Creating an output file with multi line script using echo / linux

You need to escape the backticks (`):

#!/bin/bashecho "yellow=\`tput setaf 3\`bel=\`tput bel\`red=\`tput setaf 1\`green=\`tput setaf 2\`reset=\`tput sgr0\`" >> output.txt

As a bonus:

I prefer using this method for multiline:

#!/bin/bashcat << 'EOF' >> output.txtyellow=$(tput setaf 3)bel=$(tput bel)red=$(tput setaf 1)green=$(tput setaf 2)reset=$(tput sgr0)EOF

Use single quote to prevent expansions:

echo 'yellow=`tput setaf 3`bel=`tput bel`red=`tput setaf 1`green=`tput setaf 2`reset=`tput sgr0`' >> output.txt

For more details see Difference between double and single quote.

If your text includes single quote then the above may not work. In that case using a here doc would be safer. For example, the above will break if you insert a line: var='something'.

Using a here doc it will be like this:

cat >> output.txt <<'EOF'yellow=`tput setaf 3`bel=`tput bel`red=`tput setaf 1`green=`tput setaf 2`reset=`tput sgr0`var='something'EOF

Just a late addition:

The echo 'string' >> output command is simple and great. But it may will give you the '...: Permission denied' error combined with sudo.

I recently had a lil issue with sudo echo 'string \n other string \n' > /path/to/file

What worked for me the best:
printf "Line1\nLine2\nLine3" | sudo tee --append /path/to/file

It's an extra that you actually have the string printed to the stdout too, so you will see what was written to the file.