Bio/ bioinfo tools: Entrez Direct (EDirect)...

NCBI  databases: publication, sequence, structure, gene, variation, expression, etc. These can be accessed from  a UNIX terminal window (by command-line), using EDirect.

#To install EDirect software (Jonathan Kans), following commands should be pasted into a terminal window. It needs perl and installs the folder in user home,  configuring the PATH environment variable to allow execution of programs in that location.

    cd ~
  perl -MNet::FTP -e \
    '$ftp = new Net::FTP("ftp.ncbi.nlm.nih.gov", Passive => 1); $ftp->login;
     $ftp->binary; $ftp->get("/entrez/entrezdirect/edirect.zip");'
  unzip -u -q edirect.zip
  rm edirect.zip
  export PATH=$PATH:$HOME/edirect
  ./edirect/setup.sh

Usage e.g. esearch, elink, efilter, efetch, xtract, einfo, epost, nquire 

esearch -db pubmed -query "opsin gene conversion"

esearch -db pubmed -query "opsin gene conversion" | elink -related

 esearch -db pubmed -query "opsin gene conversion" | \
 elink -related | \
 elink -target protein