“ $ line”和“ ^ $ line”有什么区别

Question

There is a while read loop:

while read line; do
    grep "^$line" file1
done < target

I should use "^$line" to get the right answer. I want to know what is the difference between "$line" and "^$line" ?

Answer 1

The while loop is not relevant here, it's the grep command that make the ^ significant. The ^ character is a identifier in regular expressions. It matches the start of the line, Therefore grep "$line" file matches any lines in the file where $line is substring but grep "^$line" only matches those that start with $line .

Answer 2

@sudo_O has the right answer for your question. However, you can omit the loop altogether:

grep -f target file1

It's way more efficient with just a single call to grep. If you need the line anchor:

grep -f <(sed 's/^/^/' target) file1