UNIX for Dummies Questions & Answers

#!/bin/bash

# Minimum age of a file to replace.
# There's a lag/jitter since time's measured in seconds,
# so set an AGE of at most 3 for a file updated every 5 seconds.
AGE=3

# File to watch for updates on
WATCH=./recent

# File to keep up to date
OUT=./complete

# Create temp files of increasing age
for ((N=0; N<AGE; N++)) ; do touch /tmp/$$-$N ; sleep 1 ; done

# Delete temp files on exit no matter what
trap "rm -f /tmp/$$-[0-9]" EXIT

while true
do
        sleep 1

        # Update oldest file
        touch /tmp/$$-$((N%AGE))
        ((N++)) # N is now the next-oldest file
        OLDEST=/tmp/$$-$((N%AGE))

        [ -f "$WATCH" ] || continue

        # File is old enough to be done
        if [ "${WATCH}" -ot "$OLDEST" ]
        then
                # do NOT replace:  Move COMPLETE files into its place.
                # incomplete files will never be seen.

                ln "${WATCH}" "${WATCH}-tmp"
                rm -f "${OUT}" "${WATCH}"
                mv "${WATCH}-tmp" "${OUT}"
        fi
done