Get a sequence file.

$ efetch -db=nuccore -format=fasta -id=AF086833 > AF086833.fa
#error report:
Can't locate HTML/Entities.pm in @INC (you may need to install the HTML::Entities module).....

Solution:

perl -MCPAN -e shell
install HTML::Parser

$ efetch -db=nuccore -format=fasta -id=AF086833 > AF086833.fa

Error report:

Can't locate URI.pm in @INC (you may need to install the URI module)

Solution:

perl -MCPAN -e shell
install LWP