I. Measurements and temporal evolutionSantiago Gonzalez-Gaitan 1 , Claudia P. Gutierrez 2, 3 , Joseph P. Anderson 4, 5 , Antonia Morales-Garoffolo 6, Lluis Galbany 3, 2 , Sabyasashi Goswami 1, 6, Ana M. Mourao1 , Seppo Mattila 7, 8 , and Mark Sullivan 91- CENTRA, Instituto Superior T´ecnico, Universidade de Lisboa, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal // e-mail: gongsale@gmail.com2- Institut d’Estudis Espacials de Catalunya (IEEC), Gran Capit`a, 2-4, Edifici Nexus, Desp. 201, E-08034 Barcelona, Spain // e-mail: cgutierrez@ice.csic.es3- Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans, s/n, E-08193 Barcelona, Spain4- European Southern Observatory, Alonso de C´ordova 3107, Casilla 19, Santiago, Chile5- Millennium Institute of Astrophysics MAS, Nuncio Monsenor Sotero Sanz 100, Off. 104, Providencia, Santiago, Chile6- Department of Applied Physics, School of Engineering, University of C´adiz, Campus of Puerto Real, E-11519 Cadiz, Spain7- Tuorla Observatory, Department of Physics and Astronomy, University of Turku, FI-20014 Turku, Finland8- School of Sciences, European University Cyprus, Diogenes Street, Engomi, 1516 Nicosia, Cyprus9- School of Physics and Astronomy, University of Southampton, Southampton, SO17 1BJ, UKABSTRACTNarrow absorption features in nearby supernova (SN) spectra are a powerful diagnostic of the slow-moving material in the line of sight: they are extensively used to infer dust extinction from the host galaxies, and they can also serve in the detection of circumstellar material originating from the SN progenitor and present in the vicinity of the explosion.Despite their wide use, very few studies have examined the biases of the methods to characterize narrow lines, and not many statistical analyses exist. This is the first paper of a series in which we present a statistical analysis of narrow lines of SN spectra of various resolutions. We develop a robust automated methodology to measure the equivalent width (EW) and velocity of narrow absorption lines from intervening material in the line of sight of SNe, including Na iD, Ca ii H&K, Ki and diffuse interstellar bands (DIBs). We carefully study systematic biases in heterogeneous spectra from the literature by simulating different signal-to-noise, spectral resolution, slit size and orientation and present the real capabilities and limitations of using low- and mid-resolution spectra to study these lines. In particular, we find that the measurement of the equivalent width of the narrow lines in low-resolution spectra is highly affected by the evolving broad P-Cygni profiles of the SN ejecta, both for core-collapse and type Ia SNe, inducing a conspicuous apparentevolution. Such pervading non-physical evolution of narrow lines might lead to wrong conclusions on the line-of-sight material, e.g. concerning circumstellar material ejected from the SN progenitors. We present thus an easy way to detect and exclude those cases to obtain more robust and reliable measurements. Finally, after considering all possible effects, we analyse the temporal evolution of the narrow features in a large sample of nearby SNe to detect any possible variation in their EWs over time. We find no time evolution of the narrow line features in our large sample for all SN types. Key words. supernovae: general, ISM: lines and bands, ISM: dust and extinction