Question

When evaluating if(-t STDIN), does the < UNIX operator count as STDIN? If not, how do I get that data?

So someone types perl example.pl < testing.txt. This doesn’t behave like data piped in via ls | ./example.pl. How can I get that behavior?

Answer 1

Test -p STDIN, which checks if the filehandle STDIN is attached to a pipe.

touch foo
perl -e 'print -p STDIN' < foo            # nothing
cat foo | perl -e 'print -p STDIN'        # 1

But I’m not sure I understand your question. In all three of these cases

1.  perl -e 'print $_=<STDIN>' < <(echo foo)
2.  echo foo | perl -e 'print $_=<STDIN>'
3.  perl -e 'print $_=<STDIN>'          # then type "foo\n" to the console

the inputs are the same and all accessible through the STDIN filehandle. In the first two cases, -t STDIN will evaluate to false, and in the second case, -p STDIN will be true.

The differences in behavior between these three cases are subtle, and usually not important. The third case, obviously, will wait until at least one line of input (terminated with “\n” or EOF) is received. The difference between the first two cases is even more subtle. When the input to your program is piped from the output of another process, you are somewhat at the mercy of that first process with respect to latency or whether that program buffers its output.

Maybe you could expand on what you mean when you say

perl example.pl < testing.txt

doesn’t behave like

ls | ./example.pl