grep list of names and information from bigger file

Question

I have two files: one with list of names (500 entries) and other having some more information for each entry in A.txt and extra entries too.

File A.txt (each line is starting with > (fasta format) and has spaces, digits and special characters too, but in single line)

>xyz, B=123 
>abc, B=231
>mnp, B=567

File B.txt (has some particular information for each entry in A.txt in next line and has extra entries than file A)

>xyz, B=123
01010101010100101
>abc, B=231
1011110000011100000000
>mnp, B=567
10101
>opq, B=678
101010101010101001
>lmn, B=123
101010100000011

Result

>xyz, B=123
01010101010100101
>abc, B=231
1011110000011100000000
>mnp, B=567
10101

I want to grep the entries in list A.txt from list B.txt

thanks

Answer 1

Assuming that you want to print just two lines for each match, with GNU or FreeBSD grep, you can pass A.txt as a list of patterns to grep and tell it to print the line after the match. This produces extra lines with just -- between each match, you can easily remove them with an extra call to grep.

grep -A1 -Ff A.txt B.txt | grep -vx -- '--'

If the number of lines per section isn't fixed, you can use awk. First read the file containing the text to look for, then for each line in B.txt that starts a new section, either start or stop printing depending on whether the section head is in A.txt or not.

awk -v patterns_file=A.txt '
  BEGIN {
    while (getline <patterns_file) patterns_array[$0] = 1;
    close(patterns_file);
  }
  /^/ { matching = $0 in patterns_array }
  matching { print }
' B.txt