Just because I like GNU parallel:
parallel -kj1 '! [[ -f "{}" ]] && echo {} || :' ::: $(jot -w %04d_A.csv - 1 501)
or if preferred parallel -kj1 '! [[ -f "{}" ]] && echo {} || :' ::: {0001..0500}_A.csv for k in $(seq -f %04.f 1 501); do
f="${k}_A.csv"
! [[ -f "$f" ]] && echo $f || :
done
or more succinctly, for k in {0001..0501}_A.csv; do
! [[ -f "$f" ]] && echo $f || :
done
and if you have GNU parallel installed: parallel -kj1 '! [[ -f "{}" ]] && echo {} || :' ::: {0001..0501}_A.csv