How to remove lines from text file not starting with certain characters (sed or grep)

Question

How do I delete all lines in a text file which do not start with the characters #, & or *? I'm looking for a solution using sed or grep.

Answer 1

Deleting lines:

With grep

From http://lowfatlinux.com/linux-grep.html :

The grep command selects and prints lines from a file (or a bunch of files) that match a pattern.

I think you can do something like this:

grep -v '^[\#\&\*]' yourFile.txt > output.txt

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You can also use sed to do the same thing (check http://lowfatlinux.com/linux-sed.html ):

sed '^[\#\&\*]/d' yourFile.txt > output.txt

It's up to you to decide

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Filtering lines:

My mistake, I understood you wanted to delete the lines. But if you want to "delete" all other lines (or filter the lines starting with the specified characters), then grep is the way to go:

grep '^[\#\&\*]' yourFile.txt > output.txt

Answer 2

sed -n '/^[#&*].*/p' input.txt > output.txt

this should work.

 sed -ni '/^[#&*].*/p' input.txt

this one will edit the input file directly, be careful +