Home ยป UNIX homework question regarding piping output, I don’t understand what it’s asking? [closed]

UNIX homework question regarding piping output, I don’t understand what it’s asking? [closed]


Always try and break things down into there simplest steps, then try and put things together after. To start the first part, I’d construct myself a sample file.

$ echo -e "line 1 ABAnline 2 ABBnline 3 CCCn" > xxx
$ cat xxx 
line 1 ABA
line 2 ABB
line 3 CCC

So now we have file xxx. Now we need to use something that can act as program yyy. Unix is full of commands that can fill in as stand ins for this, so let’s just pick grep for now. Grep will find strings that we tell it we’re looking for so let’s tell it to find strings that contain “AB”.

$ cat xxx | grep AB
line 1 ABA
line 2 ABB

So we’ve just fulfilled the first 2 requirements.

  • cat xxx sends the contents of file xxx.
  • cat xxx | grep AB sends the contents to a program, yyy.

So the next bit, “pipe the output to the input of another program zzz“, should be fairly straightforward too, since it’s more of the same, of what we’ve been doing with a command like grep. So let’s use another grep like so:

$ cat xxx | grep AB | grep BB
line 2 ABB

Great, we’re almost there. We’ve just completed the 3rd requirement.

  • cat xxx | grep AB | grep BB take the output from grep AB as input, and generates its own output.

So the last bit, we just need to append the output from zzz, i.e. grep BB. That’s easy. If you notice how we constructed the initial file, xxx, we used a file redirection > which will take the output from a command and dump it into a file. The redirector, > will overwrite a file with content, even if it already exists. But we need to append. Luckily we can make use of another redirector, >>. This will append the output from a command to a file that already exists.

Speaking of already existing, we need to construct file xyz. So let’s use our echo method we used to make xxx.

$ echo -e "previous line 1nprevious line 2" > xyz
$ cat xyz 
previous line 1
previous line 2

Now let’s append the output from our command we’ve been constructing:

$ cat xxx | grep AB | grep BB >> xyz

We can check our results:

$ cat xyz 
previous line 1
previous line 2
line 2 ABB

And we’re done.

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