I have a file of protein sequences with headers (my source file). Based on a list of IDs (which are included in some of the headers), I'd like to print out only the specified sequences, with only the ID as header.

In other words, I'd like to search source.txt for the terms in IDs.txt, and print the ID as well as the sequence. Ideally the process would continue even if an ID is not found in the source file. All headers in source.txt are of similar format.

source.txt
IDs.txt
desired output:
I am able to pull out the sequences based on the ID one-by-one, but this is slow and doesn't give me the header.

awk '/comp51893_c0_seq1/ { getline; print $0 }' source.txt

awk '{lines[NR] = $0} /comp47911_c0_seq1/ {print lines [NR]; print lines [NR+1]}' source.txt

awk ' FILENAME=="ID.txt" {arr[$0]++}
        FILENAME=="source.txt"
        {for(i in arr) {if (i ~ $0)
                             {print ">", i; getline; print $0; getline; print $0  }
                          }
        } ' ID.txt source.txt  > newfile

jim, thanks for taking a look.

Using the code you provide, I get the following in terminal:
cat newfile returns:
Just to verify, here are the input files:

Jim is correct (cool way of performing the task) but you need to switch the comparison operator in the if statement

awk 'FILENAME=="ID.txt" {arr[$0]++}
FILENAME=="source.txt" { for(i in arr) {if ($0 ~ i) {print ">", i; getline; print $0; getline; print $0  }   } }' ID.txt source.txt