ENH: easier tab completion activation: source tab-qiime (#84)

Removes the ugly and hard-to-remember `eval "$(register-qiime-completion 2> /dev/null)"` command in favor of `source tab-qiime`. Also enables tab completion out of the box for a fresh q2cli install once users run this command.

Thanks @thermokarst and @gregcaporaso for your input!

.travis.yml

@@ -15,4 +15,4 @@ install:
 script:
   - QIIMETEST= nosetests
   - flake8 q2cli setup.py
-  - eval "$(register-qiime-completion 2> /dev/null)"
+  - source tab-qiime

README.md

@@ -14,5 +14,5 @@ To get help with QIIME 2, visit http://2.qiime.org.
 To enable tab completion in Bash, run the following command or add it to your `.bashrc`/`.bash_profile`:
 
 ```bash
-eval "$(register-qiime-completion 2> /dev/null)"
+source tab-qiime
 ```

bin/tab-qiime

@@ -0,0 +1,54 @@
+#!/usr/bin/env bash
+
+# Bash completion script that defers to a cached completion script representing
+# the state of the current QIIME 2 deployment.
+#
+# This script is intended to be executed on the command-line or in
+# .bashrc/.bash_profile:
+#
+#     source tab-qiime
+#
+
+_qiime_completion()
+{
+  # Attempt to find the cached completion script. If q2cli isn't installed, or
+  # is an incompatible version, don't attempt completion.
+  local path="$(python -c "import q2cli.util; print(q2cli.util.get_completion_path())" 2> /dev/null)"
+
+  if [[ $? != 0 ]]; then
+    unset COMPREPLY
+    return 0
+  fi
+
+  # If the completion script exists, attempt completion by invoking the script
+  # in a subshell, supplying COMP_WORDS and COMP_CWORD. Capture the output as
+  # the completion reply. If the completion script failed, don't attempt
+  # completion.
+  if [[ -f "$path" ]] ; then
+    COMPREPLY=( $(COMP_WORDS="${COMP_WORDS[*]}" COMP_CWORD="${COMP_CWORD}" "$path" 2> /dev/null) )
+
+    if [[ $? != 0 ]]; then
+      unset COMPREPLY
+      return 0
+    fi
+  else
+    unset COMPREPLY
+    return 0
+  fi
+
+  return 0
+}
+
+# Enable default readline and bash completion behavior when `_qiime_completion`
+# doesn't have a reply.
+complete -F _qiime_completion -o default -o bashdefault qiime
+
+# Execute a `qiime` command (any command will do) so that tab-completion will
+# work out-of-the-box (e.g. with a fresh installation of q2cli). Running a
+# command will create or refresh the cache if necessary, which contains the
+# actual completion script.
+#
+# Ignore stdout to avoid displaying help text to users enabling tab-completion.
+# stderr displays the note about cache refreshing, as that can take a few
+# moments to complete.
+qiime > /dev/null

q2cli/__main__.py

@@ -21,7 +21,7 @@
 To enable tab completion in Bash, run the following command or add it to your \
 .bashrc/.bash_profile:
 
-    eval "$(register-qiime-completion 2> /dev/null)"
+    source tab-qiime
 
 """
 

setup.py

@@ -15,7 +15,7 @@
     packages=find_packages(),
     include_package_data=True,
     install_requires=['click', 'qiime >= 2.0.2', 'pip'],
-    scripts=['bin/register-qiime-completion'],
+    scripts=['bin/tab-qiime'],
     entry_points='''
         [console_scripts]
         qiime=q2cli.__main__:qiime