Sed to keep header(s) line(s) + matching lines

Question

I have not found (nor been able to guess) a sed command that would allow me :

• To keep : specific lines according to their index (the header for instance)
• To keep : lines matching a pattern (like grep)
• To delete : the other lines

What is the best way of doing that ?

Edit (example) :

INPUT :

Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State
tcp        0      0 188.165.65.214:53       0.0.0.0:*               LISTEN
tcp        0      0 127.0.0.2:53            0.0.0.0:*               LISTEN
tcp        0      0 127.0.0.1:53            0.0.0.0:*               LISTEN
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN
tcp        0      0 127.0.0.1:25            0.0.0.0:*               LISTEN
tcp        0      0 127.0.0.1:953           0.0.0.0:*               LISTEN
tcp        0      0 0.0.0.0:7777            0.0.0.0:*               LISTEN
tcp        0      0 0.0.0.0:3306            0.0.0.0:*               LISTEN
tcp        0      0 127.0.0.1:587           0.0.0.0:*               LISTEN
tcp6       0      0 :::53                   :::*                    LISTEN
tcp6       0      0 :::22                   :::*                    LISTEN
tcp6       0      0 ::1:953                 :::*                    LISTEN
tcp6       0      0 :::443                  :::*                    LISTEN

OUTPUT : After asking to keep line 2 and pattern 7777 :

Proto Recv-Q Send-Q Local Address           Foreign Address         State
tcp        0      0 0.0.0.0:7777            0.0.0.0:*               LISTEN

Answer 1

christian@fujiu1404:~/tmp$ cat f1
line 1 l
aaa bbb ccc ddd
111 111 111 111 abc
222 222 222 222
333 333 333 333 abc

christian@fujiu1404:~/tmp$ sed -n -e '2p' -e '/abc/p' f1
aaa bbb ccc ddd
111 111 111 111 abc
333 333 333 333 abc

Update

For your input/output

sed -n -e '2p' -e '/:7777/p'

Answer 2

You can do it easily with sed:

$ sed -e '2{
  p
  d
}
/7777/!d' file

or awk:

awk 'FNR==2 || /pattern/' file

• 2 is the line number of header, you can change it to whatever line number you want.
• pattern is the pattern you want to match.