#!/bin/sh -u
#  $0 dir file
# Create a file containing the date and a sorted list of users.
# Place the output in the file that is the second argument
# under the directory that is the first argument.
# -IAN!  idallen@ncf.ca

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

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

# Transfer the arguments to named variables for better readability.
# 2 marks
mydir="$1"
# 2 marks
myfile="$2"

# Verify that the input is a writable, searchable directory.
#
# 5 marks
if [ ! -d "$mydir" ]; then
	echo 1>&2 "$0: '$mydir' is not a directory"
	exit 1
fi
# 5 marks
if [ ! -w "$mydir" ]; then
	echo 1>&2 "$0: '$mydir' is not a writable directory"
	exit 1
fi
# 5 marks
if [ ! -x "$mydir" ]; then
	echo 1>&2 "$0: '$mydir' 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.
#
newfile="$mydir/$myfile"
# 5 marks
if [ -s "$newfile" ]; then
	echo 1>&2 "$0: '$newfile' is not an empty file"
	exit 1
fi

# Initialize the $newfile output file with the date.
# Construct a temp file name using the process ID, under the tmp
# directory.  Put the sorted "who" output into the temp file.
# Append the temp output file to the $newfile output file.
# Count the number of users in the temp file and also append that.
# Remove the temp file.
#
# 3 marks
date >"$newfile" || exit $?
# 3 marks
tmp=/tmp/mytmp$$
# 4 marks
who | sort >"$tmp" || exit $?
# 3 marks
cat "$tmp" >>"$newfile" || exit $?
# 3 marks
echo "The number of users is $(wc -l <$tmp)." >>"$newfile"
# 2 marks
rm "$tmp"