Understanding the output of Cron

Jhon Doe

12/6/22, 8:00 AM

I'm having trouble understanding the output of cron. Suppose I have a cronjob that runs every five minutes:

*/5 * * * * root /home/user/Desktop/shell.sh

shell.sh has the following contents:

#!/bin/bash
echo "hello"
bash -i

I have the following questions:

Where is the job run? E.g. is a new TTY process started where the shell script is run?
Where is the output sent? I've read that the output is sent to the mail of the owner. Does this mean root gets a mail with the echo statement?
What about the bash command? Does the script start a bash process in its controlling terminal session which remains running until the terminal session ends (at the end of the script?)?
Suppose I have reversed the shell code in the above bash script and I run a nc listener on my computer. Will this mean that every 5 minutes I will receive a connection that closes as soon as the script finishes running?

Apologies for the numerous questions. I'm new to this stuff. Thanks.

1 + 0

command-line

bash

scripts

cron

Score:0

Ubuntu

ob2

12/6/22, 8:44 AM

crontab is not exactly working like a classic shell.

The job is run as root, but without any TTY. The command is run into a non-interactive environment, so bash -i probably returns an error or just closes once executed.
Output is usually sent by mail. Ubuntu is usually shipped with a very minimal mail system called exim4. Look if file /var/mail/<your username> exists. You can try to read mail with:
```
mail -u <your username>
```
Because your script starts with #!/bin/bash, bash will be used, but this is not run in any terminal. Unless your script contains a loop, the process ends at the end of the script.
I'm not sure I understand the use case with nc, but if you run nc from crontab to connect and send a message to your computer, nc and bash will end as soon the command is done.

0 + 5

Jhon Doe

12/6/22, 8:47 AM

Hello for Q.3 I was referring to the bash -i command. For Q.4 I'm not running a listener in cron. Only the reverse shell is running in cron

ob2

12/6/22, 8:54 AM

Why didn't you described what you are trying to achieve. Are you looking for a way to take control of server with nc command ? Q3) bash -i will have no more effect, as explained, and cron is not run under interactive shell. Q4) I understood the listener is run on your computer and nc command is run a client and connect to your computer every 5 minutes.

Jhon Doe

12/6/22, 9:11 AM

Thank for the reply. So I have nc -lvnp 8000 running on computer A and suppose on computer B I have a cron job that executes with root. If I input a reverse shell code in the cron job will I have a reverse shell connection every 5 minutes that ends every time the script finishes running?

ob2

12/6/22, 10:07 AM

I'm not sure how should work the reverse shell code, do you already done it without cron by running the commands manually ?

Jhon Doe

12/6/22, 1:51 PM

Hey it's fine. Ive decide to ask a more specific question on the information security stackexchange. But thanks for your answer anyway.

Elon Musk

I sit in a Tesla and translated this thread with Ai:

EN: Understanding the output of Cron

TH: ทำความเข้าใจกับผลลัพธ์ของ Cron

RO: Înțelegerea rezultatelor lui Cron

RU: Понимание вывода Cron

VI: Hiểu đầu ra của Cron

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