An easy way to remove old SSH host keys from your ~/.ssh/known_hosts file. Useful if you often re-image systems.

forget.sh

# forget.sh: remove matching lines from ~/.ssh/known_hosts
# usage: forget <hostname|IP|substring> [...]

function forget {
  known_hosts=~/.ssh/known_hosts
  for host in $*
  do
    host=$(echo "${host}" | sed -e 's/^ssh:\/\///' -e 's/^.*@//')
    line=$(awk '{ print $1 }' ${known_hosts} | grep -n "${host}" | sed 's/:.*$//g' | xargs)
    while [ -n "${line}" ]
    do
      l=$(echo $line | awk '{ print $1 }')
      echo -n  "==> removing from ${known_hosts}: "
      awk '{ if (NR==n) print $1 }' n=$l ${known_hosts}
      sed -i '.old' "${l}d" ${known_hosts}
      line=$(awk '{ print $1 }' ${known_hosts} | grep -n "${host}" | sed 's/:.*$//g' | xargs)
    done
  done
}

I just include this in my ~/.bashrc.