extracting lines if the first field matches another list saved in a different file -- shell command - macos

Question

I have two files. One contains a list of items, e.g.,
Allie
Bob
John
Laurie
Another file (file2) contains a different list of items in a different order, but some items might overlap with the items in file 1, e.g,
Laurie 45 56 6 75
Moxipen 10 45 56 56
Allie 45 56 67 23
I want to intersect these two files and extract only those lines from file 2 whose first field matches an item in field 1.
i.e., my output should be
Allie 45 56 67 23
Laurie 45 56 6 75
(preferably in this order, but it's OK if not)
grep -f file1 file2 doesn't do what I want.
I also need something efficient because the second file is HUGE.
I also tried this:
awk -F, 'FNR==NR {a[$1]=$0; next}; $1 in a {print a[$1]}' file2 file1

Answer 1

If order doesn't matter then
awk 'FNR==NR{ arr[$1]; next }$1 in arr' file1 file2
Explanation
FNR==NR{ arr[$1]; next } Here we read first file (file1), arr is array, whose index key being first field $1.
$1 in arr we read second file ( file2), if array arr which was created while reading first file, has index key which is second file's first column ($1 in arr gives true, if index key exists), then print current record/row/line from file2
Test Results:
akshay#db-3325:/tmp$ cat file1
Allie
Bob
John
Laurie
akshay#db-3325:/tmp$ cat file2
Laurie 45 56 6 75
Moxipen 10 45 56 56
Allie 45 56 67 23
akshay#db-3325:/tmp$ awk 'FNR==NR{ arr[$1]; next }$1 in arr' file1 file2
Laurie 45 56 6 75
Allie 45 56 67 23

Answer 2

No need for complex joins, it is a filtering function
$ grep -wFf file1 file2
Laurie 45 56 6 75
Allie 45 56 67 23
has the benefit or keeping the order in file2 as well. -w option is for full word matches to eliminate sub-string matches to create false positives. Of course if your sample input is not representative and your data may contain key like entries in other fields this will not work without qualifying beginning of line.

Answer 3

This is the job that join is built for.
Providing a reproducer testable via copy-and-paste with shell functions (which you could replace with your actual input files):
cat_file1() {
printf '%s\n' Allie Bob John Laurie
}
cat_file2() {
printf '%s\n' 'Laurie 45 56 6 75' \
'Moxipen 10 45 56 56' \
'Allie 45 56 67 23'
}
join <(cat_file1 | sort) <(cat_file2 | sort)
...properly emits:
Allie 45 56 67 23
Laurie 45 56 6 75
Of course, don't cat file1 | sort -- run sort <file1 to provide a real handle for better efficiency, or (better!) store your inputs in sorted form in the first place.