Write document about command connection

docs/_sources/connection.txt

@@ -123,6 +123,9 @@ In the case 1, ``echo 'else'`` is skipped because ``echo 'if'`` returns 0. In th
 Then (?>)
 =============================
 
+Composition of if then else
+-----------------------------
+
 When two commands are connected with ``?>``, the right command runs only when the left command returns 0. When the right command returns any error, the script stops. We can implement "if-then-else" rules by combinations of ``>>, !>`` and ``?>`` as the following way.
 
 .. code-block:: bash
@@ -169,3 +172,34 @@ The clunky shape of this if-then-else can be a little bit relieved by "do blocks
 	if
 
 However, do blocks enhance computational costs.
+
+Rule of ``?>`` at errors
+-----------------------------
+
+When the command at the right side of ``?>`` returns any error, the script stops immediately. In the following example, ``false`` at the second if-then connection stops the script.
+
+.. code-block:: bash
+        :linenos:
+
+	$ cat then_stop.glue 
+	import PATH
+
+	false ?> true
+	echo 'ok'
+
+	true ?> false
+	echo 'do not come here'
+
+	$ glue then_stop.glue 
+	ok
+	Execution error at line 6, char 9
+		line6: true ?> false
+		               ^
+
+		Error at then part
+		process_level 0
+		exit_status 8
+		pid 51633
+	ERROR: 8
+
+As shown in the output, the glue command gives 8 as the exit status when it stops by the right side of ``?>``.

docs/connection.html

@@ -192,6 +192,8 @@ <h2>5.3. Or (!&gt;)<a class="headerlink" href="#or" title="Permalink to this hea
 </div>
 <div class="section" id="then">
 <h2>5.4. Then (?&gt;)<a class="headerlink" href="#then" title="Permalink to this headline">¶</a></h2>
+<div class="section" id="composition-of-if-then-else">
+<h3>5.4.1. Composition of if then else<a class="headerlink" href="#composition-of-if-then-else" title="Permalink to this headline">¶</a></h3>
 <p>When two commands are connected with <code class="docutils literal"><span class="pre">?&gt;</span></code>, the right command runs only when the left command returns 0. When the right command returns any error, the script stops. We can implement &#8220;if-then-else&#8221; rules by combinations of <code class="docutils literal"><span class="pre">&gt;&gt;,</span> <span class="pre">!&gt;</span></code> and <code class="docutils literal"><span class="pre">?&gt;</span></code> as the following way.</p>
 <div class="highlight-bash"><table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre> 1
  2
@@ -264,6 +266,53 @@ <h2>5.4. Then (?&gt;)<a class="headerlink" href="#then" title="Permalink to this
 </td></tr></table></div>
 <p>However, do blocks enhance computational costs.</p>
 </div>
+<div class="section" id="rule-of-at-errors">
+<h3>5.4.2. Rule of <code class="docutils literal"><span class="pre">?&gt;</span></code> at errors<a class="headerlink" href="#rule-of-at-errors" title="Permalink to this headline">¶</a></h3>
+<p>When the command at the right side of <code class="docutils literal"><span class="pre">?&gt;</span></code> returns any error, the script stops immediately. In the following example, <code class="docutils literal"><span class="pre">false</span></code> at the second if-then connection stops the script.</p>
+<div class="highlight-bash"><table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre> 1
+ 2
+ 3
+ 4
+ 5
+ 6
+ 7
+ 8
+ 9
+10
+11
+12
+13
+14
+15
+16
+17
+18
+19
+20</pre></div></td><td class="code"><div class="highlight"><pre><span class="nv">$ </span>cat then_stop.glue
+import PATH
+
+<span class="nb">false</span> ?&gt; <span class="nb">true</span>
+<span class="nb">echo</span> <span class="s1">&#39;ok&#39;</span>
+
+<span class="nb">true</span> ?&gt; <span class="nb">false</span>
+<span class="nb">echo</span> <span class="s1">&#39;do not come here&#39;</span>
+
+<span class="nv">$ </span>glue then_stop.glue
+ok
+Execution error at line 6, char 9
+        line6: <span class="nb">true</span> ?&gt; <span class="nb">false</span>
+                       ^
+
+        Error at <span class="k">then</span> part
+        process_level 0
+        exit_status 8
+        pid 51633
+ERROR: 8
+</pre></div>
+</td></tr></table></div>
+<p>As shown in the output, the glue command gives 8 as the exit status when it stops by the right side of <code class="docutils literal"><span class="pre">?&gt;</span></code>.</p>
+</div>
+</div>
 </div>
 
 
@@ -278,7 +327,11 @@ <h3><a href="index.html">Table Of Contents</a></h3>
 <li><a class="reference internal" href="#pipeline">5.1. Pipeline</a></li>
 <li><a class="reference internal" href="#and">5.2. And (&gt;&gt;)</a></li>
 <li><a class="reference internal" href="#or">5.3. Or (!&gt;)</a></li>
-<li><a class="reference internal" href="#then">5.4. Then (?&gt;)</a></li>
+<li><a class="reference internal" href="#then">5.4. Then (?&gt;)</a><ul>
+<li><a class="reference internal" href="#composition-of-if-then-else">5.4.1. Composition of if then else</a></li>
+<li><a class="reference internal" href="#rule-of-at-errors">5.4.2. Rule of <code class="docutils literal"><span class="pre">?&gt;</span></code> at errors</a></li>
+</ul>
+</li>
 </ul>
 </li>
 </ul>

manual_src/connection.rst

@@ -176,4 +176,30 @@ However, do blocks enhance computational costs.
 Rule of ``?>`` at errors
 -----------------------------
 
-When the command at the right side of ``?>`` returns any error, the script stops immediately.
+When the command at the right side of ``?>`` returns any error, the script stops immediately. In the following example, ``false`` at the second if-then connection stops the script.
+
+.. code-block:: bash
+        :linenos:
+
+	$ cat then_stop.glue 
+	import PATH
+	
+	false ?> true
+	echo 'ok'
+	
+	true ?> false
+	echo 'do not come here'
+
+	$ glue then_stop.glue 
+	ok
+	Execution error at line 6, char 9
+		line6: true ?> false
+		               ^
+	
+		Error at then part
+		process_level 0
+		exit_status 8
+		pid 51633
+	ERROR: 8
+
+As shown in the output, the glue command gives 8 as the exit status when it stops by the right side of ``?>``.