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      <h1 class="entry-title">Bash Adventures: Read a Single Character, Even if It&#8217;s a Newline</h1>
    
    
      <p class="meta">




<time datetime="2011-01-12 14:16:00 -0800" pubdate  updated >Jan 12<span>th</span>, 2011</time>


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<div class="entry-content"><h3>tl;dr</h3>

<div class="highlight"><pre><code class="bash">getc<span class="o">()</span> <span class="o">{</span>
  <span class="nv">IFS</span><span class="o">=</span> <span class="nb">read</span> -r -n1 -d <span class="s1">&#39;&#39;</span> <span class="s2">&quot;$@&quot;</span>
<span class="o">}</span>

getc ch
<span class="nb">echo</span> <span class="s2">&quot;$ch&quot;</span> <span class="c"># don&#39;t forget to quote it!</span>
</code></pre>
</div>

<h3>the explanation</h3>

<!-- more -->

<p>So it turns out that it&#8217;s pretty difficult to read exactly one character from a stream.
From the manual for <code>read</code> (usually in <code>man builtins</code>):</p>

<pre><code>-n nchars
       read returns after reading nchars characters rather  than
       waiting  for a complete line of input, but honor a delim‐
       iter if fewer than nchars characters are read before  the
       delimiter.
</code></pre>

<p>The naive solution, then would be:</p>

<div class="highlight"><pre><code class="bash">getc<span class="o">()</span> <span class="o">{</span>
  <span class="nb">read</span> -n1 <span class="s2">&quot;$@&quot;</span>
<span class="o">}</span>

<span class="k">while </span>getc ch; <span class="k">do</span>
<span class="k">  </span><span class="nb">echo</span> <span class="s2">&quot;[$ch]&quot;</span>;
<span class="k">done</span> <span class="s">&lt;&lt;EOF</span>
<span class="s">one</span>
<span class="s">two</span>
<span class="s">three</span>
<span class="s">four</span>
<span class="s">EOF</span>
</code></pre>
</div>

<p>The output from this is:</p>

<pre><code>[o]
[n]
[e]
[]
[t]
[w]
[o]
[]
[t]
[h]
[r]
[e]
[e]
[]
[f]
[o]
[u]
[r]
[]
</code></pre>

<p>Wait, what happened to the newlines &#8211; why are they returning empty characters?  Well, it turns out there are two things going on here.  Again from the manual:</p>

<pre><code>The characters in the value of the IFS variable are used to split the line into words.
</code></pre>

<p>&#8230;and the newline is in the IFS.  So let&#8217;s zero out <code>IFS</code>:</p>

<div class="highlight"><pre><code class="bash">getc<span class="o">()</span> <span class="o">{</span>
  <span class="nv">IFS</span><span class="o">=</span> <span class="nb">read</span> -n1 <span class="s2">&quot;$@&quot;</span>
<span class="o">}</span>
</code></pre>
</div>

<p>unfortunately, this outputs the same thing, but for a different reason.
Remember: <code>read -n nchars</code> will still &quot;honor a delimiter if fewer than <code>nchars</code> characters are read before the delimiter&quot;.
And the default delimiter (specified with <code>-d delim</code>) is the newline.
So the next solution is to use an empty delimiter.</p>

<p>Now our <code>getc</code> looks like:</p>

<div class="highlight"><pre><code class="bash">getc<span class="o">()</span> <span class="o">{</span>
  <span class="nv">IFS</span><span class="o">=</span> <span class="nb">read</span> -n1 -d <span class="s1">&#39;&#39;</span> <span class="s2">&quot;$@&quot;</span>
<span class="o">}</span>
</code></pre>
</div>

<p>(NOTE: the space between the <code>-d</code> and the <code>''</code> is not optional.  This confused me the first time I was solving the problem, and I had suggested using the EOF character.  Not only that, I suggested obtaining an EOF with <code>&quot;$(echo -e '\004')</code>.  The empty delimiter is a better way to solve this particular problem, and <code>$'\004'</code> is a better way to get an EOF character.)</p>

<p>Now we get a very different output:</p>

<pre><code>[o]
[n]
[e]
[
]
[t]
[w]
[o]
[
]
[t]
[h]
[r]
[e]
[e]
[
]
[f]
[o]
[u]
[r]
[
]
</code></pre>

<p>Our newlines are back!  Yay!</p>

<p>There is, however, one more wrinkle, as helpfully pointed out by an anonymous commenter.  <code>read</code> also likes to let you use backslashes to escape things.  For example, with the current implementation of <code>getc</code>:</p>

<div class="highlight"><pre><code class="bash"><span class="k">while </span>getc ch; <span class="k">do</span>
<span class="k">  </span><span class="nb">echo</span> <span class="s2">&quot;[$ch]&quot;</span>
<span class="k">done</span> <span class="s">&lt;&lt;EOF</span>
<span class="s">one\</span>
<span class="s">two</span>
<span class="s">EOF</span>
</code></pre>
</div>

<p>Yields the following output:</p>

<pre><code>[o]
[n]
[e]
[t]
[w]
[o]
[
]
</code></pre>

<p>Note how the backslash after <code>one</code> was interpreted as an escape for the newline that follows.  Luckily, <code>read</code> comes with an <code>-r</code> option:</p>

<pre><code>-r     Backslash does not act as an escape character.  The back‐
       slash is considered to be part of the line.  In  particu‐
       lar,  a  backslash-newline pair may not be used as a line
       continuation.
</code></pre>

<p>So the final solution is:</p>

<div class="highlight"><pre><code class="bash">getc<span class="o">()</span> <span class="o">{</span>
  <span class="nv">IFS</span><span class="o">=</span> <span class="nb">read</span> -r -n1 -d <span class="s1">&#39;&#39;</span> <span class="s2">&quot;$@&quot;</span>
<span class="o">}</span>
</code></pre>
</div>

<p>EDIT 7/19/2011:  Fixes and nuances to the original solution, as provided by <code>:)</code>.</p>
</div>


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<span class="byline author vcard">Posted by <span class="fn">Jay Adkisson</span></span>

      




<time datetime="2011-01-12 14:16:00 -0800" pubdate  updated >Jan 12<span>th</span>, 2011</time>



      


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