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cal April 2012 | cat > t | cat < t | more

Why does it showing nothing? Why isn't it showing

     April 2012       
Su Mo Tu We Th Fr Sa  
 1  2  3  4  5  6  7  
 8  9 10 11 12 13 14  
15 16 17 18 19 20 21  
22 23 24 25 26 27 28  
29 30
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  • | 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 to stdin Commented Apr 5, 2012 at 13:43
  • @knittl So do you mean cat < t is not executed at all? Commented Apr 5, 2012 at 13:45
  • so, is there any other way to do this? Commented Apr 5, 2012 at 13:45
  • @PavanManjunath typing cat < t (only) gives the calender perfectly Commented Apr 5, 2012 at 13:46
  • 1
    @knittl agreed; I sacrificed technical precision for conceptual clarity. :) I suppose describing it in terms of fd0 would have helped understand the why of the conflicting STDINs. It's actually an interesting problem: if you run the second cat using strace cal April 2012 | cat > t | strace cat < t, you'll see that in fact it reads the cal text from fd0, and writes the cal text to fd1. Bash must be shuffling something around. Commented Apr 5, 2012 at 14:31

3 Answers 3

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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
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1 Comment

Can you name a command which reads from file and give stdout? (like tee writes to file and stdout)
1

Because that first cat > t sends all its output to a file called t, leaving no more for the pipeline.

If your intent is to send it to a file and through more to the terminal, just use:

cal April 2012 | tee t | more

This | cat < t construct is very strange and I'm not even sure if it would work. It's trying to connect two totally different things to the standard input of cat and certainly unnecessary.

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this works for me if there's no existing file named t in the current directory. I'm using bash on Ubuntu Oneiric.

$ cal April 2012 | cat > t | cat < t | more
 April 2012       
 Su Mo Tu We Th Fr Sa  
 1  2  3  4  5  6  7  
 8  9 10 11 12 13 14  
 15 16 17 18 19 20 21  
 22 23 24 25 26 27 28  
 29 30
$ cal April 2012 | cat > t | cat < t | more
$ rm t
$ cal April 2012 | cat > t | cat < t | more
 April 2012       
 Su Mo Tu We Th Fr Sa  
 1  2  3  4  5  6  7  
 8  9 10 11 12 13 14  
 15 16 17 18 19 20 21  
 22 23 24 25 26 27 28  
 29 30
Improve this answer

4 Comments

Should not work (and does not work here – debian squeeze. I can test in ubuntu later)
If it's trick works for you, can you explain why it happens like this?
hmm. if I run cal April 2012 | cat > t | cat < t | more over and over, occasionally I get output and occasionally I don't. GNU bash, version 4.2.10(1)-release (i686-pc-linux-gnu)
@orzechowskid I got the same issue. It sometimes works for me, sometimes not! Totally strange!

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