How to add newlines into variables in bash script

In bash you can use the syntax

str=$'Hello World\n===========\n'

Single quotes preceded by a $ is a new syntax that allows to insert escape sequences in strings.

Also printf builtin allows to save the resulting output to a variable

printf -v str 'Hello World\n===========\n'

Both solutions do not require a subshell.

If in the following you need to print the string, you should use double quotes, like in the following example:

echo "$str"

because when you print the string without quotes, newline are converted to spaces.

You can put literal newlines within single quotes (in any Bourne/POSIX-style shell).

str='Hello World
===========
'

For a multiline string, here documents are often convenient. The string is fed as input to a command.

mycommand <<'EOF'
Hello World
===========
EOF

If you want to store the string in a variable, use the cat command in a command substitution. The newline character(s) at the end of the string will be stripped by the command substitution. If you want to retain the final newlines, put a stopper at the end and strip it away afterward. In POSIX-compliant shells, you can write str=$(cat <<'EOF'); str=${str%a} followed by the heredoc proper, but bash requires the heredoc to appear before the closing parenthesis.

str=$(cat <<'EOF'
Hello World
===========
a
EOF
); str=${str%a}

In ksh, bash and zsh, you can use the $'…' quoted form to expand backslash escapes inside the quotes.

str=$'Hello World\n===========\n'

If you need newlines in your script many times you could declare a global variable holding a newline. That way you can use it in double-quoted strings (variable expansions).

NL=$'\n'
str="Hello World${NL} and here is a variable $PATH ===========${NL}"