cal April 2012 | cat > t | cat < t | more
Why does it showing nothing? Why isn't it showing
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| (anonymous pipe) connects stdout (1) of the first process with stdin (0) of the second. After redirecting the output to a file, there is no stdout left, so there's nothing to pipe. Also, cat | cat < file does not really make sense, it gets two inputs connected to stdin (at least with bash, redirection comes later and "wins": echo uiae | cat <somefile will output the content of somefile)
If you want to display output of a command and, at the same time, write it to the file, use the tee binary. It writes to a file, but still writes to stdout
cal April 2012 | tee t | more
cat t # content of the above `cal` command