sorry for the typo, the last command should be the below:

~$ sed -e '/127.0.0.1/d' -e "1i 127.0.0.1 localhost\n127.0.0.1
$HOSTNAME.domainname $HOSTNAME" testfile
127.0.0.1 localhost
127.0.0.1 node1.domainname node1
# comment
::1     localhost ip6-localhost ip6-loopback

2017-06-14 11:48 GMT+08:00 Han Lu <luhanwa@gmail.com>:

> Hi Team,
>
> Please check this below:
>
> ~$ cat /etc/os-release
> NAME="Ubuntu"
> VERSION="14.04.5 LTS, Trusty Tahr"
> ID=ubuntu
> ID_LIKE=debian
> PRETTY_NAME="Ubuntu 14.04.5 LTS"
> VERSION_ID="14.04"
> HOME_URL="http://www.ubuntu.com/"
> SUPPORT_URL="http://help.ubuntu.com/"
> BUG_REPORT_URL="http://bugs.launchpad.net/ubuntu/"
>
> ~$ sed --version
> sed (GNU sed) 4.2.2
> Copyright (C) 2012 Free Software Foundation, Inc.
> License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.
> html>.
> This is free software: you are free to change and redistribute it.
> There is NO WARRANTY, to the extent permitted by law.
>
> Written by Jay Fenlason, Tom Lord, Ken Pizzini,
> and Paolo Bonzini.
> GNU sed home page: <http://www.gnu.org/software/sed/>.
> General help using GNU software: <http://www.gnu.org/gethelp/>.
> E-mail bug reports to: <bug-sed@gnu.org>.
> Be sure to include the word ``sed'' somewhere in the ``Subject:'' field.
>
>
> Problem reproduce process:
>
> ~$ cat testfile
> 127.0.0.1 localhost hostname
> ::1     localhost ip6-localhost ip6-loopback
>
> The below command works as expected.
> ~$ sed -e 's/127.0.0.1/127/' -e "1i 127.0.0.1 localhost\n127.0.0.1
> $HOSTNAME.domainname $HOSTNAME" testfile
> 127.0.0.1 localhost
> 127.0.0.1 node1.domainname node1
> 127 localhost hostname
> ::1     localhost ip6-localhost ip6-loopback
>
> But command below is not working as expeced:
> ~$ sed -e '/127.0.0.1/d' -e "1i 127.0.0.1 localhost\n127.0.0.1
> $HOSTNAME.domainname $HOSTNAME" testfile
> ::1     localhost ip6-localhost ip6-loopback
>
> expected output:
> 127.0.0.1 localhost
> 127.0.0.1 node1.domainname node1
> ::1     localhost ip6-localhost ip6-loopback
>
> If I add a comment at first line. then it's working.
> ~$ cat testfile
> # comment
> 127.0.0.1 localhost hostname
> ::1     localhost ip6-localhost ip6-loopback
>
> ~$ sed -e '/127.0.0.1/d' -e "1i 127.0.0.2 localhost\n127.0.0.2
> $HOSTNAME.domainname $HOSTNAME" testfile
> 127.0.0.2 localhost
> 127.0.0.2 node1.domainname node1
> # comment
> ::1     localhost ip6-localhost ip6-loopback
>
>