How can I both extract a specific line in a text file as well as multiple lines containing a specific string?

Question

I have a collection of text files containing more data than I need. Each file's first line contains a comma-separated string that looks like this:

stop_id,stop_code,stop_name,stop_desc,stop_lat,stop_lon,location_type,parent_station,zone_id

Then, below those keys is all the data. I need to extract a subset of that data into a new text file so I can work with the subset (I don't need all the data, it's too much).

I'm using this command to extract the first line:

sed -n '1p' source.txt > destination.txt

I'm also using this command to extract the specific lines I need:

grep "string" source.txt > destination.txt

The challenge is that when I run the two commands in the same script (pretty much as is, separated by a line or &&), the grep output overwrites the sed output. How can I run both in sequence and have the combined output of both?

I noticed a question that seems similar and involves using a more complex grep command to locate the one line, followed by a range of lines. That won't work here because the first line of each of the files I need to extract data from is different.

Ideally, I want to write a function that I can run against each of the files I need to work with but I need to chain these commands and combine their outputs first.

Answer 1

sed can do both jobs (print first line and all lines containing string):

sed -n '1p; /string/p' source.txt > destination.txt

or longer version:

sed -n -e '1p' -e '/string/p' source.txt > destination.txt

Answer 2

Just change the grep output to append,

grep "string" source.txt >> destination.txt

Answer 3

There are many ways of doing this. You can use a single command to get both lines, as @Cyrus's sed solution does. Here are a couple of other tools that can do this:

awk 'NR==1 || /string/' source.txt > destination.txt
perl -ne 'print if /string/ || $. ==1' source.txt > destination.txt

You can of course also run both commands you were running and simply change the second to append to the file as BANJOSA suggested. Alternatively, you could group the two commands in a subshel and redirect the subshell's output to a file:

(sed -n '1p' file; grep string file) source.txt > destination.txt

or

{ sed -n '1p' file; grep string file; } source.txt > destination.txt

So if what you want to do is make a function out of these, it would be as simple as adding this to your shell's initialization file (e.g. ~/.bashrc):

foo(){ sed -n '1p' file; grep string file; } 

And now you can run the function foo to do what you want:

foo source.txt > destination.txt