#!/bin/sh -u
#  $0  file [dir]
# Run a difference between the passwd file and its sorted self. 
# Place the output in the file that is the first (mandatory) argument
# under the directory that is the second (optional) argument.
# Prompt for a missing directory name.
# -IAN!  idallen@ncf.ca

export PATH=/bin:/usr/bin
umask 022

# 6 marks
# Validate the number of arguments; issue a good error message.
#
if [ $# -eq 0 ]; then
	echo 1>&2 "$0: Expecting 1 or 2 arguments; found $# ($*)"
	exit 1
fi

# 2 marks
# Transfer the argument to a named variable for better readability.
#
filnam="$1"

# 7 marks
# Prompt for a missing directory argument.
# Put the directory name into a named variable.
#
if [ $# -eq 1 ]; then
	echo 1>&2 "$0: Please enter a directory name:"
	read dirnam
else
	dirnam="$2"
fi

# 3 marks
echo "You are processing file '$filnam' and directory '$dirnam'."

# Verify that the input is a writable, searchable directory.
#
# 5 marks
if [ ! -d "$dirnam" ]; then
	echo 1>&2 "$0: '$dirnam' is not a directory"
	exit 1
fi
# 5 marks
if [ ! -w "$dirnam" ]; then
	echo 1>&2 "$0: '$dirnam' is not a writable directory"
	exit 1
fi
# 5 marks
if [ ! -x "$dirnam" ]; then
	echo 1>&2 "$0: '$dirnam' is not a searchable directory"
	exit 1
fi

# 2 marks
# Build the output file name in another variable.
# Make sure it is either nonexistent or is zero size.
#
diffout="$dirnam/$filnam"
# 5 marks
if [ -s "$diffout" ]; then
	echo 1>&2 "$0: '$diffout' is not an empty file"
	exit 1
fi

# 3 marks
# Construct a temp file name using the process ID, under the tmp
# directory.  Sort the password file into the temp file.
#
tfile=/usr/tmp/mytmp$$
# 4 marks
sort /etc/passwd >"$tfile" || exit $?

# 3 marks
# Run diff against the sorted copy; save the results.
# Record the exit status of diff.
#
diff /etc/passwd "$tfile" >"$diffout"
# 2 marks
dstat="$?"

# 6 marks
# Append the output status to a non-empty results file.
#
if [ -s "$diffout" ] ; then
	echo "Difference exit status was $dstat." >>"$diffout" || exit $?
fi