Display a matching line and the closest preceding line matching another pattern

Question

I have this requirement where show mac address table of a switch will list 1000 entries .

file1:

switch_1#show mac address table
AA:AA:AA:AA:AA:AA port 47
BB:BB:BB:BB:BB:BB port 48
.
.
.
DD:DD:DD:DD:DD:DD port 50

file2:

switch_2#show mac address table
AA:AA:AA:AA:AA:AA port 47
BB:BB:BB:BB:BB:BB port 48
.
.
.
DD:DD:DD:DD:DD:DD port 45

I need to find a way to display the line containing DD:DD:DD:DD:DD:DD and once this occurs, it should search few lines above and print the hostname containing line as well

So the output should look like:

switch_1#show mac address table
DD:DD:DD:DD:DD:DD port 50
switch_2#show mac address table
DD:DD:DD:DD:DD:DD port 45

file1, file2are stored in the same directory (these are just examples, there is like 100+ files/switches and MAC needs to be filtered along with the switch names)

Answer 1

awk solution

If we get a line with a hash, store it in variable a.

If we get a line with a matching MAC address, print variable a, along with the current line.

$ awk '/#/{a=$0}/^DD:DD:DD:DD:DD:DD/{print a"\n"$0}' file1 file2
switch_1#show mac address table
DD:DD:DD:DD:DD:DD port 50
switch_2#show mac address table
DD:DD:DD:DD:DD:DD port 45
$

sed solution

If we get a line with a hash, place it in the hold space (h).

If we get a line with a matching MAC address, append to the pattern space (H), copy hold space to pattern space (g), print pattern space (p)

$ sed -n '/#/h;/^DD:DD:DD:DD:DD:DD/{H;g;p}' file1 file2
switch_1#show mac address table
DD:DD:DD:DD:DD:DD port 50
switch_2#show mac address table
DD:DD:DD:DD:DD:DD port 45
$