I have a very similar question but I need to delete one line in a file which matches one very precise instance of a string only.

Let's assume I have a file composed of thousands of lines and let's call the file chap-secrets. Let's take the following sample entries:

I would now like to delete the line containing the username "pp" only. The following suggestion has been made in a related question in this thread:

However, this doesn't work here, for it would delete (or rather not store) all these lines, because all lines with the usernames "cpp", "hopp" and "flopps" are valid matches, all lines with the server name "pptpd" are valid matches and all lines where the password contains a "pp" (such as "ppbbbbpp") are valid matches, too, and these lines all would be deleted (or rather not stored) as well.

Note that the number of spaces between the fields differ as well all the time, so I can't search for a pattern like

either.

I suppose this problem is very simple for some of you, but for me it's not, thus every help would be very much appreciated.

Thank you!

Hi Black Sun, welcome to LQ!

You have resurrected a LONG dead zombie thread here and I have reported it for a possible move onto its own fresh thread instead.

Meanwhile though, assuming the username is always the first entry, and is always followed by some spaces, you could do this:

Sasha

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Thanks for the warm welcome, Sasha!

Yes, I noticed that it's a very old thread, but old or not, the question seemed to fit to the topic, and Google seems to love pages which are stable and exist for a long time. mistake? Sorry in this case.

Your solution works like a charm, by the way.

Thank you very much -- I wouldn't have expected to get an answer so soon.

I still have one related question, maybe you have a solution to this one as well. As you might notice, I try to get the deletion of users automated, because using a text editor all the time to go through half a dozen of files is a real hassle.

My other file, let's call it users.allowed looks like this:

The usernames are unique again, there is no space after the usernames, and always exactly one space between an "allow" and a username. Again, I would like to get the line containing "allow pp" deleted without lynching the other users who happen to have a "pp" in their names, too.

(That's the last file, I promise.)

EDIT: Actually, I think

should just work fine in this case? Maybe there is a more elegant solution, though? I'll give it a shot, anyway.

2nd EDIT: No, it doesn't work. I had tried this before, doh! It also deletes the line "allow ppv" that's why I had to dump it ... and because there is no space after the usernames I couldn't use "allow pp " (with a space after the username) either.

So I'd still be looking for a solution.

Thank you!

You can use sed as well to extract information:
sed '/allow pp$/w newfilename' filename

or

sed -n '/allow pp$/p' filename >newfilename

You could even use grep for this one as well.
grep '^allow pp$' filename >newfilename

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Why not just use grep?

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Hello, jschiwal!

Thanks for your proposal. Based on the sed commands Sasha and you gave me, I rewrote yours and both

and

seem to be exactly doing what I want: namely to delete the entry called "allow pp" from the file but not entries like "allow ppv" or "allow ppreacher", etc. So I suppose the "$" sign has the function of a "stop", "EOL" or "line must match exactly" mark?

But what's about the (optional?) space character in the two commands I pasted above: "/ d" vs. "/d"? Are the two commands above functionally completely identical or has the space some meaning or function as well?

Cuz' I'm really a total newbie when it comes to awk, sed, grep & co.

And I believe that

would have the same shortcoming as

in that it would delete not only the line "allow pp" but entries like "allow pp2" or "allow ppreacher" as well?

I cannot speak about the AWK code at the end of your above post, since I'd need to play with it to see precisely what it's doing. I can say though, that while awk is fabulous, sometimes (like in this case) there are semantically simpler solutions (such as sed).

The space before the d in "/d" or "/ d" is not significant. The regex goes inside the /slashes/ and the "d" is the command for "delete".

the $ sign signifies the end of the line. a "^" carat sign means the beginning of a line.

With grep, the -v switch means "invert the match", so it greps for everything EXCEPT the regex (it gives you everything EXCEPT the item you use -v on) You are correct: grep -v "allow pp" would also toss out "allow pppreacher" so you would need to fine tune the regex to match for ONLY "allow pp". It isn't a shortcoming, it's just a result of not using the correct regex.

To learn more about all this, you really ought to check `man 7 regex` as well as the excellent online tutorials on regular expressions, sed, awk, and grep

Best regards,
Sasha

if you want to match exactly, use == or !=. since username is column 1, then use $1 instead of $0
similar for your allow file