DOS solution:

ren *.log *.txt
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bash solution:

for f in /tmp/*.log ; do mv $f ${f%%.*}.txt; done
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File::Find::Rule solution:

my @files = File::Find::Rule->file()->name('*.log')->maxdepth(1)->in($
+destinationDirectory);
foreach my $src ( @files ){
  my $dest = $src; $dest =~ s/[^.]+$/txt/;
  rename $src, $dest;
}
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