Print help if pager is not on PATH

This checks to see if the PAGER is available on POSIX systems
before attempting to run it. If it is not found, then the help
information will just be printed. This adds support for the
rare system that does not have `less` installed.

Fixes aws#1957

.changes/next-release/bugfix-help.json

@@ -0,0 +1,5 @@
+{
+  "description": "Write help content to stdout if less is not installed. Fixes `#1957 <https://github.com/aws/aws-cli/issues/1957>`__",
+  "category": "help",
+  "type": "bugfix"
+}

awscli/help.py

@@ -12,6 +12,7 @@
 # language governing permissions and limitations under the License.
 import logging
 import os
+import sys
 import platform
 import shlex
 from subprocess import Popen, PIPE
@@ -62,6 +63,8 @@ class PagingHelpRenderer(object):
     a particular platform.
 
     """
+    def __init__(self, output_stream=sys.stdout):
+        self.output_stream = output_stream
 
     PAGER = None
 
@@ -114,6 +117,12 @@ def _convert_doc_content(self, contents):
 
     def _send_output_to_pager(self, output):
         cmdline = self.get_pager_cmdline()
+        if not self._exists_on_path(cmdline[0]):
+            LOG.debug("Pager '%s' not found in PATH, printing raw help." %
+                      cmdline[0])
+            self.output_stream.write(output.decode('utf-8') + "\n")
+            self.output_stream.flush()
+            return
         LOG.debug("Running command: %s", cmdline)
         with ignore_ctrl_c():
             # We can't rely on the KeyboardInterrupt from

tests/unit/test_help.py

@@ -19,6 +19,7 @@
 
 import mock
 
+from awscli.compat import six
 from awscli.help import PosixHelpRenderer, ExecutableNotFoundError
 from awscli.help import WindowsHelpRenderer, ProviderHelpCommand, HelpCommand
 from awscli.help import TopicListerCommand, TopicHelpCommand
@@ -40,11 +41,15 @@ def _popen(self, *args, **kwargs):
 
 
 class FakePosixHelpRenderer(HelpSpyMixin, PosixHelpRenderer):
-    pass
+    def __init__(self, output_stream=sys.stdout):
+        HelpSpyMixin.__init__(self)
+        PosixHelpRenderer.__init__(self, output_stream)
 
 
 class FakeWindowsHelpRenderer(HelpSpyMixin, WindowsHelpRenderer):
-    pass
+    def __init__(self, output_stream=sys.stdout):
+        HelpSpyMixin.__init__(self)
+        WindowsHelpRenderer.__init__(self, output_stream)
 
 
 class TestHelpPager(unittest.TestCase):
@@ -96,10 +101,23 @@ def test_pager_with_args(self):
     def test_no_groff_exists(self):
         renderer = FakePosixHelpRenderer()
         renderer.exists_on_path['groff'] = False
-        with self.assertRaisesRegexp(ExecutableNotFoundError,
-                                     'Could not find executable named "groff"'):
+        expected_error = 'Could not find executable named "groff"'
+        with self.assertRaisesRegexp(ExecutableNotFoundError, expected_error):
             renderer.render('foo')
 
+    @skip_if_windows('Requires POSIX system.')
+    def test_no_pager_exists(self):
+        fake_pager = 'foobar'
+        os.environ['MANPAGER'] = fake_pager
+        stdout = six.StringIO()
+        renderer = FakePosixHelpRenderer(output_stream=stdout)
+        renderer.exists_on_path[fake_pager] = False
+
+        renderer.exists_on_path['groff'] = True
+        renderer.mock_popen.communicate.return_value = (b'foo', '')
+        renderer.render('foo')
+        self.assertEqual(stdout.getvalue(), 'foo\n')
+
     def test_shlex_split_for_pager_var(self):
         pager_cmd = '/bin/sh -c "col -bx | vim -c \'set ft=man\' -"'
         os.environ['PAGER'] = pager_cmd
@@ -109,6 +127,7 @@ def test_shlex_split_for_pager_var(self):
     def test_can_render_contents(self):
         renderer = FakePosixHelpRenderer()
         renderer.exists_on_path['groff'] = True
+        renderer.exists_on_path['less'] = True
         renderer.mock_popen.communicate.return_value = ('rendered', '')
         renderer.render('foo')
         self.assertEqual(renderer.popen_calls[-1][0], (['less', '-R'],))
@@ -133,6 +152,7 @@ def _is_pager_call(self, args):
         renderer = CtrlCRenderer()
         renderer.mock_popen.communicate.return_value = ('send to pager', '')
         renderer.exists_on_path['groff'] = True
+        renderer.exists_on_path['less'] = True
         renderer.render('foo')
         last_call = renderer.mock_popen.communicate.call_args_list[-1]
         self.assertEqual(last_call, mock.call(input='send to pager'))