  This paper presents a search for hypothetical massive, charged, long-lived
particles with the ATLAS detector at the LHC using an integrated luminosity of
139 fb$^{-1}$ of proton-proton collisions at $\sqrt{s}=13$ TeV. These particles
are expected to move significantly slower than the speed of light and should be
identifiable by their high transverse momenta and anomalously large specific
ionisation losses, ${\mathrm{d}}E/\mathrm{d}x$. Trajectories reconstructed
solely by the inner tracking system and a ${\mathrm{d}}E/\mathrm{d}x$
measurement in the pixel detector layers provide sensitivity to particles with
lifetimes down to ${\cal O}(1)$ $\text{ns}$ with a mass, measured using the
Bethe--Bloch relation, ranging from 100 GeV to 3 TeV. Interpretations for
pair-production of $R$-hadrons, charginos and staus in scenarios of
supersymmetry compatible with these particles being long-lived are presented,
with mass limits extending considerably beyond those from previous searches in
broad ranges of lifetime.
