How to Monitor Changes Within a File

Question

I am looking to monitor content changes within a file and when that content changes, produce a notification displaying the new text.

Details

Let us say the file (connection.txt) contained only the words "You are connected to the internet".

However, when the computer disconnects from the internet, the contents of the file change to "You are now disconnected from the internet".

How would you monitor the contents of the file and, only when the contents change, display the new text in a desktop notification (I am using Manjaro).

To add to the problem, this file is updated every four seconds, often with exactly the same text.

My research

I have searched for solutions to this with no avail. I have found many examples of monitoring files and directories for event changes such as files created, modified or deleted, but nothing on real-time monitoring of text inside a file.

If this is possible, are there similar techniques for monitoring changes to the output of a command?

Answer 1

For the general case you want to monitor a file, and send a desktop notification with the new file content only when the content has changed, you can use inotifywait (from inotify-tools) with the -m, --monitor option to execute indefinitely.

--format "%e" will print only the type of the event to the next command.

notify-send, from libnotify for desktop notifications, is being used to send the notification only if the file content is modified.

#!/bin/bash

f="filename"
curr=$(<"$f")

inotifywait -m -e modify "$f" --format "%e" | while read -r event; do
    if [ "$event" == "MODIFY" ]; then
        prev="$curr"
        curr=$(<"$f")
        [ "$curr" == "$prev" ] || notify-send "Title" "$curr"
    fi
done

---

For your specific case, I would not monitor changes to files, if your goal is to display a desktop notification with a text like "you are connected" or "you are disconnected". I would modify the place where you print that text (every N seconds as you say) into that file, to something like this:

while true; do
    prev="$curr"
    curr=$( <here you output the new text> )
    [ "$curr" == "$prev" ] || notify-send "Title" "$curr"
    sleep 4
done

Answer 2

I would like to share a skeleton code. You might expand on this idea.

#!/bin/bash 

# monitor_changes
#
#     notifies changes to FILE passed as first parameter $1
#     uses tail -1 to return last line of the file

# first run -- save last line on variable old
old=$(tail -1 $1) 

# infinite loop 
while : ; do
    sleep 1
    # read again last line
    new=$(tail -1 $1) 

    # this is where the magic should happen
    [[ "$old" != "$new" ]] && echo "NOTIFY: $old --> $new"

    # save for next round
    old=$new
done

Answer 3

inotify to catch the (potential) changes. Just because the file is modified, does not mean that is is changed.

make to act of file changes. It works with file date, so this needs to be reliable. You also need an output file, this can be a dummy that you create after writing to the screen.

Other tools to do the 2nd part: e.g. Using file hashes (instead of dates), if the file is small, keeping a copy and using cmp.

an example of usage

#!/bin/bash

while true
do
    inotify-wait --event modify source-file.txt
    make $(basename source-file .txt).stamp
    #there is a race hazard here: if the file changes again before we get back to the wait, then it may not be picked up.
done

# A make file template
%.stamp: %.txt
«tab» do_it $<
«tab» touch $@

Replace «tab» with a tab. Replace do_it with the code that does it. The $< will be replaced with the source file-name.