what's this bash redirection operator? "<<!"

Question

I saw the following bash code:at 19:00 <<! echo "job 1". I have two problems:

1. What's this redirection operator: <<!?

2. I wrote the following script code:

   at 19:00 <<!
       echo "job 1"
   
   at 20:00 <<!
       echo "job 2"
   

   When I executed this script, atq command only showed one job, the first one. What's the matter? And how should I submit the two jobs via this script correctly?

Answer 1

From bash reference manual

3.6.6 Here Documents

This type of redirection instructs the shell to read input from the current source until a line containing only word (with no trailing blanks) is seen. All of the lines read up to that point are then used as the standard input for a command.

The format of here-documents is: 
     <<[−]word
             here-document
     delimiter

So

1. You shouldn't need to specify anything after word(in your case !)
2. Then you should specify at job on one or more lines
3. Finally, add a line containing word(again, in your case !)