You can make straight lines by stroking the edges of a selection, but this requires the line to be connected to others forming a figure. There is no tool for just painting a simple straight line. I find it strange that this option is not included, as it, in my opinion, is one of the most fundamental functions in Photoshop. I am talking about what happens when you hold shift while using the brush in Photoshop for Mac and PC. The pen tool and the simple line tool do the job as well.
It would be preferable if the line tool would give an option to continue with a new line from the end of the first one - like the polygon selection tool, only with stroked lines and not with the necessity of the lines to connect in the end. It would also be a convenient addition to give the option for the user to add eg. an arrowhead to the end of the line or just round the corners of it. And so on.
This simple tool would be the last bit to fulfil my personal needs of a portable Photoshop, and I am sure that I am not the only one it would satisfy.