Why don't word splitting and filename expansion apply to the conditional expression within `[[ ... ]]`?

Question

From Bash Reference Manual

Rule from Word Splitting section:

The shell scans the results of parameter expansion, command substitution, and arithmetic expansion that did not occur within double quotes for word splitting.

Rule from Filename Expansion section:

After word splitting, unless the -f option has been set (see Section 4.3.1 [The Set Builtin], page 58), Bash scans each word for the characters ‘*’, ‘?’, and ‘[’. If one of these characters appears, then the word is regarded as a pattern, and replaced with an alphabetically sorted list of filenames matching the pattern

So after parameter expansion, command substitution, and arithmetic expansion, word splitting happens unless on the parts within double quotes.

1. In [[ ... ]], Giles and John1024 both said that word splitting and filename expansion don't apply to the conditional expression within [[ ... ]]. Which rules in the Bash Reference Manual or POSIX 7 Specifications govern that?

   • The conditional expression within [[ ... ]] isn't double quoted, so why doesn't word splitting apply?

   • The -f option isn't set. Why does filename expansion not apply either?

2. Besides [[ ... ]], are there other cases where word splitting, filename expansion, or both don't apply? Are their reasons that one or both of the two don't apply the same as [[..]]?

3. Do word splitting and filename expansion always go hand in hand, in the sense that they either both apply or both don't apply to each case?

Answer 1

In the documentation for the [[ command, you'll see

Word splitting and filename expansion are not performed on the words between the [[ and ]]; tilde expansion, parameter and variable expansion, arithmetic expansion, command substitution, process substitution, and quote removal are performed.

(emphasis mine)

Also the case statement has exemptions

The word undergoes tilde expansion, parameter expansion, command substitution, arithmetic expansion, and quote removal before matching is attempted. Each pattern undergoes tilde expansion, parameter expansion, command substitution, and arithmetic expansion.

Notable by their absence are word splitting and filename expansion.

Additionally, variable assignment (see Shell Parameters)

A variable may be assigned to by a statement of the form

name=[value]

If value is not given, the variable is assigned the null string. All values undergo tilde expansion, parameter and variable expansion, command substitution, arithmetic expansion, and quote removal

So this is safe:

a="hello world"
b=$a

Other places where word splitting is not performed:

• Arithmetic Expansion
• Here-strings

My trick: search for the word "undergo" in the bash manual.

Answer 2

The words within [[ and ]] are an extension, which bash uses (among other things) to provide regular expressions:

An additional binary operator, ‘=~’, is available, with the same precedence as ‘==’ and ‘!=’. When it is used, the string to the right of the operator is considered an extended regular expression and matched accordingly (as in regex3)).

Doing filename expansion on a regular expression would not be helpful, since both use the same * and ? meta characters for different purposes.

Further reading:

• 3.2.4.2 Conditional Constructs (Bash Reference Manual)