Recursively compress files with xz using multiple threads

compress.sh

# With Bash 4.3 I'd rather use wait -n to keep N
# processes running at all times. As it is now you
# unfortunately have to wait for the last process
# to finish. 

# Recursively compress all files in $dir
compress () {
  dir=$1
  count=${2:-2}
  num_procs=${3:-0}

  if [ $count -le 0 ]; then
    exit 1
  fi

  if [ ! -d "$dir" ]; then
    return 1
  fi

  for file in $dir/*; do
    if [ -f "$file" ]; then
      xz "$file" &
      if [ $! -gt 0 ]; then
        ((num_procs++))
      fi

      if [ $num_procs -ge $count ]; then
        wait
        num_procs=0
        # In bash 4.3 I would try:
        #wait -n
        #((num_procs--))
      fi
    fi
    if [ -d "$file" ]; then
      compress "$file" $count $num_procs
    fi
  done
}

# Example:
# compress /var/log/remote/2016/11