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

connection: so far we've looked at basics and building blocks of shell
programs (statements), now we'll concentrate on more specialized
techniques.

* advanced shell programming

advert: we see how to write polished shell programs,
how to handle interrupts, and new ways to provide programs with input
and to read input in the shell.

	it's also worth reading the manual page for sh to see other things...

** process id of current shell
	each shell has it's process id as the value of $$
	---------------
	$ echo $$
	16150
	$ ps
	   PID TTY      TIME COMMAND
	 16184 ttyt0    0:00 ps
	 16150 ttyt0    0:01 sh
	---------------

	Q: How do name a temporary file?
		use /tmp/cmdname.$$, for example
		Important so different copies of same program don't try to use
			same temporary file

		don't use /tmp/$0.$$,
			because $0 may be /home/cs218/cs218a23/bin/foo


** interrupts (pages 489-490)
	Interrupts (e.g., hangup, ^C) send signals
		so does the kill command

	Q: What is the default response of a shell script to a signal?

	to handle them otherwise, use trap

	------------
	# turn off interrupts
	trap '' 1 2 3 15 

	trap 'echo "$0: quitting early" >&2; exit 1' 1 2 3 15
	------------
		1=hangup
		2=interrupt (^C)
		3=quit (core dump, ^G)
		15=software termination (kill command)

	You can even nest trap handlers:
		recall 0 is the file descriptor for stdin

	------------
	# get password from user
	if test -t 0
	then
		# a terminal user
		trap 'trap "" 2 3 15; stty echo; exit 1' 2 3 15
		stty -echo		# turn off echoing
		echo "Password: \\c"
		read PASSWORD || { stty echo; exit 1; }
		stty echo		# turn echoing back on
		trap 2 3 15
		echo			# echo user's newline
	else
		# not a terminal
		read PASSWORD || exit 1
	fi
	------------	

		note the \c string passed to echo makes no newline

** here documents (page 493)
	a way to get longish texts into a command's standard input
		in a way that allows for the various shell substitutions.

	Q: What is the standard input of try_hailstone below?
	------------
	try_hailstone <<-!
	n
	n
	$1
	quit
	!
	------------

		the - sign makes the leading white space stripped from the doc.

	Q: How would you do the above without using a here document?

		{ echo n; echo n; echo $1; echo quit;} |try_hailstone

		(echo n; echo n; echo $1; echo quit) | try_hailstone

		explain { cmd-list; } vs. ( subshell ) idea
			$ echo $x
			$ (x=2)
			$ echo $x

			$ { x=2; }
			$ echo $x
			2

	Another syntactic form
	cat <<! | try_hailstone
	n
	n
	$1
	quit
	!

** read (pages 496-500)
	----------------
	$ read line
	some text on a line
	$ echo $line
	some text on a line
	$ read word1 word2 rest
	some text on a line
	$ echo $word1
	some
	$ echo $word2
	text
	$ echo $rest
	on a line
	----------------
	see example of reading a password above.

	read in the shell is usually *slow*
		if you need to do this, often better to write a C program.

	Q: can you redirect the input of read?
		no, but can use a statement such as
			for i in x; do read y; done <file

	from my .profile ...
	----------------
	while read ignore name value
	do
		eval "${name}=${value}"
		export "$name"
	done < $HOME/.login-environment
	----------------
		note: reading in a loop maintains a position in the file
			read fails (exit value is non-zero) when gets eof

	-----------------
	$ read foo
	$ echo $?
	1
	-----------------
** customization (.profile)
	to customize how your (ksh) behaves
		for similar kinds of stuff in csh, see the Vincent .login...
	Note: there is a standard login sequence that is run BEFORE your login
		it's found in /etc/profile (for /bin/sh or ksh)
		for csh it's found in /etc/login (if at all)
			may set default erase, kill chars, etc.
			prints message of the day (/etc/motd)
			checks to see if you have mail
			sets your timezone...

*** setting environment variables (for export)

	Q: What envrionment variables are set by $PUB/profile?

	---------------------
	if test "X"${user}  = X
	then
		user=`expr "$HOME" : '.*/\([^/][^/]*\)'`
		export user
	fi
	--------------------------

**** set the PATH
	-------------
	PATH=:$HOME/bin$PATH
	PATH=:$HOME/bin/$MACHINE:$HOME/bin/$OS:$HOME/bin:$PATH:/usr/fred/bin
	-------------

*** setting the terminal parameters
	may be handled for you (as it is on zippy/zaphod)
	-------------
       stty erase '^?' quit '^G' intr '^C'
       eval "`tset -s -Q -m 'network:?vt102' -m 'dumb:?sun' -m 'unknown:?sun'`"
	-------------

*** setting your umask
	-------------
	umask 022
	-------------

*** other stuff
	I like to check the system clock and the disk usage
	----------
	date
	df |more  # or something more sophisticated...
	----------

	May want to set the shell's prompt
	----------------
	PS1="`uname -n` \$ "
	----------------
	in csh I use:
		set prompt="`hostname` \!% "
	in tcsh I use
		set prompt='%m %\!%% '

** debugging (see section 13.9)
	If invoke shell by sh -x cmdname args, get debugging output
		another way to do this is to use set -x
	
	Q: What does set -x do to the shell?

	[ -n "$echo" ] && set -x
	at the beginning of each script

	when your shell doesn't work do

	echo=true
	export echo

** checklist
	Q: What things should you look for when checking over a shell script?
	check error cases for
		messages to stderr
		return code non-zero (exit 2)

	check all exit conditions for exit code

	have I checked for proper invocation?

	comments