Quiz 1

Today's quiz asked you to write a really simple shell script -- I promise next quiz will get more sophisticated. But, for today, we'll keep it simple:

• Write a program that lists the contests of a directory. The name of the directory should be supplied as a command-line argument.

Solution:

  #!/bin/sh

  ls ${1}
  

Handing In Assignments

We spent a good bit of time going over the logistics of handing in assignments. The gist of the discussion and help session is this:

• The course directory is "/afs/andrew/course/15/123-kesden"
• The handin directory is "/afs/andrew/course/15/123-kesden/handin"
• Underneath the handin directory, you'll find a subdirectory for the assignment/quiz. If it doesn't happen to be there, you have the power to create it. For example, "/afs/andrew/course/15/123-kesden/handin/quiz1"
• Create a subdirectory for yourself, e.g., "/afs/andrew/course/15/123-kesden/handin/quiz1/gkesden"
• Copy your files, using the command line or file transfer client into this directory
• You have permissions to insert new files and to list the files -- but not permission to read files or to modify them once written
• You can verify the correctness of your submission using "ls -l" -- the file size should match the file size of the original and should certainly not be zero.
• Should you need to resubmit, you can do this as many times as you'd like, just create a new, sequentially numbered directory each time, such as "gkesden.1", then "gkesden.2", then "gkesden.3". We will grade only the most recent -- and throw away the rest.

Warnings:

• Please spell your andrewid correctly. Our grading system is script driven. If you misspell your andrewid, you won't get credited with your score.
• If you don't get feedback within a week, please let us know. There is likely a problem. We return all assignments within a week.
• Do not use Apple's Finder to Drag-and-Drop files -- they'll surely have a size of zero. Instead, try using "scp" as we did in class, or fetch, or some other ftp client or ssh-based tool. In fact, I'd be sckeptical of drag-and-drop, even on most windows systems.

In class, we did the following a "Terminal" window:

• ssh unix.andrew.cmu.edu
• cd /afs/andrew/course/15/123-kesden/quiz1
• mkdir gkesden {your userid, not mine, please}
• exit {now we are back to the local iMac}
• cd Desktop {or wherever you've got your files}
• scp quiz1.c gkesden@unix.andrew.cmu.edu:/afs/andrew/course/15/123-kesden/handin/quiz1/gkesden {again, use yourid, not mine}

Setting Positionals

Unlike other variables, positions can't be assigned values using the = operator. Instead, they can only be changed in a very limited way.

The set command sets these values. Consider the following example:

  set a b c
  # $1 is now a
  # $2 is now b
  # $3 is now c
  

One thing that should be noted about the set command. It accepts arguments, itself. These begin with the - sign. As a result, it can get confused and begin to interprete values that it should be assigning to positionals. To avoid this, the -- flag can be used:

  set -- -a- -b- -c- 
  # $1 is now -a-
  # $2 is now -b-
  # $3 is now -c-
  

If there are more than 9 command-line arguments, there is a bit of a problem -- there are onyl 9 positionals: $1, $2, ..., $9. $0 is special and is the shell script's name.

To address this problem, the shift command can be used. It shifts all of the arguments to the left, throwing away $1. What would otherwise have been $10 becomes $9 -- and addressible. We'll talk more about shift after we've talked about while loops.

Quotes, Quotes, and More Quotes

Shell scripting has three different styles of quoting -- each with a diffent meaning:

• unquoted strings are normally interpreted
• "quoted strings are basically literals -- but $variables are evaluated"
• 'quoted strings are absolutely literally interpreted'
• `commands in quotes like this are executed, their output is then inserted as if it were assigned to a variable and then that variable was evaluated`

I think "quotes" and 'quotes' are pretty straight-forward -- and will be constantly reinforced. But, I do want to show an example using `quotes`:

  day=`date | cut -d" " -f1`
  printf "Today is %s.\n" $day
  

expr

The expr program can be used to manipulate variables, normally interpreted as strings, as integers. Consider the following "adder" script:

  sum=`expr $1 + $2`

  printf "%s + %s = %s\n" $1 $2 $sum
  

A Few Other Special Variables

We'll talk a bit more about these as we get into more complex examples. For now, I'd just like to mention them:

• $? - the exit status of the last program to exit
• $$ - The shell's pid