Shell Programming and Scripting

I have a file with two columns and each pair in the rows denote 2 connected nodes in the network file, edge_list.txt. Given a query file, input.txt, I want to retrieve the nodes that are directly connected (first degree neighbors) to the nodes present in the input.txt. Kindly help.

edge_list.txt

Code:

A  B
B  J
J  H
C  A
G  H
H  A
K  G

input.txt

Code:

A
G
H

Desired output:

Code:

A  B
C  A
H  A
G  H
K  G
J  H

Please note:
edge_list does not have any duplicate entries. A H and H A are same. The output file cannot have any duplicate entries (i.e. only A H or H A should be present)
Thanks.

Output order will be same as that of edge_list.txt
Try :

Code:

$ awk 'FNR==NR{A[$1];next}($1 in A || $2 in A)' input.txt edge_list.txt  
A  B
J  H
C  A
G  H
H  A
K  G

Do you say that edge_list can't have H A and A H? Then there won't be duplicates in the output anyway. Should above be possible though, remove duplicates with

Code:

awk 'FNR==NR{A[$1];next}($1 in A || $2 in A) && !N[$1,$2] {N[$1,$2]++;N[$2,$1]++; print}' file2 file1

Code:

grep -Ff input.txt edge_list.txt

Regards,
Alister