Ubuntu – How to send a notification using at command

You are using two different things here and should be using a third. Let's see:

1. | : This is the pipe operator, it serves to pass the output of one process as input to another:

   foo | bar
   

   This runs the program foo and passes its output as input to the program bar.

2. >,<,>> and <<: These are the redirection operators, they serve to send data to/from files:

   • foo > bar : runs the program foo and saves its output to the file bar, overwriting¹ its contents and creating it if it does not exist.

   • foo >> bar : runs the program foo and saves its output to the file bar, appending to its contents and creating it if it does not exist.

   • foo < bar : runs foo, telling it to read input from the file bar.

   • The << is a special case, since there is no point in "appending" input to a command, the << is primarily (exclusively AFAIK) used for Here Documents:

     $ cat << EOF > file.txt
     > Hello World!
     > EOF
     

     The construct << SomeStringHere > Out.file will redirect all text written until it encounters the ending string (EOF in the example above) to the target file. Here docs allow you to easily format multi-line strings and include variables and special characters.

3. The <<< operator, the Here String, is like a Here Document but it expands variables. So, for example:

   grep foo <<< "$bar"
   

   The command above is equivalent to echo "$bar" | grep foo.

4. What you are actually looking for is called process substitution and is another way to pass the output of a command to another. It consists of <(command).

   foo <(bar) 
   

   So, for your at example, you could do

   at now < <(echo "notify-send HELLO")
   

   The above works because process substitution actually creates a file (read the link above for more details) and it is the file descriptor of that file that is passed with < to at now.

¹ The default behavior is to overwrite, this can be modified by setting the noclobber option to bash. If set, echo foo > bar will fail if bar exists. In that case, it can be forced using echo foo |> bar instead. See section 3.6.2 here.

You format text with html code

notify-send -i terminal "<font size=16 color=blue><b><i>'bash -c ls'</b></i></font>"  "execution complete"

change what is in the single quotes with the command you wish, feel free to change the size and color to what you wish.

If you need assistance, post your code and more detailed information on what you wish to change