diff --git a/README.md b/README.md
index be6695d..846b88e 100644
--- a/README.md
+++ b/README.md
@@ -31,18 +31,18 @@ To show all known classes, semesters, and assignments:
 
     rake list
 
-To load a roster (a newline-delimited list of student identifiers) for a
-specific semester:
+To load a roster (a newline-delimited list of student identifiers) for the
+current semester of a particular course (e.g. "cs123"):
 
-    rake roster cs123 myroster.csv semester=sp16
+    rake roster cs123 myroster.csv
 
-To load a roster for the current semester:
+You can also specify the semester:
 
-    rake roster cs123 myroster.csv
+    rake roster cs123 myroster.csv sp16
 
 ### Grading
 
-To grade a particular assignment (e.g., "mp1" for the "cs123" course):
+To grade a particular assignment (e.g., "mp1" for "cs123"):
 
     rake grade cs123 mp1
 
@@ -88,5 +88,5 @@ repo has changed since the last run. To forcibly regrade:
 
     rake grade cs123 mp1 regrade=yes
 
-You can combine these arguments in any meaningful way. `semester` is available
-for `roster` and `commit`, and and `students` is available for `commit`.
+You can combine these arguments in any meaningful way. `semester` and
+`students` are also available options for `commit`.
diff --git a/lib/kgrader/cli.rb b/lib/kgrader/cli.rb
index b385e0c..9ab35e5 100644
--- a/lib/kgrader/cli.rb
+++ b/lib/kgrader/cli.rb
@@ -8,19 +8,15 @@ module KGrader
       @fs = Filesystem.new dir
     end
 
-    def list
-      @fs.courses.each do |name|
-        puts "course: #{name}"
-        course = Course.new(@fs, name)
-
-        puts "  rosters:"
-        course.rosters.each do |roster|
-          puts "    - #{roster.semester} (#{roster.students.size} students)"
-        end
-
-        puts "  assignments:"
-        course.assignments.each do |assignment|
-          puts "    - #{assignment}"
+    def list(course, semester)
+      if semester
+        puts Course.new(@fs, course).roster(semester).students
+      elsif course
+        list_course course, 0
+      else
+        @fs.courses.each do |name|
+          puts "course: #{name}"
+          list_course name, 1
         end
       end
     end
@@ -58,6 +54,21 @@ module KGrader
     end
 
     private
+    def list_course(name, indent = 0)
+      course = Course.new(@fs, name)
+      pad = '  ' * indent
+
+      puts "#{pad}rosters:"
+      course.rosters.each do |roster|
+        puts "#{pad}  - #{roster.semester} (#{roster.students.size} students)"
+      end
+
+      puts "#{pad}assignments:"
+      course.assignments.each do |assignment|
+        puts "#{pad}  - #{assignment}"
+      end
+    end
+
     def reset_jail
       FileUtils.rm_rf @fs.jail
       FileUtils.mkdir @fs.jail
diff --git a/lib/kgrader/util.rb b/lib/kgrader/util.rb
index 1f86683..0280377 100644
--- a/lib/kgrader/util.rb
+++ b/lib/kgrader/util.rb
@@ -1,7 +1,7 @@
 require 'time'
 
 module KGrader
-  def self.parse_args(raw, num, keywords)
+  def self.parse_args(raw, range, keywords)
     args = []
     options = {}
 
@@ -27,8 +27,9 @@ module KGrader
       end
     end
 
-    raise ArgumentError, "too few arguments" if args.size < num
-    raise ArgumentError, "too many arguments" if args.size > num
+    raise ArgumentError, "too few arguments" if args.size < range.begin
+    raise ArgumentError, "too many arguments" if args.size > range.end
+    args[range.end - 1] = nil unless args.size == range.end
     return args, options
   end
 
diff --git a/rakefile b/rakefile
index dd4af74..dbb412b 100644
--- a/rakefile
+++ b/rakefile
@@ -12,7 +12,7 @@ end
 
 def parse_args(num, keywords = {})
   args, options = KGrader::parse_args ARGV.drop(1), num, keywords
-  args.each { |arg| task arg.to_sym {} }
+  args.each { |arg| task arg.to_sym {} unless arg.nil? }
   args + [options]
 rescue KGrader::KGraderError => err
   die err
@@ -22,8 +22,8 @@ task :default => :help do ; end
 
 task :help do
   puts %{usage:
-- rake list
-- rake roster <course> <rosterfile> [semester=<...>]
+- rake list   [<course>] [<semester>]
+- rake roster <course> <rosterfile> [<semester>]
 - rake grade  <course> <assignment> [semester=<...>] [students=<...>]
               [due=<...>] [fetch=<yes/no>] [regrade=<yes/no>]
 - rake commit <course> <assignment> [semester=<...>] [students=<...>]
@@ -32,23 +32,24 @@ task :help do
 end
 
 task :list do
-  run { |cli| cli.list }
+  course, semester = parse_args 0..2
+  run { |cli| cli.list course, semester }
 end
 
 task :roster do
-  course, rosterfile, options = parse_args 2, { :semester => :string }
-  run { |cli| cli.roster course, options[:semester], rosterfile }
+  course, rosterfile, semester = parse_args 2..3
+  run { |cli| cli.roster course, semester, rosterfile }
 end
 
 task :grade do
-  course, assignment, options = parse_args 2,
+  course, assignment, options = parse_args 2..2,
     { :semester => :string, :students => :array, :due => :time,
       :fetch => :bool, :regrade => :bool }
   run { |cli| cli.grade course, options[:semester], assignment, options }
 end
 
 task :commit do
-  course, assignment, options = parse_args 2,
+  course, assignment, options = parse_args 2..2,
     { :semester => :string, :students => :array }
   run { |cli| cli.commit course, options[:semester], assignment, options }
 end