Wait for a number of files to be present (including 0, the use case!) and start a command

wait-for-files.sh

#!/bin/sh

# Number of files to wait for, 0 is usually what you want
TARGET=$1

# Pattern to match path
PATTERN=$2

# Command to execute once target has been reached
COMMAND=$3

while true; do
    NFILES=$(ls -1 ${PATTERN}|wc -l)
    WHEN=$(date +'%Y%m%d %H:%M:%S')
    echo "[${WHEN}] $NFILES file(s) matching $PATTERN"
    if [ "$NFILES" -eq "$TARGET" ]; then
        exec $3
    fi
    sleep 5
done