Real functions/Linear/Exercise

Which of the following functions ${}f\colon \mathbb {R} \rightarrow \mathbb {R}$ are linear?

1. The real exponential function.
2. The zero function.
3. The constant function with value ${}7$.
4. The squaring function ${}x\mapsto x^{2}$.
5. The function which halves every real number.
6. The function which subtracts ${}1$ from every real number.

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