Shell Programming and Scripting

#!/bin/ksh
IAm="${0##*/}"
Usage() {
	case "$1" in
	(2)	printf "%s: Can't read input_file: %s\n" "$IAm" "$2";;
	(4)	printf '%s: Invalid date in "|T" line: %s\n' "$IAm" "$2";;
	(5)	printf '%s: Invalid ID in ".T" line: %s\n' "$IAm" "$2";;
	(6)	printf '%s: Invalid tag: %s\n' "$IAM" "$2";;
	(7)	printf '%s: Invalid line length: %s\n' "$IAm" "$2";;
	esac >&2
	printf 'Usage: %s input_file output_file\n' "$IAm" >&2
	exit $1
}

[ $# -ne 2 ] && Usage 1
[ ! -r "$1" ] && Usage 2 "$1"
> "$2" || Usage 3 "$2"
while read -r tag data
do	case ${#tag} in
	(2)	case "$tag" in
		("|T")	[ ${#data} -ne 8 ] && Usage 4 "$tag $data"
			out="$tag $data"
			copy=0;;
		(".T")	[ "$data" = '' ] && Usage 5 "$tag"
			if [ $copy -eq 1 ]
			then	printf '%s\n%s %s\n' "$out" "$tag" "$data" > "$2"
			fi;;
		(*)	Usage 6 "$tag $data";;
		esac;;
	(17)	case "$tag" in
		(10000000000000000)
			;;
		(10*)	copy=1;;
		esac
		out=$(printf '%s\n%s' "$out" "$tag");;
	(*)	Usage 7 "$tag $data";;
	esac
done < "$1"