APL, 4 characters

/⍨⍳⎕

How it works:

⎕ reads user input. As for output, APL by default prints the result from every line.

⍳n is the integers from 1 to n. Example: ⍳3 ←→ 1 2 3

/ means replicate. Each element from the right argument is repeated as many times as specified by its corresponding element from the left argument. Example: 2 0 3/'ABC' ←→ 'AACCC'

⍨ is the switch operator. When it occurs to the right of a function invoked with a single argument, the switch operator provides it as both left and right argument, so f⍨ A ←→ A f A. (It can also swap arguments: A f⍨ B ←→ B f⍨ A, hence "switch", but that's irrelevant to this solution.)

Bonus:

6-∊⌽⍳¨⍳⎕ (8 characters, thanks @phil-h)

⍳5 (iota five) is 1 2 3 4 5.

⍳¨ ⍳5 (iota each iota five) is (,1)(1 2)(1 2 3)(1 2 3 4)(1 2 3 4 5), a vector of vectors. Each (¨) is an operator, it takes a function on the left and applies it to each item from the array on the right.

⌽ reverses the array, so we get (1 2 3 4 5)(1 2 3 4)(1 2 3)(1 2)(,1).

∊ is enlist (a.k.a. flatten). Recursively traverses the argument and returns the simple scalars from it as a vector.

APL, 4 characters

/⍨⍳⎕

How it works:

⎕ reads user input. As for output, APL by default prints the result from every line.

⍳n is the integers from 1 to n. Example: ⍳3 ←→ 1 2 3

/ means replicate. Each element from the right argument is repeated as many times as specified by its corresponding element from the left argument. Example: 2 0 3/'ABC' ←→ 'AACCC'

⍨ is the commute operator. When it occurs to the right of a function, it modifies its behaviour, so it either swaps the arguments (A f⍨ B ←→ B f A, hence "commute") or provides the same argument on both sides (f⍨ A ←→ A f A, a "selfie"). The latter form is used in this solution.

Bonus:

6-∊⌽⍳¨⍳⎕ (8 characters, thanks @phil-h)

⍳5 (iota five) is 1 2 3 4 5.

⍳¨ ⍳5 (iota each iota five) is (,1)(1 2)(1 2 3)(1 2 3 4)(1 2 3 4 5), a vector of vectors. Each (¨) is an operator, it takes a function on the left and applies it to each item from the array on the right.

⌽ reverses the array, so we get (1 2 3 4 5)(1 2 3 4)(1 2 3)(1 2)(,1).

∊ is enlist (a.k.a. flatten). Recursively traverses the argument and returns the simple scalars from it as a vector.

APL, 4 characters

/⍨⍳⎕

How it works:

⎕ reads user input. As for output, APL by default prints the result from every line.

⍳n is the integers from 1 to n. Example: ⍳3 ←→ 1 2 3

/ means replicate. Each element from the right argument is repeated as many times as specified by its corresponding element from the left argument. Example: 2 0 3/'ABC' ←→ 'AACCC'

⍨ is the switch operator. When it occurs to the right of a function invoked with a single argument, the switch operator provides it as both left and right argument, so f⍨ A ←→ A f A. (It can also swap arguments: A f⍨ B ←→ B f⍨ A, hence "switch", but that's irrelevant to this solution.)

So, /⍨ is a (derived) function and ⍳ is a function. A "train" of two functions in isolation forms a so-called atop. An atop with a single argument is equivalent to composition: (f g)A ←→ f g A, so g is applied to A, then f is applied to the result.

Bonus:

6-∊⌽⍳¨⍳⎕ (8 characters, thanks @phil-h)

⍳5 (iota five) is 1 2 3 4 5.

⍳¨ ⍳5 (iota each iota five) is (,1)(1 2)(1 2 3)(1 2 3 4)(1 2 3 4 5), a vector of vectors. Each (¨) is an operator, it takes a function on the left and applies it to each item from the array on the right.

⌽ reverses the array, so we get (1 2 3 4 5)(1 2 3 4)(1 2 3)(1 2)(,1).

∊ is enlist (a.k.a. flatten). Recursively traverses the argument and returns the simple scalars from it as a vector.

APL, 4 characters

/⍨⍳⎕

How it works:

⎕ reads user input. As for output, APL by default prints the result from every line.

⍳n is the integers from 1 to n. Example: ⍳3 ←→ 1 2 3

/ means replicate. Each element from the right argument is repeated as many times as specified by its corresponding element from the left argument. Example: 2 0 3/'ABC' ←→ 'AACCC'

⍨ is the switch operator. When it occurs to the right of a function invoked with a single argument, the switch operator provides it as both left and right argument, so f⍨ A ←→ A f A. (It can also swap arguments: A f⍨ B ←→ B f⍨ A, hence "switch", but that's irrelevant to this solution.)

Bonus:

6-∊⌽⍳¨⍳⎕ (8 characters, thanks @phil-h)

⍳5 (iota five) is 1 2 3 4 5.

⍳¨ ⍳5 (iota each iota five) is (,1)(1 2)(1 2 3)(1 2 3 4)(1 2 3 4 5), a vector of vectors. Each (¨) is an operator, it takes a function on the left and applies it to each item from the array on the right.

⌽ reverses the array, so we get (1 2 3 4 5)(1 2 3 4)(1 2 3)(1 2)(,1).

∊ is enlist (a.k.a. flatten). Recursively traverses the argument and returns the simple scalars from it as a vector.

APL, 4 characters

/⍨⍳⎕

How it works:

⎕ reads user input. As for output, APL by default prints the result from every line.

⍳n is the integers from 1 to n. Example: ⍳3 ←→ 1 2 3

/ means replicate. Each element from the right argument is repeated as many times as specified by its corresponding element from the left argument. Example: 2 0 3/'ABC' ←→ 'AACCC'

⍨ is the switch operator. When it occurs to the right of a function invoked with a single argument, the switch operator provides it as both left and right argument, so f⍨ A ←→ A f A. (It can also swap arguments: A f⍨ B ←→ B f⍨ A, hence "switch", but that's irrelevant to this solution.)

So, /⍨ is a (derived) function and ⍳ is a function. A "train" of two functions in isolation forms a so-called atop. An atop with a single argument is equivalent to composition: (f g)A ←→ f g A, so g is applied to A, then f is applied to the result.

Bonus: the shortest I could come up with is (∊⌽-1-⍳¨)⍳⎕, 11 characters.

APL, 4 characters

/⍨⍳⎕

How it works:

⎕ reads user input. As for output, APL by default prints the result from every line.

⍳n is the integers from 1 to n. Example: ⍳3 ←→ 1 2 3

/ means replicate. Each element from the right argument is repeated as many times as specified by its corresponding element from the left argument. Example: 2 0 3/'ABC' ←→ 'AACCC'

⍨ is the switch operator. When it occurs to the right of a function invoked with a single argument, the switch operator provides it as both left and right argument, so f⍨ A ←→ A f A. (It can also swap arguments: A f⍨ B ←→ B f⍨ A, hence "switch", but that's irrelevant to this solution.)

So, /⍨ is a (derived) function and ⍳ is a function. A "train" of two functions in isolation forms a so-called atop. An atop with a single argument is equivalent to composition: (f g)A ←→ f g A, so g is applied to A, then f is applied to the result.

Bonus:

6-∊⌽⍳¨⍳⎕ (8 characters, thanks @phil-h)

⍳5 (iota five) is 1 2 3 4 5.

⍳¨ ⍳5 (iota each iota five) is (,1)(1 2)(1 2 3)(1 2 3 4)(1 2 3 4 5), a vector of vectors. Each (¨) is an operator, it takes a function on the left and applies it to each item from the array on the right.

⌽ reverses the array, so we get (1 2 3 4 5)(1 2 3 4)(1 2 3)(1 2)(,1).

∊ is enlist (a.k.a. flatten). Recursively traverses the argument and returns the simple scalars from it as a vector.