Class 2: Homework

1. What is the difference between a program and a process?

2. For each process created by the following command line: 1) identify which portion of the command-line created that process, 2) give the argv vector for that process, 3) describe stdin, stdout and stderr for that process. (See this example from the lecture.)

   
   
   
   
   	  cat eliza  |  tr -s a
   
   
   
   
   
   	

3. For each process created by the following command line: 1) identify which portion of the command-line created that process, 2) give the argv vector for that process, 3) describe stdin, stdout and stderr for that process. (See this example from the lecture.)

   
   
   
   
   	  cd tmp  ;  ls -l foo 
   
   
   
   
   
   	

4. For each process created by the following command line: 1) identify which portion of the command-line created that process, 2) give the argv vector for that process, 3) describe stdin, stdout and stderr for that process. (See this example from the lecture.)

   
   
   
   
   	  ps -ef | grep -v crabbe > tmp.txt
   
   
   
   
   
   	

5. For each process created by the following command line: 1) identify which portion of the command-line created that process, 2) give the argv vector for that process, 3) describe stdin, stdout and stderr for that process. (See this example from the lecture.)

   
   
   
   
   	  ./foo -v < dat1 > res1
   
   
   
   
   
   	

6. Write a single command line that would write out the number of processes currently running on a system. Note that ps -ef writes a list of all processes currently on the system. Also note that I won't hold it against you if you're off by one. Finally, you must actually run this command and see the result. If you haven't tried it out in a terminal, please don't write down an answer.

7. Suppose your user name is tonks, and there are three different files called wc are readable by you: /bin/wc, /home/tonks/bin/wc, and a wc in your current working directory ~/foo/wc. which gets executed by the shell if your path is:

   /bin:/usr/bin:/home/tonks/bin
   
   	

   How would you change your path so that ~/foo/wc gets executed instead?

8. There is a special "file" in the filesystem called /dev/null. It's essentially a black-hole for data. If you want to run a command but not see any output, you can redirect the command to /dev/null. Now, why would you want to do something like that? Here's an example: Suppose I want to time how long it takes for the system to get a list of all the files in my homedirectory -- including subdirectories, -- -- sub-subdirectories, etc. Well, the command ls -R ~ lists every file, that's fine. And the command time ls -R ~ actually displays how long ls -R ~ takes. However, it prints loads of stuff to the screen, and I don't really want to see the list, I simply want to know how long it takes to generate. I can run this instead:

   time ls -R ~ > /dev/null
   	

   ... and I'll get the timing information but nothing will print, no files will get created, nothing ... all the results just die in the black hole that is /dev/null. Here's another reason to use /dev/null. Suppose I don't want error messages for a command to print? I could accomplish this by redirecting stderr to /dev/null. The command rm ~wcbrown/ggg prints an error message:

   bear[~]> rm ~wcbrown/ggg 
   rm: /home/wcbrown/ggg: No such file or directory
   	

   Do a Google search to find out how to redirect stderr in bash, and write down a modified version of rm ~wcbrown/ggg in which no error messages are printed because stderr is redirected to /dev/null.
   Note: You must actually try this out on an actual command line and verify that the error message was supressed. If you don't actually try it out on the command line and see that it works, I don't want you to give me an "answer".