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<div class="section-level-extent" id="Environments">
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<hr>
<h3 class="section" id="Environments-1">5.31 Environments</h3>
<a class="index-entry-id" id="index-environments"></a>
<p>As discussed in <a class="ref" href="Deferring-Output.html">Deferring Output</a>, environments store most of the
parameters that determine the appearance of text. A default environment
named ‘<samp class="samp">0</samp>’ exists when GNU <code class="code">troff</code> starts up; it is modified by
formatting-related requests and escape sequences.
</p>
<a class="index-entry-id" id="index-stack"></a>
<p>You can create new environments and switch among them. Only one is
current at any given time. Active environments are managed using a
<em class="dfn">stack</em>, a data structure supporting “push” and “pop”
operations. The current environment is at the top of the stack.
The same environment name can be pushed onto the stack multiple times,
possibly interleaved with others. Popping the environment stack does
not destroy the current environment; it remains accessible by name and
can be made current again by pushing it at any time. Environments
cannot be renamed or deleted, and can only be modified when current. To
inspect the environment stack, use the <code class="code">pev</code> request; see
<a class="ref" href="Debugging.html">Debugging</a>.
</p>
<p>Environments store the following information.
</p>
<ul class="itemize mark-bullet">
<li>a partially collected line, if any
</li><li>data about the most recently output glyph and line (registers
<code class="code">.cdp</code>, <code class="code">.cht</code>, <code class="code">.csk</code>, <code class="code">.n</code>, <code class="code">.w</code>)
</li><li>typeface parameters (size, family, style, height and slant, inter-word
and inter-sentence space sizes)
</li><li>page parameters (line length, title length, vertical spacing, line
spacing, indentation, line numbering, centering, right-alignment,
underlining, hyphenation parameters)
</li><li>filling enablement; adjustment enablement and mode
</li><li>tab stops; tab, leader, escape, control, no-break control, hyphenation,
and margin characters
</li><li>input line traps
</li><li>stroke and fill colors
</li></ul>
<dl class="first-deffn">
<dt class="deffn" id="index-_002eev"><span class="category-def">Request: </span><span><strong class="def-name"><code class="t">.ev</code></strong> <var class="def-var-arguments">[<span class="r"><i class="slanted">ident</i></span>]</var><a class="copiable-link" href='#index-_002eev'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-ev"></a>
</dd><dt class="deffnx def-cmd-deffn" id="index-_005cn_005b_002eev_005d"><span class="category-def">Register: </span><span><strong class="def-name"><code class="t">\n[.ev]</code></strong><a class="copiable-link" href='#index-_005cn_005b_002eev_005d'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-_002eev-1"></a>
<a class="index-entry-id" id="index-switching-environments-_0028ev_0029"></a>
<a class="index-entry-id" id="index-environment_002c-switching-_0028ev_0029"></a>
<a class="index-entry-id" id="index-environment-number_002fname-register-_0028_002eev_0029"></a>
<p>Enter the environment <var class="var">ident</var>, which is created if it does not
already exist, using the same parameters as for the default environment
used at startup. With no argument, GNU <code class="code">troff</code> switches to the
previous environment.
</p>
<p>Invoking <code class="code">ev</code> with an argument puts environment <var class="var">ident</var> onto
the top of the environment stack. (If it isn’t already present in the
stack, this is a proper push.) Without an argument, <code class="code">ev</code> pops the
environment stack, making the previous environment current. It is an
error to pop the environment stack with no previous environment
available. The read-only string-valued register <code class="code">.ev</code> contains the
name of the current environment—the one at the top of the stack.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">.ev footnote-env
.fam N
.ps 6
.vs 8
.ll -.5i
.ev
<span class="r">…</span>
.ev footnote-env
\[dg] Observe the smaller text and vertical spacing.
.ev
</pre></div></div>
<p>We can familiarize ourselves with stack behavior by wrapping the
<code class="code">ev</code> request with a macro that reports the contents of the
<code class="code">.ev</code> register to the standard error stream.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">.de EV
. ev \\$1
. tm environment is now \\n[.ev]
..
.
.EV foo
.EV bar
.EV
.EV baz
.EV
.EV
.EV
</pre></div></div>
<div class="example">
<div class="group"><pre class="example-preformatted"> error→ environment is now foo
error→ environment is now bar
error→ environment is now foo
error→ environment is now baz
error→ environment is now foo
error→ environment is now 0
error→ error: environment stack underflow
error→ environment is now 0
</pre></div></div>
</dd></dl>
<dl class="first-deffn">
<dt class="deffn" id="index-_002eevc"><span class="category-def">Request: </span><span><strong class="def-name"><code class="t">.evc</code></strong> <var class="def-var-arguments">environment</var><a class="copiable-link" href='#index-_002eevc'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-evc"></a>
<a class="index-entry-id" id="index-copying-environment-_0028evc_0029"></a>
<a class="index-entry-id" id="index-environment_002c-copying-_0028evc_0029"></a>
<p>Copy the contents of <var class="var">environment</var> to the current environment.
</p>
<p>The following environment data are not copied.
</p>
<ul class="itemize mark-bullet">
<li>a partially collected line, if present;
</li><li>the interruption status of the previous input line (due to use of the
<code class="code">\c</code> escape sequence);
</li><li>the count of remaining lines to center, to right-justify, or to
underline (with or without underlined spaces)—these are set to zero;
</li><li>the activation status of temporary indentation;
</li><li>input line traps and their associated data;
</li><li>the activation status of line numbering (which can be reactivated with
‘<samp class="samp">.nm +0</samp>’<!-- /@w -->); and
</li><li>the count of consecutive hyphenated lines (set to zero).
</li></ul>
</dd></dl>
<dl class="first-deffn">
<dt class="deffn" id="index-_005cn_005b_002ew_005d"><span class="category-def">Register: </span><span><strong class="def-name"><code class="t">\n[.w]</code></strong><a class="copiable-link" href='#index-_005cn_005b_002ew_005d'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-_002ew"></a>
</dd><dt class="deffnx def-cmd-deffn" id="index-_005cn_005b_002echt_005d"><span class="category-def">Register: </span><span><strong class="def-name"><code class="t">\n[.cht]</code></strong><a class="copiable-link" href='#index-_005cn_005b_002echt_005d'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-_002echt"></a>
</dd><dt class="deffnx def-cmd-deffn" id="index-_005cn_005b_002ecdp_005d"><span class="category-def">Register: </span><span><strong class="def-name"><code class="t">\n[.cdp]</code></strong><a class="copiable-link" href='#index-_005cn_005b_002ecdp_005d'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-_002ecdp"></a>
</dd><dt class="deffnx def-cmd-deffn" id="index-_005cn_005b_002ecsk_005d"><span class="category-def">Register: </span><span><strong class="def-name"><code class="t">\n[.csk]</code></strong><a class="copiable-link" href='#index-_005cn_005b_002ecsk_005d'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-_002ecsk"></a>
<a class="index-entry-id" id="index-environment_002c-dimensions-of-last-glyph-_0028_002ew_002c-_002echt_002c-_002ecdp_002c-_002ecsk_0029"></a>
<a class="index-entry-id" id="index-width_002c-of-last-glyph-_0028_002ew_0029"></a>
<a class="index-entry-id" id="index-height_002c-of-last-glyph-_0028_002echt_0029"></a>
<a class="index-entry-id" id="index-depth_002c-of-last-glyph-_0028_002ecdp_0029"></a>
<a class="index-entry-id" id="index-skew_002c-of-last-glyph-_0028_002ecsk_0029"></a>
<a class="index-entry-id" id="index-last-glyph_002c-dimensions-_0028_002ew_002c-_002echt_002c-_002ecdp_002c-_002ecsk_0029"></a>
<a class="index-entry-id" id="index-glyph_002c-last_002c-dimensions-_0028_002ew_002c-_002echt_002c-_002ecdp_002c-_002ecsk_0029"></a>
<p>The <code class="code">\n[.w]</code> register contains the width of the last glyph
formatted in the environment.
</p>
<p>The <code class="code">\n[.cht]</code> register contains the height of the last glyph
formatted in the environment.
</p>
<p>The <code class="code">\n[.cdp]</code> register contains the depth of the last glyph
formatted in the environment. It is positive for glyphs extending below
the baseline.
</p>
<p>The <code class="code">\n[.csk]</code> register contains the <em class="dfn">skew</em> (how far to the
right of the glyph’s center that GNU <code class="code">troff</code> should place an
accent) of the last glyph formatted in the environment.
</p></dd></dl>
<dl class="first-deffn">
<dt class="deffn" id="index-_005cn_005b_002en_005d"><span class="category-def">Register: </span><span><strong class="def-name"><code class="t">\n[.n]</code></strong><a class="copiable-link" href='#index-_005cn_005b_002en_005d'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-_002en"></a>
<a class="index-entry-id" id="index-environment_002c-previous-line-length-_0028_002en_0029"></a>
<a class="index-entry-id" id="index-line-length_002c-previous-_0028_002en_0029"></a>
<a class="index-entry-id" id="index-length-of-previous-line-_0028_002en_0029"></a>
<a class="index-entry-id" id="index-previous-line-length-_0028_002en_0029"></a>
<p>The <code class="code">\n[.n]</code> register contains the length of the previous output
line emitted in the environment.
</p></dd></dl>
</div>
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