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<P><FONT SIZE="5">9.6 Diff</FONT><BR>
<P>The <SAMP>diff</SAMP> command reads in two text files and
<I>appends</I> a report to a log file containing a list of
the differences between the two input files. If the log file
does not already exist, an empty one is created first.
<P>
<PRE>
Syntax :
diff = "diff" s filename s filename s
filename s ["ABORT"]
Examples:
diff result.tex answer.tex diff.log
diff $Otest23.out $Atest23.out $Ldiff.log ABORT
</PRE>
<P>The <SAMP>diff</SAMP> command performs a full line-based
differences operation. It will identify different sections
in a file, even if they are of differing length.
<P>The implementation of the <SAMP>diff</SAMP> command is
quite complicated. To be sure that it is at least getting
its same/different proclamation right, the <SAMP>diff</SAMP>
command performs a binary comparison as an extra check.
<P>The following points describe the rules for determining
the result status.
<P> <OL>
<LI> <SAMP>diff</SAMP> aborts with a severe error if the log
file cannot be opened or created for appending.
<LI> An ordinary error is generated if either or both of
the input files cannot be opened.
<LI> If, at the end of the run, the two input files have
not been proven to be identical, and the <SAMP>ABORT</SAMP>
keyword is present, <SAMP>diff</SAMP> returns severe status.
<LI> <SAMP>diff</SAMP> returns success status if none of the
above conditions (or similar conditions) occur, even if the
two files are different.
</OL>
<P>The <SAMP>diff</SAMP> command <I>appends</I> its
differences report rather than merely writing it. This
allows a regression test script to perform a series of
regression tests and produce a report for the user.
<P>The <SAMP>diff</SAMP> command was added to the shell after
it had become apparent that the simpler <SAMP>compare</SAMP>
command was not yielding enough information. Whereas early
on, regression testing was treated mainly as a tool to
ensure that FunnelWeb was being ported to other machines
correctly, it began to place an increasing role during
development in identifying the effects of changes made to
the code. The <SAMP>diff</SAMP> command supports this
application of regression testing by pinpointing the
differences between nearly-identical text files.
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