Get the summary as a tabular text file.
curl -O ftp://ftp.ncbi.nlm.nih.gov/genomes/refseq/bacteria/assembly_summary.txt

# Filter for complete genomes.
awk -F "\t" '$12=="Complete Genome" && $11=="latest"{print $20}' assembly_summary.txt > ftpdirpaths

# Identify the FASTA files (.fna.) other files may also be downloaded here.
awk 'BEGIN{FS=OFS="/";filesuffix="genomic.fna.gz"}{ftpdir=$0;asm=$10;file=asm"_"filesuffix;print ftpdir,file}' ftpdirpaths > ftpfilepaths

# Download everything in parallel
mkdir -p all
cat ftpfilepaths | parallel -j 20 --verbose --progress "cd all && curl -O {}"