code: function(cb_) {
      common.log.out('Hash generation: ' + base_name + '.tar.gz.sha1sum');
      var sha1sum = require('child_process').spawn('sha1sum', 
        [base_name + '.tar.gz'], {
        cwd: out_path
      });
      var out = fs.createWriteStream(path.join(out_path, 
                                               base_name + '.tar.gz.sha1sum'));
      sha1sum.stdout.on('data', function(data) {
        out.write(data);
      });
      sha1sum.stderr.on('data', function(data) {
        console.log('stderr: ' + data);
      });
      sha1sum.on('close', function(code) {
        out.end();
        if(code !== 0) {
          return cb_(common.err('`sha1sum` failed with code: ' + code,
                                'auto_updater:failed_sha1sum'));
  
        }
        return cb_();
      });
    }
docs: /* Generate sha1sum.