Hanna-
 Unix files have permissions associated with them, so a user may not have permission to open a particular file. 
 FSL is a popular and free tool for brain imaging. One of the many tools it includes is 'fslhd' that will read a brain imaging header and describe the contents. Check Google to find out how to install and use fsl.
 I strongly recommend the fsl training course - they not only describe their own tools, but provide an introduction in how to use Unix computers. It has a lot of useful information for beginners as well as experts (I went this year, even though I have been in the field for decades):
   https://fsl.fmrib.ox.ac.uk/fslcourse/
 I also suggest you look at Martin Lindquist's book and course
  http://www.biostat.jhsph.edu/~mlindqui/main.html
 Finally, all the material and slides for my own course is here
   https://www.mccauslandcenter.sc.edu/crnl/psyc589888