How to add newlines into variables in bash script

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'

Are you using "echo"? Try "echo -e".

echo -e "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}"

From all discussion, here is the simplest way for me:

bash$ str="Hello World
==========="
bash$ echo "$str"
Hello World
===========

The echo command must use double quotes.

To complement the great existing answers:

If you're using bash and you prefer using actual newlines for readability, read is another option for capturing a here-doc in a variable.

With trailing (and leading) \n instances REMOVED ($IFS set to \n):

IFS=$'\n' read -r -d '' str <<'EOF'
Hello World
===========
EOF

# Test
printf %q "$str" # -> $'Hello World\n==========='

With trailing (and leading) \n instances PRESERVED ($IFS set to empty):

IFS= read -r -d '' str <<'EOF'
Hello World
===========
EOF

# Test
printf %q "$str" # -> $'Hello World\n===========\n'

• -r ensures that read doesn't interpret the input (by default, it would treat backslashes special)
• -d '' sets the "record" delimiter to an empty string, causing read to read the entire input at once (instead of just a single line).
• By setting the set of internal field separator characters, $IFS:
  • to an empty string,the entire here-doc is read as is.
  • to $'\n' (an newline expressed as an ANSI-C quoted string), leading and trailing \n instances are automatically trimmed.
• Note that prepending IFS=... directly to the read command means that the assignment is in effect during that command only, so there is no need to restore the previous value.

you need to do it this way:

STR=$(echo -ne "Hello World\n===========\n")

Update:

As Fred pointed it out, this way you will loose trailing "\n". To assign variable with backslash sequences expanded, do:

STR=$'Hello World\n===========\n\n'

let's test it:

echo "[[$STR]]"

gives us now:

[[Hello World
===========

]]

Note, that $'' is different than $"". The second does translation according to current locale. For deital see QUOTING section in man bash.