Charlie Harvey

Bash Tip: Remove a line from middle of file

This is a thing that I’ve wanted to do for ages, and it came up for me during this week'd episode of Hak5, which I am getting to be a big fan of by the way. The material under discussion was what to do when one of your known hosts changes. Darren was saying he was a bit slack and just did an rm of his whole known_hosts file.

As you may know, when a host key has changed, you'll get a warning like this telling you so in no uncertain terms $ ssh -o stricthostkeychecking=ask ssh.example.com @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ @ WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! @ @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY! Someone could be eavesdropping on you right now (man-in-the-middle attack)! It is also possible that the RSA host key has just been changed. The fingerprint for the RSA key sent by the remote host is 4a:ad:fe:96:de:02:95:f1:76:34:23:12:e4:78:12:bb. Please contact your system administrator. Add correct host key in /home/xahria/.ssh/known_hosts to get rid of this message. Offending key in /home/xahria/.ssh/known_hosts:8 RSA host key for localhost has changed and you have requested strict checking. Host key verification failed. As you can see the offending key is on line 8 of our known hosts file. So I’d like to be able to remove that line.

Now there is probably a tool for this or something, but I thought it was a nice quick one liner to do in bash, a-la Commandline Fu. My first shot used nested backticks and piping the output of wc -l to bc. Fugly. I think this version is a little nicer.

My test data is a file t with contents1 2 3 4 5 Exciting stuff. My goal is to rewrite the file so that it loses line 3.

Here’s how I do it. (x=3; head -n$(($x-1)) t; tail -n$((`wc -l t | cut -d ' ' -f 1`-$x)) t) > t2 && cat t2 > t And lets just check it works: $ cat t 1 2 4 5

There is quite a lot going on so let's step through it.

In the real world, you would use the "Offending key" line number from the SSH warning as your x and ~/.ssh/known_hosts as your t. I suppose you could make a proper script of this if you wanted. Being a perl geek, I would perl -nle 'print if(++$i!=3)' known_hostsBut playing Bash is fun sometimes, eh?


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