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<p class="MsoNormal">I’m trying to debug a shell script that uses dialog(1), which has the unfortunate habit of overwriting the debug output I wanted to see.<o:p></o:p></p>
<p class="MsoNormal">After quite some time banging my head against the wall, I grabbed the source to dialog to look at their examples.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Their examples follow a pattern I have NOT been following; I’m now looking at this trying to understand WTF it does:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal" style="text-indent:.5in">1 exec 3>&1<o:p></o:p></p>
<p class="MsoNormal" style="text-indent:.5in">2 returntext=`$DIALOG --title "RANGE BOX" --rangebox "Please set the volume..." 0 60 0 123 5 2>&1 1>&3`<o:p></o:p></p>
<p class="MsoNormal" style="text-indent:.5in">3 returncode=$?<o:p></o:p></p>
<p class="MsoNormal" style="text-indent:.5in">4 exec 3>&-<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">so… ok. <o:p></o:p></p>
<p class="MsoNormal">Line 1 duplicates FD 1 (stdout) into FD 3. That’s usually so we can mess with FD 1 later on and still have a handle to stdout. Good so far.<o:p></o:p></p>
<p class="MsoNormal">Line 2 runs /usr/bin/dialog in a subshell, the options to dialog(1) are irrelevant here. I’m looking at the redirects.
<o:p></o:p></p>
<p class="MsoNormal"> “2>&1” duplicates FD 1 into FD 2, effectively throwing away the handle to the inherited /dev/stderr and forcing stderr output onto stdout. That’s a good thing because dialog(1) reports its results onto stderr by default to avoid messing
with curses(3) output.<o:p></o:p></p>
<p class="MsoNormal"> “1>&3” however, throws away stdout (FD 1), replacing it with FD 3 which is… already a copy of FD 1 ???<o:p></o:p></p>
<p class="MsoNormal">Line 3 is obvious.<o:p></o:p></p>
<p class="MsoNormal">Line 4 closes FD 3. Not sure that we need to bother, but OK, cleaning up behind ourselves is usually a good thing.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I get the need for swapping stdin/stderr, as dialog(1) – well, really curses(3) - uses stdout to paint the screen, so dialog(1) spits its results out on stderr to avoid conflicting. If we’re in a subshell trying to capture that output,
we need to grab it just as dialog(1) exits – and $() assignment only captures stdout.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">But what’s the point of having FD 3 in this setup, if all we do is assign it to stdin inside the subshell and then forget about it?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Can anyone see what I’m missing here?<o:p></o:p></p>
<p class="MsoNormal">-Adam<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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