How do I output multiple lines to a file within a while loop?

Question

I'm writing a Bash script and I'm trying to print out multiple lines of output and print them to a file. Here's what I've tried so far, this is just an example, but very similar to what I'm trying to accomplish.

I'm trying to print out "Hello World" to hello.txt every 2 seconds and be able to see hello.txt updated every 2 seconds. I would do this by running tail -f hello.txt. Here's what I've tried so far.

echo "Hello" >> hello.txt
echo "World" >> hello.txt

But I would need to run " >> hello.txt " after each line in my loop. So I learned to run the following to output a block of text to a file

cat >> hello.txt << EOL
echo "Hello"
echo "World"
EOL

Then I applied the while loop.

while true
do
    echo >> hello.txt << EOL
    echo "Hello"
    echo "World"
    EOL
    sleep 2
done

But then I got the following error.

./test.sh: line 10: warning: here-document at line 7 delimited by end-of-file (wanted `EOL')
./test.sh: line 11: syntax error: unexpected end of file

Then I tried putting the file-output outside the while loop

echo >> hello.txt << EOL
while true
do
    echo "Hello"
    echo "World"
    sleep 2
done
EOL

But this printed out the actual code and not what it was intended to do. How can I print out multiple lines in a loop to a text file without having to write " >> hello.txt " after every line?

You can't indent the here-document or its end delimiter (EOF in your script), unless you use "-" (as in <<-EOF), in which case you can indent with tabs (but not spaces). Also, you don't use shell commands inside (like echo) inside a here-document, just the text you want to pass. — Gordon Davisson
– Gordon Davisson, Commented Oct 14, 2019 at 4:12

Diego Torres Milano · Accepted Answer · 2019-10-14 04:10:17Z

1

You can redirect the output of a subshell

while true
do
    (
    echo "Hello"
    echo "World"
    ) >> hello.txt
    sleep 2
done

answered Oct 14, 2019 at 4:10

Diego Torres Milano

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Comments

Yuji · Accepted Answer · 2019-10-14 04:14:12Z

0

You can use cat instead of echo.

while true
do
    cat << EOL >> hello.txt
Hello
World
EOL
    sleep 2                                                                     
done

answered Oct 14, 2019 at 4:14

Yuji

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