The shell fully determines the list of values to loop on before it starts running the loop body. That is:
- The shell builds a path using value of the variable
- The shell collects the list of file names in the specified directory, to build the list of matches for the wildcard pattern.
- The shell executes the loop body, in turn, with each element of the list of matches.
You can change the value of the variable
search_dir, and change the content of the directory, while the loop is running. This won't affect what files the loop acts on.
If a file is removed while the loop is processing other files, then once it gets to that file, that file won't exist. Depending on what you do in the loop, it may or may not matter. If there is a concurrent process that may remove files, note that testing whether the file exists before processing it won't really solve that problem, since the file could be removed between the time you test and the time you start processing.
If you need to mark a file as processed to make sure that you don't process it twice, then this script should move the files to another directory once it's processed them. Moving a file to another directory (on the same filesystem) is atomic: either it isn't done yet or it's done, there's no intermediate state. But once again, if a different process (possibly another instance of that scripts) moves files, then the loop will sometimes hit files that are moved while the loop is executing.
If you want to process new files as they are created, then you'll need to loop again. Obviously files could be created during the execution of the loop, or after all the prior files have been processed, so the script would need to keep running forever. There are tools to wait until a file is created in a directory. On Linux, the basic facility for that is inotify; if you need to process files as they are created, then
inotifywait or incron should help you. Remember that inotify only notifies you of files created (or modified or accessed depending on your trigger) after the inotify-based commands start; you'll also need to take care of prior existing files, and you can't just do
for entry in *; do …; done; inotifywait … because files can be created during the execution of the loop or even while the
inotifywait command is starting up.