CS/CE 218 Lecture -*-Outline-*-

Connection: already seen single statements in shell programming
	now to look at more complex programs, using compound statements.

* statements of the Bourne shell

** testing success (exit value)pages 467-471
	each command execution can be tested for success or failure (error)
	Q: How do you know if a Unix command is successful?
		returns 0 exit code
		should return >0 if unsuccessful

	Q: How do you control the exit code of a shell script?

	this is the most common mistake of novice Unix programmers:
		incorrect return code makes success of command difficult
			to tell
		unfortuanately, book routinely makes this mistake
			(its semantics are hard to get right)

*** test command (pages 471-478)
	Q: what kinds of things can "test" test?
		numeric values  (-eq)
		file types
		character strings (=)

	---------------
	if test $# -eq 0
	then
		echo "$USAGE" >&2
		exit 1
	fi
	---------------

	Q: How do you test to see if a file is readable?
		test -r file

	Another syntax for test is [ -r file ]

	usually best to enclose arguments to test in double quotes,
		in case user called it wrong

	if test "$1" = '-?'
	then
		...
	fi

*** if statement and if-then-else

	Q: If you think of true as 1, how does "if" act strangely?

	recall dviprint example...
	---------------
	RET=0
	if dvialw -q $f
	then lpr -P${PRINTER-lw1} \#${f}-alw
	else RET=1
	fi
	exit $RET
	---------------

	different syntax in cshell

*** && and || combinations
	---------------
	# almost equivalent
	RET=0
	dvialw -q $f && lpr -P${PRINTER-lw1} \#${f}-alw || RET=1
	exit $RET
	---------------		
		the RET=1 is done if *either* fails

*** while and until loops
	------------------
	while true			# poke $owcpid every so often
	do 
	  sleep "$sleepytime"
	  kill -IO "$owcpid" || exit 
	done > /dev/null 2>&1
	------------------
		note, you can redirect output from a loop as a whole

** for loops (section 13.6)
	Q: Do you know in advance how many times a for loop will execute?

	different syntax in cshell
	------------------
	for f
	do
		dvialw -q $f && lpr -P${PRINTER-lw1} \#${f}-alw || RET=1
	done
	------------------
		this is for all command line arguments
		can also do
			for f in *

** break and continue statements (page 481)
	often have a "last option" -- or -

	Q: What does the following do?
	------------------
	while test -n "$1"
	do
	   case "$1" in
	   -[bB]) base_option=TRUE
		  shift;;
	   --)	shift
	     	break;;
	   *) break ;;
	   esac
	done
	------------------

	Q: How is "continue" like a goto?

** case statement (page 484-485)

	pattern matching on strings (not numeric as in Pascal)
	uses shell meta characters
		can also use | to mean "or"

		case "$1" in
		-b|-B) ... ;;
		...
		esac

	Q: Do you understand the following command's details?
	---------------------
# eclean -- clean up emacs backup files
#
# SYNOPSIS:  Remove files whose names match *~ from current working directory
#	Giving prefixes causes only files with names starting in that list
#	to be removed.
#
# AUTHOR:	Gary T. Leavens

USAGE='Usage: '"$0"' [prefix ...]'

case "$1" in
-*)
	echo "$USAGE" >&2
	exit 2
	;;
esac

# all files if no prefix specified
if test $# = 0
then
	set '*'
fi

# remove the files
for p
do
	rm -f ${p}~
done

exit 0
	---------------------

	Why is it set '*' above instead of set *?