Bash cannot do this easily. You’d have to basically get a list of all the files in the folder and periodically get a new list and compare them to see whats changed.

What you’re looking for is called inotify. Its built into the linux kernel and you can basically sit there waiting for something to happen at which point inotify comes back and says ‘hey, theres a new file called foobar’

To accomplish what you want you’d have to switch to something like perl and use Linux::Inotify2 (python probably supports inotify as well, but I’m a perl person).

I am assuming the target folder (I’ll call it isempty just for convenience) is empty and you are waiting for one or more files to be dropped there.

You can use the following command:

ls -1A isempty | wc -l

just to check if the folder is still empty, in fact it will return a 0 if there is no new file (hence the isempty folder is still empty) or, on the other hand, it will return a value greater than 0 (actually the number of files currently in the folder).

That said a silly if/then test can make the rest of the work:

if [ $(ls -1A isempty | wc -l) -gt 0 ] ; then do_something ; fi

Of course the do_something function will have to manipulate the file(s) within the isempty folder and then remove it(them) from the folder itself after processing.

Adding a line like the following in your crontab will run the check once a minute and will trigger the do_something action if the folder is not empty of course:

* * * * *     if [ $(ls -1A isempty | wc -l) -gt 0 ] ; then do_something ; fi

I prefer incron, as its easier to manage. Essentially it’s a service that leverages inotify and you can setup configurations to take action based on file change operations.

Ex:

<directory> <file change mask> <command or action>  options
/var/www/html IN_CREATE /root/scripts/backup.sh

You can see a full example here:
http://www.cyberciti.biz/faq/linux-inotify-examples-to-replicate-directories/

I just cooked up this, and see no huge problems with it, other than a tiny chance of missing files in between checks.

while true
do
       touch  ./lastwatch
       sleep 10
       find /YOUR/WATCH/PATH -cnewer ./lastwatch -exec SOMECOMMAND {} ;
done

If your file processing doesn’t take too long, you should not miss any new file. You could also background the activities…
It’s not bullet proof, but it serves some purposes without external tools like inotify.

If you want to detect new files, then process them and at the end delete proceeded files you can use systemd.path. This method bases on inotify. There is an option DirectoryNotEmpty, so systemd can run your script always when it detects any files in directory. You have to remember it will work only if you can delete proceeded files and script leaves directory empty.

First prepare mymonitor.service file

[Unit]
Description=Start the script

[Service]
Type=oneshot
ExecStart=/path/to/your/script

next go to mymonitor.path to define the path

[Unit]
Description= Triggers the service

[Path]
DirectoryNotEmpty=/path/to/monitor

[Install]
WantedBy=multi-user.target

If the name of the .path file is the same as the name of the service there is no need to specify the service name in .path file.

It bases on Monitoring File Access for Dummies

This works in cygwin and Linux. Some of the previous solutions which write a file will cause the disk to thrash. This scipt does not have that problem:

SIG=1
SIG0=$SIG
while [ $SIG != 0 ] ; do
 while [ $SIG = $SIG0 ] ; do
   SIG=`ls -1 | md5sum | cut -c1-32`
   sleep 10
 done
 SIG0=$SIG
 ls -lrt | tail -n 1
done

Below is an abridged version of example on stackoverflow that I’ve tested and incorporated into one of my projects that requires monitoring of specific directories.

Var_dir="${1:-/tmp}"
Var_diff_sleep="${2:-120}"
Var_diff_opts="--suppress-common-lines"
Func_parse_diff(){
    _added="$(grep -E '>' <<<"${@}")"
    if [ "${#_added}" != "0" ]; then
        mapfile -t _added_list <<<"${_added//> /}"
        _let _index=0
        until [ "${#_added_list[@]}" = "${_index}" ]; do
            _path_to_check="${Var_dir}/${_added_list[${_index}]}"
            if [ -f "${_path_to_check}" ]; then
                echo "# File: ${_path_to_check}"
            elif [ -d "${_path_to_check}" ]; then
                echo "# Directory: ${_path_to_check}"
            if [ -p "${_path_to_check}" ]; then
                echo "# Pipe: ${_path_to_check}"
            fi
            let _index++
        done
        unset _index
    fi
}
Func_watch_bulk_dir(){
    _current_listing=""
    while [ -d "${Var_dir}" ]; do
        _new_listing="$(ls "${Var_dir}")"
        _diff_listing="$(diff ${Var_dec_diff_opts} <(${Var_echo} "${_current_listing}") <(${Var_echo} "${_new_listing}"))"
        if [ "${_diff_listing}" != "0" ]; then
            Func_parse_diff "${_diff_listing}"
        fi
        _current_listing="${_new_listing}"
        sleep ${Var_diff_sleep}
    done
}

Here’s a link to a script that uses a modified version of above to automatically decrypt files or directories found in its sshfs mount point; the afore mentioned project.

In case you came here for a simple, fast, handy solution reading the title, you could use watch:

watch -n 0.1 ls <your_folder>

This monitors your folder and lists everything in it every 0.1 seconds.

Drawbacks

• Non-scriptable. For scripting options, have a look at other answers.
• Not real-time, so if a file was created and deleted in less than 0.1 second, then this would not work, watch only supports minimum of 0.1 seconds.

entr

Using entr is the new way to do this (it’s cross platform). Note entr doesn’t use polling giving it a huge advantage over many of the alternatives.

Uses kqueue(2) or inotify(7) to avoid polling. entr was written to make rapid feedback and automated testing natural and completely ordinary.

On BSD it uses pledge(2)

You can install it with

apt-get install entr
dnf install entr
brew install entr

You can track a directory for new additions using

while $(true); do
  # echo ./my_watch_dir | entr -dnr echo "Running trigger..."
  echo ./my_watch_dir | entr -dnr ##MY COMMAND##
done;

Options explained (from the docs),

• -d Track the directories of regular files provided as input and exit if a new file is added. This option also enables directories to be specified explicitly. Files with names beginning with ‘.’ are ignored.
• -n Run in non-interactive mode. In this mode entr does not attempt to read from the TTY or change its properties.
• -r Reload a persistent child process. As with the standard mode of operation, a utility which terminates is not executed again until a file system or keyboard event is processed. SIGTERM is used to terminate the utility before it is restarted. A process group is created to prevent shell scripts from masking signals. entr waits for the utility to exit to ensure that resources such as sockets have been closed. Control of the TTY is not transferred the child process.

Note entr isn’t polling there. It’s after there is an operation on an inode it hasn’t seen. Note this isn’t a failsafe mechanism because the inode could be recycled to a different file name entirely. Ie,

mkdir foo;
cd foo;
touch bar;
touch baz;
ls -1i;
echo "delete bar; add quz"
rm bar;
touch quz;
ls -1i;

See the inotifywait answer for a better, and more powerful method of doing this. Albeit with a much worse interface.

After searching the answers here and the ones at the other question mentioned as possible duplicate in the comments above, I think I’d go with fswatch since it is has cross-platform support.

It uses different kinds of monitors for different OS and can choose the appropriate one automatically, or allow one to specify which one to use and even pass custom platform-specific parameters to the respective monitor.

The list of monitors it currently supports is:

• A monitor based on the File System Events API of Apple OS X.
• A monitor based on kqueue, an event notification interface introduced in FreeBSD 4.1 and supported on most *BSD systems
  (including OS X).
• A monitor based on inotify, a Linux kernel subsystem that reports file system changes to applications.
• A monitor based on File Events Notification, a Solaris/Illumos kernel API that reports file events.
• A monitor based on ReadDirectoryChangesW, a Microsoft Windows API that reports changes to a directory.
• A monitor which periodically stats the file system, saves file modification times in memory and manually calculates file system
  changes, which can work on any operating system where stat (2)
  can be used.

It seems to be available via apt-get on Debian/Ubuntu Linuxes. See how to install via apt-get and use fswatch.

FreeBSD and OS-X package-based installation support for fswatch is provided by its author.

Can also build and install it at other OSes, found an article+video showing how to make and install fswatch on CentOS.

There is also an other article that shows the same manual process to build/install (and use) fswatch for Linux.

Windows-based package-based installation support doesn’t seem to be available yet (e.g. no package for Chocolatey yet, and no package for Vcpkg)

There is also extensive documentation for fswatch, though docs for the latest 1.5 version point to 1.4 ones currently.
See how to convert commands for fswatch 0.x to fswatch 1.x

Tips for manually choosing a monitor (currently not updated to mention all monitors)

Read about fswatch usage here, here and here and a tutorial intro here

A library named libfswatch is kept in sync with the fswatch tool. See here and a newer doc here.
Note that the library is versioned differently from fswatch utility itself. Specifically, the 1.14.0 library doc states:

"libtool’s versioning scheme is described by three integers:
current:revision:age.

• current is the most recent interface number implemented by the library.
• revision is the implementation number of the current interface.
• age is the difference between the newest and the oldest interface that the library implements.

Beware that there is also another similar s/w called fswatch (that is go-related I think).

See also Watchman (based on inotify), especially when complex monitoring and action-triggering is involved. It’s available for not just Linux, but also Windows and Mac. See more at A: Watchman Q: How to monitor a complete directory tree for changes in Linux?.

Found this related blog post which has a really useful inotifywait section. Here is the cool demo script from this blog that is a variation on the selected answer.

#!/bin/bash

file_to_wait=$1
directory=$(dirname ${file_to_wait})
filename=$(basename ${file_to_wait})
 
while read f
do 
  if [ “$f” = ${filename} ]
  then 
    break
  fi
done < <(inotifywait –m -q -e create –-format %f ${directory})

echo “${file_to_wait} has been created”