I'm trying to learn regular expressions, and so I created a file with some characters in it. When I use the cat command and pipe it to rgrep command with an argument to filter out text, the resulting output is "filename:filtered_text".

cat regex.txt | rgrep -E '^a'

Not only that it also checks for other files in the folder and if they match the regular expression it shows them as well, even though I only piped a single file.

How do I get a normal output with only the contents of the file I've passed into cat?

  • Can I ask what you are intending with the -R flag?
    – BowlOfRed
    Sep 28, 2018 at 17:36
  • 1
    cat doesn't output the file names.
    – RalfFriedl
    Sep 28, 2018 at 17:38
  • @BowlOfRed That was a mistake. I needed to use the -E option (for more options according to the video I'm watching). The output is still the same without any options.
    – Skaper
    Sep 28, 2018 at 17:40
  • Have you tried using cat < regex.txt?
    – unxnut
    Sep 28, 2018 at 17:55
  • @unxnut I did, just now, and got the same output. This time it even checked subdirectories.
    – Skaper
    Sep 28, 2018 at 17:58

2 Answers 2


You need to use the -h flag with rgrep, as it is the rgrep that prints the file names. This flags stops this.

I can only guess why you use rgrep as opposed to the simple grep and cat. So, first, you don't need cat. grep PATTERN FILE is enough. Second rgrep is equivalent to grep -r. And here's what the -r flag to grep does:

-r, --recursive Read all files under each directory, recursively, following symbolic links only if they are on the command line. Note that if no file operand is given, grep searches the working directory. This is equivalent to the -d recurse option.

For a simple solution, try:

grep '^a' regex.txt

(It's unclear with this pattern, why you would use the -E switch.)

  • That took out the file name but it still reads from all the files in the folder. I used '^a' and got lines starting with 'a' from all the files.
    – Skaper
    Sep 28, 2018 at 17:53
  • @Skaper Use normal grep
    – user147505
    Sep 28, 2018 at 17:58
  • I used it but it gives me the whole output of the file with the characters matching the regular expression highlighted as red color. Shouldn't it only output the lines matching the regex?
    – Skaper
    Sep 28, 2018 at 18:03
  • @Skaper You need to ask a full question with what you do, what you expect and why, and what you get. I attempted a better answer with my edit.
    – user147505
    Sep 28, 2018 at 18:06
  • @Skaper See a solution added.
    – user147505
    Sep 28, 2018 at 18:11

rgrep is a shortcut (on my ubuntu 18.04: a script) for grep -r. man grep:

   -r, --recursive
          Read  all  files  under  each directory, recursively,

In fact, the use of cat is pointless, as rgrep seems to ignore it.

  • I'm using Linux 18.04.1 LTS. Are you talking about predefined aliases? If yes, then I don't have any for rgrep command.
    – Skaper
    Sep 28, 2018 at 17:49
  • No, it's a script in /usr/bin.
    – RudiC
    Sep 28, 2018 at 17:52

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