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$ echo Hello >t1.txt

$ echo World >t2.txt
...
$ perl -MIPC::System::Simple=system -wle " $ret = system q[diff -u t1.
+txt t2.txt >NUL 2>&1] ; print $ret; print $?"
"diff -u t1.txt t2.txt >NUL 2>&1" unexpectedly returned exit value 1 a
+t -e line 1

$ diff --help |grep -i exit
Exit status is 0 if inputs are the same, 1 if different, 2 if trouble.

$ perl -MIPC::System::Simple=system -wle " $ret = system [0,1], q[diff
+ -u t1.txt t2.txt >NUL 2>&1] ; print $ret; print $?"
1
...
$ perl -Mautodie=system -wle " $ret = system [0,1], q[diff -u t1.txt t
+2.txt >NUL 2>&1] ; print $ret; print $?"
1
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