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If the PAGER environment variable is set but contains an empty string, psql would pass it to "sh" which would silently exit, causing whatever query output we were printing to vanish entirely. This is quite mystifying; it took a long time for us to figure out that this was the cause of Joseph Brenner's trouble report. Rather than allowing that to happen, we should treat this as another way to specify "no pager". (We could alternatively treat it as selecting the default pager, but it seems more likely that the former is what the user meant to achieve by setting PAGER this way.) Nonempty, but all-white-space, PAGER values have the same behavior, and it's pretty easy to test for that, so let's handle that case the same way. Most other cases of faulty PAGER values will result in the shell printing some kind of complaint to stderr, which should be enough to diagnose the problem, so we don't need to work harder than this. (Note that there's been an intentional decision not to be very chatty about apparent failure returns from the pager process, since that may happen if, eg, the user quits the pager with control-C or some such. I'd just as soon not start splitting hairs about which exit codes might merit making our own report.) libpq's old PQprint() function was already on board with ignoring empty PAGER values, but for consistency, make it ignore all-white-space values as well. It's been like this a long time, so back-patch to all supported branches. Discussion: https://postgr.es/m/CAFfgvXWLOE2novHzYjmQK8-J6TmHz42G8f3X0SORM44+stUGmw@mail.gmail.com
<!-- doc/src/sgml/README.links -->
Linking within SGML documents can be confusing, so here is a summary:
Intra-document Linking
----------------------
<xref>
use to get chapter/section number from the title of the target
link, or xreflabel if defined at the target, or refentrytitle if target
is a refentry; has no close tag
http://www.oasis-open.org/docbook/documentation/reference/html/xref.html
<link>
use to supply text for the link, requires </link>
http://www.oasis-open.org/docbook/documentation/reference/html/link.html
linkend=
controls the target of the link/xref, required
endterm=
for <xref>, allows the text of the link/xref to be taken from a
different link target title
External Linking
----------------
<ulink>
like <link>, but uses a URL (not a document target); requires
</ulink>; if no text is specified, the URL appears as the link
text
http://www.oasis-open.org/docbook/documentation/reference/html/ulink.html
url=
used by <ulink> to specify the URL, required
Guidelines
----------
o If you want to supply text, use <link>, else <xref>
o Do not use text with <ulink> so the URL appears in printed output
o Specific nouns like GUC variables, SQL commands, and contrib modules
usually have xreflabels