Shell script implementation of a Queue.

queue.sh

#!/bin/bash
#
# This script encapsulates the functionality of a queue. It requires there to be
# an input file with the data in the queue being separated on different lines.
#

INPUT=queue.txt
OUTPUT=trash.txt
CMD=/usr/bin/vlc

if [ ! -f "$INPUT" ] ; then
	touch "$INPUT"
fi

#
# poll() removes the top line from $INPUT and appends it to the $OUTPUT file
#
poll() {
	if [ ! -f "$CMD" ] ; then
		echo "$CMD does not exist" 1>&2
		exit 1
	fi
	if [ ! -x "$CMD" ] ; then
		echo "$CMD is not executable" 1>&2
		exit 1
	fi
	head -1 "$INPUT" >> "$OUTPUT" && sed -i '1d' "$INPUT"
	ENTRY="`tail -1 "$OUTPUT"`"
	$("$CMD" "$ENTRY")
}

#
# offer() appends the given arguments to the $INPUT file
#
offer() {
	for arg in ${@:2} ; do
		if [ -f "$arg" ] ; then
			echo "$arg"
			echo "$arg" >> "$INPUT"
		fi
	done
}

#
# peek() prints the next entry in the queue
#
peek() {
	head -1 "$INPUT"
}

#
# random() passes a random line from $INPUT to $CMD
#
random() {
	if [ ! -f "$CMD" ] ; then
		echo "$CMD does not exist" 1>&2
		exit 1
	fi
	if [ ! -x "$CMD" ] ; then
		echo "$CMD is not executable" 1>&2
		exit 1
	fi

	COUNT=`cat "$INPUT" | wc -l`
	if [ "$COUNT" -lt 1 ] ; then
		exit 0
	fi
	RANDOM_LINE_NO=$[ ( $RANDOM % $COUNT ) + 1 ]

	RANDOM_LINE=`sed -n ''"$RANDOM_LINE_NO"'p' "$INPUT"`
	$("$CMD" "$RANDOM_LINE")
}

print_help() {
	cat << EOF
USAGE:
  queue COMMAND

COMMANDS:
  poll
	removes the top line from \$INPUT and appends it to the \$OUTPUT file
  offer
	appends the given arguments to the \$INPUT file
  peek
	prints the next entry in the queue
  random
	passes a random line from \$INPUT to \$CMD

EXAMPLES:
  To offer a list of files to the queue:

	find /path/to/files | xargs ./queue.sh offer
EOF
}

if [ "$1" == "poll" ]
then
	poll
elif [ "$1" == "offer" ]
then
	offer $*
elif [ "$1" == "peek" ]
then
	peek
elif [ "$1" == "random" ]
then
	random
else
	print_help
	exit 1
fi