Monitor Pre-existing and new files in a directory with bash

Question

I have a script using inotify-tool.
This script notifies when a new file arrives in a folder. It performs some work with the file, and when done it moves the file to another folder. (it looks something along these line):

inotifywait -m -e modify "${path}" |
    while read NEWFILE
       work on/with NEWFILE
       move NEWFILE no a new directory
    done 

By using inotifywait, one can only monitor new files. A similar procedure using for OLDFILE in path instead of inotifywait will work for existing files:

for OLDFILE in ${path} 
do 
   work on/with OLDFILE 
   move NEWFILE no a new directory
done

I tried combining the two loops. By first running the second loop. But if files arrive quickly and in large numbers there is a change that the files will arrive wile the second loop is running. These files will then not be captured by neither loop.

Given that files already exists in a folder, and that new files will arrive quickly inside the folder, how can one make sure that the script will catch all files?

Answer 1

By using inotifywait, one can only monitor new files.

I would ask for a definition of a "new file". The man inotifywait specifies a list of events, which also lists events like create and delete and delete_self and inotifywait can also watch "old files" (beeing defined as files existing prior to inotifywait execution) and directories. You specified only a single event -e modify which notifies about modification of files within ${path}, it includes modification of both preexisting files and created after inotify execution.

... how can one make sure that the script will catch all files?

Your script is just enough to catch all the events that happen inside the path. If you have no means of synchronization between the part that generates files and the part that receives, there is nothing you can do and there always be a race condition. What if you script receives 0% of CPU time and the part that generates the files will get 100% of CPU time? There is no guarantee of cpu time between processes (unless using certified real time system...). Implement a synchronization between them.

You can watch some other event. If the generating sites closes files when ready with them, watch for the close event. Also you could run work on/with NEWFILE in parallel in background to speed up execution and reading new files. But if the receiving side is slower then the sending, if your script is working on NEWFILEs slower then the generating new files part, there is nothing you can do...

If you have no special characters and spaces in filenames, I would go with:

inotifywait -m -e modify "${path}" |
while IFS=' ' read -r path event file ;do
    lock "${path}" 
    work on "${path}/${file}"
    ex. mv "${path}/${file}" ${new_location}
    unlock "${path}"
done

where lock and unlock is some locking mechanisms implemented between your script and the generating part. You can create a communication between the-creation-of-files-process and the-processing-of-the-files-process.

I think you can use some transaction file system, that would let you to "lock" a directory from the other scripts until you are ready with the work on it, but I have no experience in that field.

I tried combining the two loops. But if files arrive quickly and in large numbers there is a change that the files will arrive wile the second loop is running.

Run the process_new_file_loop in background prior to running the process_old_files_loop. Also it would be nice to make sure (ie. synchronize) that inotifywait has successfully started before you continue to the processing-existing-files-loop so that there is also no race conditions between them.

Maybe a simple example and/or startpoint would be:

work() {
    local file="$1"
    some work "$file"
    mv "$file" "$predefiend_path"
}

process_new_files_loop() {
    # let's work on modified files in parallel, so that it is faster

    trap 'wait' INT
    inotifywait -m -e modify "${path}" |
    while IFS=' ' read -r path event file ;do
        work "${path}/${file}" &
    done
}

process_old_files_loop() {
    # maybe we should parse in parallel here too?
    # maybe export -f work; find "${path} -type f | xargs -P0 -n1 -- bash -c 'work $1' -- ?

    find "${path}" -type f |
    while IFS= read -r file; do
        work "${file}"
    done
}

process_new_files_loop &
child=$!

sleep 1

if ! ps -p "$child" >/dev/null 2>&1; then
    echo "ERROR running processing-new-file-loop" >&2
    exit 1
fi
process_old_files_loop
wait # wait for process_new_file_loop

If you really care about execution speeds and want to do it faster, change to python or to C (or to anything but shell). Bash is not fast, it is a shell, should be used to interconnect two processes (passing stdout of one to stdin of another) and parsing a stream line by line while IFS= read -r line is extremely slow in bash and should be generally used as a last resort. Maybe using xargs like xargs -P0 -n1 sh -c "work on $1; mv $1 $path" -- or parallel would be a mean to speed things up, but an average python or C program probably will be nth times faster.

Answer 2

Once inotifywait is up and waiting, it will print the message Watches established. to standard error. So you need to go through existing files after that point.

So, one approach is to write something that will process standard error, and when it sees that message, lists all the existing files. You can wrap that functionality in a function for convenience:

function list-existing-and-follow-modify() {
  local path="$1"
  inotifywait --monitor \
              --event modify \
              --format %f \
              -- \
              "$path" \
    2> >( while IFS= read -r line ; do
            printf '%s\n' "$line" >&2
            if [[ "$line" = 'Watches established.' ]] ; then
              for file in "$path"/* ; do
                if [[ -e "$file" ]] ; then
                  basename "$file"
                fi
              done
              break
            fi
          done
          cat >&2
        )
}

and then write:

list-existing-and-follow-modify "$path" \
| while IFS= read -r file
    # ... work on/with "$file"
    # move "$file" to a new directory
  done

Notes:

• If you're not familiar with the >(...) notation that I used, it's called "process substitution"; see https://www.gnu.org/software/bash/manual/bash.html#Process-Substitution for details.
• The above will now have the opposite race condition from your original one: if a file is created shortly after inotifywait starts up, then list-existing-and-follow-modify may list it twice. But you can easily handle that inside your while-loop by using if [[ -e "$file" ]] to make sure the file still exists before you operate on it.
• I'm a bit skeptical that your inotifywait options are really quite what you want; modify, in particular, seems like the wrong event. But I'm sure you can adjust them as needed. The only change I've made above, other than switching to long options for clarity/explicitly and adding -- for robustness, is to add --format %f so that you get the filenames without extraneous details.
• There doesn't seem to be any way to tell inotifywait to use a separator other than newlines, so, I just rolled with that. Make sure to avoid filenames that include newlines.

Answer 3

A simpler solution is to add an ls in front of the inotifywait in a subshell, with awk to create output that looks like inotifywait.

I use this to detect and process existing and new files:

(ls ${path} | awk '{print "'${path}' EXISTS "$1}' && inotifywait -m ${path} -e close_write -e moved_to) |
  while read dir action file; do
    echo $action $dir $file
    # DO MY PROCESSING
  done
  

So it runs the ls, format the output and sends it to stdout, then runs the inotifywait in the same subshell sending the output also to stdout for processing.