Why can't I run .tif file with GDAL?

I am trying to extract data from a geotiff file in the Ubuntu terminal. My professor used the command “gdal NDVI.tif” and was able to write the data to a text file. When I use that command, I get the error “gdal: command not found.”

I type in “gdalinfo –version” and it gives me the version number. I type in “whereis gdal” and it shows me what directories it’s in. I used the command “sudo apt-get install python-gdal” to install and already have python 2.7.

Does anyone have any idea why I am getting this error? Google searches have been fruitless.

Best,
Sean