Linux Fu: Easier File Watching

In an earlier installment of Linux Fu, I mentioned how you can use inotifywait to efficiently watch for file system changes. The comments had a lot of alternative ways to do the same job, which is great. But there was one very easy-to-use tool that didn’t show up, so I wanted to talk about it. That tool is entr. It isn’t as versatile, but it is easy to use and covers a lot of common use cases where you want some action to occur when a file changes.

The program is dead simple. It reads a list of file names on its standard input. It will then run a command and repeat it any time the input files change. There are a handful of options we’ll talk about in a bit, but it is really that simple. For example, try this after you install entr with your package manager.

1. Open two shell windows
2. In one window, open your favorite editor to create an empty file named /tmp/foo and save it
3. In the second window issue the command: echo "/tmp/foo" | entr wc /tmp/foo
4. Back in the first window (or your GUI editor) make some changes to the file and save it while observing the second window

If you can’t find entr, you can download it from the website.

Frequently, you’ll feed the output from find or a similar command to entr.

What It Isn’t

I had mentioned incron before as a way to attach actions to file changes. It also makes things easier, although perhaps not as easy as entr. As the name suggests, incron does for filesystem changes what cron does for time. That is, it causes some action to occur any time the specified file system changes happen. That survives a reboot or anything else short of you canceling it. This is very different from entr. With entr, the command runs like a normal application. As long as it is running, changes in the target files will trigger the specified action. When you stop the program, that’s the end of it. Usually, you’ll stop with a Control+C or a “q” character.

Speaking of the keyboard, you can also press space to trigger the action manually, as though a file changed. So, unlike incron, entr is an interactive tool. You’ll want it running in the foreground.

Options

There are several command line options:

• -c – Clear screen before executing command
• -d – Track directories that do not start with “.”
• -p – Do not execute the command until an event occurs
• -r – Kill previously run command before executing command
• -s – Use shell on first argument

The /_ placeholder gets the name of the first file that caused a trigger, although that doesn’t seem to work properly with -s. For example:

find /tmp/t/data -name '*.txt' | entr cp /_ /tmp/archive

When one of the .txt files changes, it will copy to /tmp/archive.

The -d option has a peculiarity. With it you can use a directory name or a file name and the program will watch that directory along with any files. The file changes behave as normal. However, any new files in the directory will cause entr to stop. This lets you write things in a loop like this:

while true; do
  ls -d src/*.[ch] | entr -d make
done

The loop ensures that entr is always looking at the right list of files. This will also cause an error exit if you delete one of the files. The ls command provides all the .c and .h files in the src directory. The command is smart enough to infer the directory, so you don’t need to set it explicitly.

Missing Changes

The -r option is good if you are running a program that persists — for example, you might use kdiff3 to show the differences between a recently changed file and an original copy. This option causes entr to kill the program before starting a new one. Without this flag, it is possible, too, for entr to miss a file change. For example, make a file called foo and try this:

echo foo | entr -ps 'echo change; sleep 20'

In another shell, change foo.  You’ll see the change message print on the original shell. If you wait 20 seconds you’ll see something like “bash returned exit code 0.” Now change foo again, but before you see the bash message, change it again. Once the 20 second timer expires, entr will go back to waiting and the new change will not cause a trigger!

Exit entr and start it again with the options -psr instead of -ps. Now do the same test again. You’ll see that the change registers and the original bash script never completes. However, if there are no changes for 20 seconds, the last script will exit normally.

Examples

There are plenty of examples in the tool’s man page. For example:

find src/ | entr -s 'make | head -n 20'

This gets a list of all files in the source tree (including subdirectories) and when any change, you run make. The head command only shows the top 20 lines of output.

A lot of editors make automatic backup of files, but if yours doesn’t it would be pretty simple to make an auto archive with entr, although, honestly, use git or something if you want real version control:

echo testfile.txt | entr -s 'AFN=/tmp/testfile-$(date).txt; cp testfile.txt "$AFN"; zip -j archive.zip "$AFN"'

This stores each version of testfile.txt in archive.zip along with a timestamp on the file name:

It was tempting to use /_ in this script, but it doesn’t seem to work with the -s option very well.

Embrace Change

I doubt I’ll use entr as much as I use incron. However, for little one-off projects, it is pretty handy and I could see making some use of in that case. However, as usual, for any given task there are usually many ways to accomplish it. Having entr in your toolbox can’t hurt.

There are still other ways. For example, last time, someone mentioned in the comments that systemd can take a “path unit” that can trigger when a file or directory appears or changes. This still uses inotify internally, so it is really just another wrapper. Still, if you like systemd, it is a consistent way to set up something similar to incron but under the tentacles of systemd.

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