This week we'll continue with some more advanced shell scripting control strucutres, including conditionals, loops, etc. You'll be building upon the shell script file you made last week, add.sh.

if-then-else

Just like other programming languages, shell scripts can have conditional statements (if-then-else) and loops (while and foreach).

Now we're going to add an extra bit of code to the add.sh script from the previous lab. That script took in two numbers as arguments from the command-line and added them. But we never checked that the user had entered at least two arguments; if they ran the script and didn't enter two arguments, it would just output an unrecognizable error. Here, we'll use an if statement to output a better error if at least two arguments aren't passed:

if ($#argv != 2) then
    echo "You must give exactly two parameters!"
else
    set num1 = $argv[1]
    set num2 = $argv[2]
endif

Example with a for loop

#! /bin/csh -f

######################################################################
#
# Script to find specified file
#
######################################################################

set f = $1
foreach d (*)
  if (-e $d/$f) then
      echo "FOUND: $d/$f"
      exit(0)
  endif
end
echo "$f not found in subdirectories."

In the line foreach d (*), the * is a wild card, so it would expand to a list of all files in the current directory.

In the line if (-e $d/$f) then, the -e means existance; in other words, we are asking if the file $d/$f exists. If we type ./find_file.sh test.txt, as in the example above, $f would be test.txt and $d would start out as bin. So, we would be asking if the file bin/test.txt exists. Because it's in a for each loop, it'll try the if statement either until it finds one and it exits(0) or until there are no more directories to try.

Writing your own script

Using what you've learned in all the labs, you are now going to write a script yourself that does the following:

• changes to your home directory
• creates a file with a name that's passed in as the first argument to the script (Hint: the touch command creates a new file). Also check that at least one argument was passed to the script (Hint: use the same method as used above). If not print an error.
• lists the current contents of the directory (using the long listing flag) and pipes them into the file
• zips the file using gzip
• changes the permissions of the file so that it's readable by everyone.
• copies the file to the home/backups directory while adding .bak to the end of the filename

Hand In: This lab handout with the answers filled in attached to a listing of your final programs
(use the enscript command from your Programming Style Sheet to print it out:
enscript -E -G -2rj -M Letter -PECT2_PS <filename>. You can also use a2ps as in: a2ps <filename>).