How to stop cat command from outputting file name if it's piped to another command?

Question

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'
regex.txt:a
regex.txt:aa
regex.txt:aaa
regex.txt:aaaa
regex.txt:aaaaa
regex.txt:aaaaaa
file:acpi
file:adduser.conf
file:aliases
file:aliases.db
file:alternatives
file:anacrontab
file:apache2
file:apg.conf
file:apm
file:apparmor
file:apparmor.d
file:apport
file:appstream.conf
file:apt
file:avahi

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?

Answer 1

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.)

Answer 2

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.