使用grep提取文件之间的行

Question

I have two files, file1.txt and file2. txt

file1.txt contains multiple strings such as:

Little.1
Little.2
Little.3

and the file2.txt contains strings such as:

XYQ Little.1 AGTAGT
ABC Little.12 ATCGCT
GDT Little.3 CGTAGT

How do I use grep so that when I use file1 to grep file2 I only return:

XYG Little.1 AGTAGT
GDT Little.3 CGTAGT

Because they match exactly?

Answer 1

grep -w and -f are done for this:

$ grep -wf file1 file2
XYQ Little.1 AGTAGT
GDT Little.3 CGTAGT

From man grep :

-f FILE, --file=FILE

Obtain patterns from FILE, one per line. The empty file contains zero patterns, and therefore matches nothing. (-f is specified by POSIX.)

-w, --word-regexp

Select only those lines containing matches that form whole words. The test is that the matching substring must either be at the beginning of the line, or preceded by a non-word constituent character. Similarly, it must be either at the end of the line or followed by a non-word constituent character. Word-constituent characters are letters, digits, and the underscore.

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Note that without -w it will match other cases:

$ grep -f file1 file2
XYQ Little.1 AGTAGT
ABC Little.12 ATCGCT
GDT Little.3 CGTAGT