For instance, Symfony's configuring-environment-variables-in-env-files documents provides examples:
DATABASE_URL="mysql://db_user:[email protected]:3306/db_name"
DB_USER=root
Why are quotes (aka parentheses) used for the first example and not the second?
If the applicable answer is dependent upon the application parsing the .env file, please base it on Symfony.
TL;DR solution: Use the quotation marks when the string contains spaces or certain special character and certain syntaxes. These include:
space and other whitespace,
backslash (escapes a space and newline – \ gives space even in unquoted string),
quotation marks (but you can combine multiple quotation mark\ 'styles like'"this"),
pound sign (#) that marks start of comment (if it is not in quotes string or $(…)),
dollar sign (that is used to expand a variable – see below),
parentheses (( and )) – depending on context,
shell redirection chars (>, <, 2>, | etc.),
asterisk (*) and question mark (?), since it is used in globs,
square brackets (because they list characters),
comma-separated text in {…} (because it provides multiple variants of text – {foo,bar}baz expands to foobaz barbaz),
maybe others,
and of course, the newline.
.env files are, according to the page linked by you, regular bash scripts. That means that:
One string can't contain multiple words (space-separated parts), unless it is enclosed in quotes.¹
FOO_VAR='multiple words' # This works.
ANOTHER_VAR="foo bar" # This works, too.
BAR_VAR=this does not work # Executes “does” with args
# “not work” and variable
# BAR_VAR=“this”.
Variable expansion is performed if the text is enclosed in double quotes or it is not enclosed in any quotes.
my_var=42
VARIABLES="foo ${my_var}" # Gives “foo 42”.
Shell commands can be executed to produce the strings.²
CURRENT_DATE="$(date)" # Executes “date” and uses its
# stdout as the value.
DO_NOT_DO_THIS=$(date) # First expands the command, and we
# then get multiple unquoted words.
AVOID_THIS="`date`" # Non-standard syntax, accepted by bash.
¹ In the other case, it will run the second and all later “words” as a shell command with the given variable. Refer to bash(1) manpage.
² According to the docs, it is not supported on Microsoft Windows. It does not say anything about variable expansion syntax on Windows.