
+++ b/segments/read_only.py
@@ -1,9 +1,7 @@
 import os
 
-def add_read_only_segment():
+def add_read_only_segment(powerline):
     cwd = powerline.cwd or os.getenv('PWD')
 
     if not os.access(cwd, os.W_OK):
         powerline.append(' %s ' % powerline.lock, Color.READONLY_FG, Color.READONLY_BG)
-
-add_read_only_segment()
diff --git a/segments/root.py b/segments/root.py
index 9cd5e8c2..49404ecf 100644
--- a/segments/root.py
+++ b/segments/root.py
@@ -1,4 +1,4 @@
-def add_root_indicator_segment():
+def add_root_segment(powerline):
     root_indicators = {
         'bash': ' \\$ ',
         'zsh': ' %# ',
@@ -10,5 +10,3 @@ def add_root_indicator_segment():
         fg = Color.CMD_FAILED_FG
         bg = Color.CMD_FAILED_BG
     powerline.append(root_indicators[powerline.args.shell], fg, bg)
-
-add_root_indicator_segment()
diff --git a/segments/ruby_version.py b/segments/ruby_version.py
index 82ba7f27..040f5334 100644
--- a/segments/ruby_version.py
+++ b/segments/ruby_version.py
@@ -1,7 +1,7 @@
 import subprocess
 
 
-def add_ruby_version_segment():
+def add_ruby_version_segment(powerline):
     try:
         p1 = subprocess.Popen(["ruby", "-v"], stdout=subprocess.PIPE)
         p2 = subprocess.Popen(["sed", "s/ (.*//"], stdin=p1.stdout, stdout=subprocess.PIPE)
@@ -13,5 +13,3 @@ def add_ruby_version_segment():
         powerline.append(version, 15, 1)
     except OSError:
         return
-
-add_ruby_version_segment()
diff --git a/segments/set_term_title.py b/segments/set_term_title.py
index 4d1d5cd8..e9954ae8 100644
--- a/segments/set_term_title.py
+++ b/segments/set_term_title.py
@@ -1,4 +1,4 @@
-def add_term_title_segment():
+def add_set_term_title_segment(powerline):
     term = os.getenv('TERM')
     if not (('xterm' in term) or ('rxvt' in term)):
         return
@@ -13,5 +13,3 @@ def add_term_title_segment():
 
     powerline.append(set_title, None, None, '')
 
-
-add_term_title_segment()
diff --git a/segments/ssh.py b/segments/ssh.py
index e90397a3..5361bc5e 100644
--- a/segments/ssh.py
+++ b/segments/ssh.py
@@ -1,8 +1,6 @@
 import os
 
-def add_ssh_segment():
+def add_ssh_segment(powerline):
 
     if os.getenv('SSH_CLIENT'):
         powerline.append(' %s ' % powerline.network, Color.SSH_FG, Color.SSH_BG)
-
-add_ssh_segment()
diff --git a/segments/svn.py b/segments/svn.py
index a378a87a..89168c01 100644
--- a/segments/svn.py
+++ b/segments/svn.py
@@ -1,6 +1,7 @@
 import subprocess
 
-def add_svn_segment():
+
+def _add_svn_segment(powerline):
     is_svn = subprocess.Popen(['svn', 'status'],
                               stdout=subprocess.PIPE, stderr=subprocess.PIPE)
     is_svn_output = is_svn.communicate()[1].decode("utf-8").strip()
@@ -17,9 +18,20 @@ def add_svn_segment():
         changes = output.strip()
         powerline.append(' %s ' % changes, Color.SVN_CHANGES_FG, Color.SVN_CHANGES_BG)
 
-try:
-    add_svn_segment()
-except OSError:
-    pass
-except subprocess.CalledProcessError:
-    pass
+
+def add_svn_segment(powerline):
+    """Wraps _add_svn_segment in exception handling."""
+
+    # FIXME This function was added when introducing a testing framework,
+    # during which the 'powerline' object was passed into the
+    # `add_[segment]_segment` functions instead of being a global variable. At
+    # that time it was unclear whether the below exceptions could actually be
+    # thrown. It would be preferable to find out whether they ever will. If so,
+    # write a comment explaining when. Otherwise remove.
+
+    try:
+        _add_svn_segment(powerline)
+    except OSError:
+        pass
+    except subprocess.CalledProcessError:
+        pass
diff --git a/segments/time.py b/segments/time.py
index 68431504..8c0313f6 100644
--- a/segments/time.py
+++ b/segments/time.py
@@ -1,4 +1,4 @@
-def add_time_segment():
+def add_time_segment(powerline):
     if powerline.args.shell == 'bash':
         time = ' \\t '
     elif powerline.args.shell == 'zsh':
@@ -8,5 +8,3 @@ def add_time_segment():
         time = ' %s ' % time.strftime('%H:%M:%S')
 
     powerline.append(time, Color.HOSTNAME_FG, Color.HOSTNAME_BG)
-
-add_time_segment()
diff --git a/segments/uptime.py b/segments/uptime.py
index 8ea229f4..4c7711ce 100644
--- a/segments/uptime.py
+++ b/segments/uptime.py
@@ -1,15 +1,7 @@
 import subprocess
 import re
 
-# uptime output samples
-# 1h:   00:00:00 up 1:00,  2 users,  load average: 0,00, 0,00, 0,00             
-# 10+h: 00:00:00 up 10:00,  2 users,  load average: 0,00, 0,00, 0,00            
-# 1+d:  00:00:00 up 1 days, 1:00,  2 users,  load average: 0,00, 0,00, 0,00     
-# 9+d:  00:00:00 up 12 days, 1:00,  2 users,  load average: 0,00, 0,00, 0,00    
-# -1h   00:00:00 up 120 days, 49 min,  2 users,  load average: 0,00, 0,00, 0,00
-# mac:  00:00:00 up 23  3 day(s), 10:00,  2 users,  load average: 0,00, 0,00, 0,00
-
-def add_uptime_segment():
+def add_uptime_segment(powerline):
     try:
         output = subprocess.check_output(['uptime'], stderr=subprocess.STDOUT)
         raw_uptime = re.search('(?<=up).+(?=,\s+\d+\s+user)', output).group(0)
@@ -22,5 +14,3 @@ def add_uptime_segment():
         powerline.append(uptime, Color.CWD_FG, Color.PATH_BG)
     except OSError:
         return
-
-add_uptime_segment()
diff --git a/segments/username.py b/segments/username.py
index 4d8e5755..3d73a12c 100644
--- a/segments/username.py
+++ b/segments/username.py
@@ -1,5 +1,5 @@
 
-def add_username_segment():
+def add_username_segment(powerline):
     import os
     if powerline.args.shell == 'bash':
         user_prompt = ' \\u '
@@ -14,5 +14,3 @@ def add_username_segment():
         bgcolor = Color.USERNAME_BG
 
     powerline.append(user_prompt, Color.USERNAME_FG, bgcolor)
-
-add_username_segment()
diff --git a/segments/virtual_env.py b/segments/virtual_env.py
index d8937e63..710efc69 100644
--- a/segments/virtual_env.py
+++ b/segments/virtual_env.py
@@ -1,6 +1,6 @@
 import os
 
-def add_virtual_env_segment():
+def add_virtual_env_segment(powerline):
     env = os.getenv('VIRTUAL_ENV') or os.getenv('CONDA_ENV_PATH')
     if env is None:
         return
@@ -9,5 +9,3 @@ def add_virtual_env_segment():
     bg = Color.VIRTUAL_ENV_BG
     fg = Color.VIRTUAL_ENV_FG
     powerline.append(' %s ' % env_name, fg, bg)
-
-add_virtual_env_segment()
diff --git a/test/segments_test/git_test.py b/test/segments_test/git_test.py
new file mode 100644
index 00000000..982289e3
--- /dev/null
+++ b/test/segments_test/git_test.py
@@ -0,0 +1,69 @@
+import unittest
+import mock
+import tempfile
+import shutil
+import sh
+import segments.git as git
+
+
+class GitTest(unittest.TestCase):
+
+    def setUp(self):
+        self.powerline = mock.MagicMock()
+        git.Color = mock.MagicMock()
+
+        self.dirname = tempfile.mkdtemp()
+        sh.cd(self.dirname)
+        sh.git("init", ".")
+
+    def tearDown(self):
+        shutil.rmtree(self.dirname)
+
+    def _add_and_commit(self, filename):
+        sh.touch(filename)
+        sh.git("add", filename)
+        sh.git("commit", "-m", "add file " + filename)
+
+    def _new_branch(self, branch):
+        sh.git("checkout", "-b", branch)
+
+    def _get_commit_hash(self):
+        return sh.git("rev-parse", "HEAD")
+
+    @mock.patch('segments.git.get_PATH')
+    def test_git_not_installed(self, get_PATH):
+        get_PATH.return_value = "" # so git can't be found
+        git.add_git_segment(self.powerline)
+        self.assertEqual(self.powerline.append.call_count, 0)
+
+    def test_non_git_directory(self):
+        shutil.rmtree(".git")
+        git.add_git_segment(self.powerline)
+        self.assertEqual(self.powerline.append.call_count, 0)
+
+    def test_big_bang(self):
+        git.add_git_segment(self.powerline)
+        self.assertEqual(self.powerline.append.call_args[0][0], ' Big Bang ')
+
+    def test_master_branch(self):
+        self._add_and_commit("foo")
+        git.add_git_segment(self.powerline)
+        self.assertEqual(self.powerline.append.call_args[0][0], ' master ')
+
+    def test_different_branch(self):
+        self._add_and_commit("foo")
+        self._new_branch("bar")
+        git.add_git_segment(self.powerline)
+        self.assertEqual(self.powerline.append.call_args[0][0], ' bar ')
+
+    def test_detached(self):
+        self._add_and_commit("foo")
+        commit_hash = self._get_commit_hash()
+        self._add_and_commit("bar")
+        sh.git("checkout", "HEAD^")
+        git.add_git_segment(self.powerline)
+
+        # In detached mode, we output a unicode symbol and then the shortened
+        # commit hash.
+        self.assertIn(self.powerline.append.call_args[0][0].split()[1],
+                      commit_hash)
diff --git a/test/segments_test/uptime_test.py b/test/segments_test/uptime_test.py
new file mode 100644
index 00000000..d2dacd2c
--- /dev/null
+++ b/test/segments_test/uptime_test.py
@@ -0,0 +1,29 @@
+import unittest
+import mock
+import segments.uptime as uptime
+
+test_cases = {
+    # linux test cases
+    "00:00:00 up 1:00,  2 users,  load average: 0,00, 0,00, 0,00": "1h",
+    "00:00:00 up 10:00,  2 users,  load average: 0,00, 0,00, 0,00": "10h",
+    "00:00:00 up 1 days, 1:00,  2 users,  load average: 0,00, 0,00, 0,00": "1d",
+    "00:00:00 up 12 days, 1:00,  2 users,  load average: 0,00, 0,00, 0,00": "12d",
+    "00:00:00 up 120 days, 49 min,  2 users,  load average: 0,00, 0,00, 0,00": "120d",
+
+    # mac test cases
+    "00:00:00 up 23  3 day(s), 10:00,  2 users,  load average: 0,00, 0,00, 0,00": "3d",
+}
+
+
+class UptimeTest(unittest.TestCase):
+
+    def setUp(self):
+        self.powerline = mock.MagicMock()
+        uptime.Color = mock.MagicMock()
+
+    @mock.patch('subprocess.check_output')
+    def test_all(self, check_output):
+        for stdout, result in test_cases.items():
+            check_output.return_value = stdout
+            uptime.add_uptime_segment(self.powerline)
+            self.assertEqual(self.powerline.append.call_args[0][0].split()[0], result)