  Sum-rank-metric codes have wide applications in universal error correction,
multishot network coding, space-time coding and the construction of partial-MDS
codes for repair in distributed storage. Fundamental properties of
sum-rank-metric codes have been studied and some explicit or probabilistic
constructions of good sum-rank-metric codes have been proposed. In this paper
we give three simple constructions of explicit linear sum-rank-metric codes. In
finite length regime, numerous larger linear sum-rank-metric codes with the
same minimum sum-rank distances as the previous constructed codes can be
derived from our constructions. For example several better linear
sum-rank-metric codes over ${\bf F}_q$ with small block sizes and the matrix
size $2 \times 2$ are constructed for $q=2, 3, 4$ by applying our construction
to the presently known best linear codes. Asymptotically our constructed
sum-rank-metric codes are close to the Gilbert-Varshamov-like bound on
sum-rank-metric codes for some parameters. Finally we construct a linear MSRD
code over an arbitrary finite field ${\bf F}_q$ with various square matrix
sizes $n_1, n_2, \ldots, n_t$ satisfying $n_i \geq n_{i+1}^2+\cdots+n_t^2$ ,
$i=1, 2, \ldots, t-1$, for any given minimum sum-rank distance. There is no
restriction on the block lengths $t$ and parameters $N=n_1+\cdots+n_t$ of these
linear MSRD codes from the sizes of the fields ${\bf F}_q$. \end{abstract}
