Bash中的<<<和<<（）是什么？

Question

I'm studying bash but I'm not able to understand this example code:

while IFS= read -r line2; 
    do
        if [[ "$line1" == "$line2" ]]
        then
            (( i++ ))
        fi
    done <<< "$lines"

in particular the line done <<< "$lines" what do mean <<< ?

I find other example for example this:

while read line
  do
      echo "Word count per line: $line"
  done < <(cat list.txt | wc -w)

in this case because it uses < < and not <<<

Answer 1

<<< specifies a here string .

In your case, the content of $lines is sent to the standard input of the while loop.

---

<(...) is a kind of process substitution .

In your case, the output of cat list.txt | wc -w cat list.txt | wc -w is sent to the standard input of the while loop.

Process substitution is very useful when a program expects a file name as an argument.

Answer 2

Whenever you write a loop, you can feed it with the content of a file with the expression:

while ...; do
   # things
done < file

Then, you can take advantage of this by using a process substitution instead of a file itself. This way, you can feed a while loop with the result of a process without having to pipe. Internally, it kind of creates a temporary file:

while ...; do
   # things
done < <(find -type f -name ".htaccess")

But, why is it important to use this approach instead of cat file | wc -w | while ... cat file | wc -w | while ... cat file | wc -w | while ... ? Because this one will open a sub-shell, so that whatever variables you set within the while loop will be lost once it finishes.

For example, if you say:

$ var=5
$ seq 10 | while read v; do var=$v; done

Then the variable $var is not set to 10 at the end of the loop, but it remains unchanged:

$ echo $var
5

You can read more about it in I set variables in a loop that's in a pipeline. Why do they disappear after the loop terminates? Or, why can't I pipe data to read? .

---

Then, you have:

while ...; do
   # things
done <<< "$variable"

This feeds the while loop with a here string . That is, the while loop keeps reading from the content of $variable until it is done.

For example, the following code:

while IFS=: read -r product price; do
   echo "$product cost $price euros"
done <<< "potatos:23
kiwis:45"

Will return:

potatos cost 23 euros
kiwis cost 45 euros