Trouble with latex makefile

Question

I've been having trouble getting my makefile to produce a pdf. Here's the code

file=assignment

#Same as pdf
all: pdf

#makes the .dvi file
dvi: $(file).tex $(file).bbl
        @latex $(file)

#makes the .ps and .bib file
ps: dvi
        @dvips -Ppdf -GO $(file).dvi
        @bibtex $(file)
#makes the .pdf
pdf: ps
        @ps2pdf $(file).ps $(file).pdf

I've been scouring the internet trying to find other example latex makefiles, but I can't find the important difference that causes my makefile to not output a pdf. It outputs all the other files like the .ps and .dvi without problem.