Using shell with cron

Question

I have an app controlling a piece of connected hardware that displays a few lines of info and requires user input (on stdio). The app runs fine from a shell.

I set up the RPi to auto-login with CLI and run the app as a cron job shortly after reboot. It runs (as evidenced by it interacting properly with the attached hardware) but I cannot see the output and there is no way to enter input; it's running in the background, not interacting with the open shell.

How can I make it interact with the open shell, or start a new one?

Answer 1

If you want it to keep a process running so you can use it then install the screen program then use screen at the start command in the cron job. This is where you use the /usr/bin/screen -d -m -fa -S Screen_Name_For_Instance /usr/bin/name_of_server_program to start it up. You can test the command at your command line. Once happy it runs correctly then the CTRL key held down then the a and next d keys pressed while it is held down will detach the screen and leave it running for you. A screen -r will get you back into a running single session screen instance if more than one you need to know the process ID number it tells you when detaching to use screen -r 12345 for process 12345 or if you use the naming option when starting it then screen -r Name. The CTRL key held down then the a and k it is held down will kill off the running session you are in and the program running in it. Or at the command line knowing the process ID number kill 12345. A nice page on it here.

https://stackoverflow.com/questions/4847691/how-do-i-get-out-of-screen-without-typing-exit

Answer 2

Since you are using stdio, a fifo might be a solution for your input. For example, open two terminals and do:

terminal 1:                               |terminal 2:
$ mkfifo fifo                             |
$ tail -f fifo | sed 's/a/A/'             |$ echo hoppa > fifo
hoppA                                     |

So, you could create a fifo in your application directory

mkfifo /your/application/directory/fifo

and use that as to give commands to your application.

Normally, for background jobs, you would redirect the output to some log file, which you could then cat, tail -f or whatever you want.

In all, your crontab entry would be:

@reboot tail -f /your/application/directory/fifo | your_app | logger

When you want to provide input to your application, you do:

echo "Do something" > /your/application/directory/fifo

Here, threw the output on the syslog, so

tail /var/log/messages

would give you the output.