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I have a Python script, EulerianCycle.py, and an input file, euleriancycle.txt.

I am able to get the correct results by doing py EulerianCycle euleriancycle.txt > cat euleriancycleout.txt into the current folder (py is an alias for python3).

However, I have another folder in this current one called outputs, to which I want all my output files be directed.

I've tried py EulerianCycle.py euleriancycle.txt | cd outputs/ | cat > euleriancycleout.txt

And py EulerianCycle.py euleriancycle.txt | cat >cd outputs/euleriancycleout.txt

which gives me the broken pipe error.

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3 Answers 3

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If py EulerianCycle.py euleriancycle.txt writes to the standard output stream (which I assume it does, since otherwise you wouldn't be able to pipe it to cat) then cat is entirely superfluous here - you can redirect standard output directly, specifying either absolute or relative path to your output file:

py EulerianCycle.py euleriancycle.txt > outputs/euleriancycleout.txt

(note: the directory outputs/ must already exist).

Neither of your other commands works the way you might imagine.

  • in py EulerianCycle euleriancycle.txt > cat euleriancycleout.txt, the shell creates a file named cat in the current directory, and redirects the output of py EulerianCycle to it, passing both euleriancycle.txt and euleriancycleout.txt to it as input arguments.

  • in py EulerianCycle.py euleriancycle.txt | cat >cd outputs/euleriancycleout.txt, the shell creates a file named cd in the current directory, cat reads outputs/euleriancycleout.txt and writes it to file cd, ignoring standard input from the pipe (cat only reads standard input when it is given no input files, or an explicit -).

Perhaps what you were aiming for here was to pipe the output to a subshell like:

py EulerianCycle.py euleriancycle.txt | (cd outputs; cat > euleriancycleout.txt)

or

py EulerianCycle.py euleriancycle.txt | (cd outputs && cat > euleriancycleout.txt)

Here, cat reads the subshell's standard input - which is provided by the pipe - after changing to the target directory. The second version only creates euleriancycleout.txt if the cd command succeeds; the first creates it in the current directory if the cd fails.

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  • okay yes this makes sense. Your code works as intended. I also did see a random file named "cd" in the cwd. Thanks for explaining the unintentional actions of the commands I tried as well. for @mook765, the broken pipe error is as follows: py EulerianCycle.py euleriancycle.txt | cat >cd outputs/euleriancycleout.txt Traceback (most recent call last): File "/home/user/.../EulerianCycle.py", line 39, in <module> print('->'.join(eulerian_cycle(graph))) BrokenPipeError: [Errno 32] Broken pipe
    – trinity
    Jul 4, 2021 at 15:36
  • @Rukhan so presumably that's because cat is reading (instead of writing to) the intended output file, so there's noting actually reading what your python program is sending down the pipe
    – steeldriver
    Jul 4, 2021 at 15:40
  • @steeldriver Understood! Did not realize that cat's use is for stdin explicitly like cat EulerianCycle.py, where I was trying to make it read stdout and send it down a pipe
    – trinity
    Jul 4, 2021 at 15:47
  • @Trinity cat will do that - if you either give it no input files, or give it - as a pseudo input file. But it is not needed here.
    – steeldriver
    Jul 4, 2021 at 15:57
  • @steeldriver noted with thanks, have a great one!
    – trinity
    Jul 4, 2021 at 16:01
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Additionally, you can use:

EulerianCycle.py | tee euleriancycleout.txt

to send the content to the text file and to stdout concurrently in 2 seperate streams. In other words the content will end up both in the text file and printed in the terminal.

I always do it this way as it shows me what wrote, and saves me having to open up vim to check my work.

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  • Note: This is meant only to provide additional context, in support of the answer above.
    – Nate T
    Jul 6, 2021 at 11:38
  • Thanks for the info, really useful!
    – trinity
    Jul 7, 2021 at 17:05
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Figured out I can use py EulerianCycle.py euleriancycle.txt | cat > euleriancycleout.txt | mv -t outputs/ euleriancycleout.txt

OR

py EulerianCycle.py euleriancycle.txt | cat > euleriancycleout.txt & mv -t outputs/ euleriancycleout.txt

Still open to a possible command that will do this more concisely. :)

edit: code presented is not efficient, see green check marked answer

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