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Tim R avatar Ubuntu

Filtering content of command with grep

kn flag
Tim R

I am trying to filter out a the output of ssh-keyscan. The goal of this is to filter the output so I can use it in my python code to identify hosts connected to my VPN. Normally I would use grep to filter, one of my greps is filtering properly, but the other is not. The first grep is working to get just the ed25519 ID, but not sure why I am getting the SSH-2.0... lines also. The command I ran along with the output is below:

user@host# ssh-keyscan 10.xx.xx.xx | grep ed25519 | grep -v "#"
  # 10.xx.xx.xx:22 SSH-2.0-OpenSSH_8.2p1 Ubuntu-4ubuntu0.3
  # 10.xx.xx.xx:22 SSH-2.0-OpenSSH_8.2p1 Ubuntu-4ubuntu0.3
  # 10.xx.xx.xx:22 SSH-2.0-OpenSSH_8.2p1 Ubuntu-4ubuntu0.3
  # 10.xx.xx.xx:22 SSH-2.0-OpenSSH_8.2p1 Ubuntu-4ubuntu0.3
  # 10.xx.xx.xx:22 SSH-2.0-OpenSSH_8.2p1 Ubuntu-4ubuntu0.3
  10.xx.xx.xx ssh-ed25519 <host key>

I need it to print only the ed25519 line. I have also tried AWK to filter, but still not getting the output I need.

Any idea how to filter only the ed25519 line?

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ssh
hr flag
steeldriver
The `#` lines are likely going to stderr not stdout - try `ssh-keyscan 10.xx.xx.xx 2>&1 | grep ed25519 | grep -v "#"`
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Tim R avatar
kn flag
Tim R
You were absolutely correct on that. If you want to put it in as answer so I can give you credit for it as the solution. Thank you so much. Didn't need the second grep with that either since the 2>&1 corrected it
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hr flag
steeldriver
OK done - I also made some suggestions to eliminate the grep(s)
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Score:3
avatar Ubuntu
hr flag
steeldriver

The lines starting with # are going to stderr (standard error) rather than standard output (stdout). To get grep -v to exclude them, you need to redirect stderr to stdout:

ssh-keyscan 10.xx.xx.xx 2>&1 | grep ed25519 | grep -v "#"

Note that you can eliminate the first grep by adding a type filter to the ssh-keyscan command itself:

ssh-keyscan -t ed25519 10.xx.xx.xx 2>&1 | grep -v "#"

If you prefer, you could redirect all error to null instead of filtering it:

ssh-keyscan -t ed25519 10.xx.xx.xx 2>/dev/null
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Score:0
Marrium Sharif avatar Ubuntu
us flag
Marrium Sharif

Well if you know the output is going to include "ssh-ed25519" you should grep on that.

Or pipe to that.

Or if you know it will be the last line of the output, pipe to the appropriate "tail" command.

Have you looked at the sed command?

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in flag
matigo
Welcome to AskUbuntu. When suggesting a person do A or B to fix a problem, it's usually beneficial to include examples of the solution. Fortunately, these already exist with the accepted answer
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