system() not working in C on cron job

Question

The system() function does not work when the program is run from a cron job, but it works fine when I manually execute the program.

C Code:

#include <time.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(){

char command[256];

strcpy(command, "mpg123 /home/vlad/MIA/Alarm/test.mp3");
printf("Commnd Executed");
system(command);

return 0;
}

Crontab File:

*/1 * * * * /home/vlad/MIA/Alarm/test >> /home/vlad/MIA/debug_test.txt

Not at all, I have a bigger program that has the issue, but I can not even seem to get a simple program to work,that is why I posted that small c program. — Vlad, Apr 01 '17 at 00:41
This sounds like a permission or context issue. I doubt you can directly access audio from cron. — Tatsuyuki Ishi, Apr 01 '17 at 00:44
I do not think it is a permission issue, I just gave it 777 and still nothing — Vlad, Apr 01 '17 at 00:47
To see whether your program is being executed at all, try adding this line to your program: `system("echo Job is running >> cron.out");` — Keith Thompson, Apr 02 '17 at 20:07
Most of cron debugging can be solved by `echo date >>/tmp/omg.date` — wildplasser, Apr 02 '17 at 22:40
If an answer solved your problem, you can accept it by clicking the green check mark. — Keith Thompson, Apr 03 '17 at 20:19
@KeithThompson is there anyway to export env variables in C like you would in a bash script? — Vlad, Apr 04 '17 at 20:31
`man 3 setenv`, `man 3 putenv` (both are POSIX-specific, not standard C, but that's not likely to be an issue). You can also set environment variables in your crontab; `man 5 crontab` for details.It looks like your original problem was solved; are you planning to accept an answer? — Keith Thompson, Apr 04 '17 at 20:34

Keith Thompson · Answer 1 · 2017-04-02T22:17:14.913

Cron jobs run with a limited set of environment variables.

To see what the environment looks like, you can create a temporary cron job like this:

* * * * * printenv > cron-env

The most likely culprit is the $PATH variable. It's likely that the mpg123 command is in a directory that's in your $PATH in an interactive shell, but is not in the default $PATH provided to cron jobs.

UPDATE : I see that the mpg123 command is provided (on Ubuntu) by the mpg321 package, which presumably installs it as /usr/bin/mpg123. But I don't know what the command does, or how you can tell that it didn't run. Your first step in debugging this should be to try running a simple command so that you can clearly tell whether it ran or not. And just be sure, you are feeding your crontab to the crontab command, yes? If so, it should appear in the output of crontab -l.

Turns out you were right, it was the environmental variables, I did more research and I was able to get it all to work but have a script set the variables then launch the program. Here is how I got it to work: export DISPLAY=:0.0 export XAUTHORITY=/home/vlad/.Xauthority #Runs main Alarm check C program. /home/vlad/MIA/Alarm/Alarm_Main — Vlad, Apr 03 '17 at 17:40

score 0 · Accepted Answer · answered Apr 04 '17 at 20:54

It seems it was a combination, the cron job was not running, I was able to figure that out with the help of Keith Thompson, as well as cron job needed some evnormental variales. The work around I did was to lanuch the C program from a script with the variables needed.

.sh file

 #/bin/bash 

##Needed to have cron play an audio file
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin
export DISPLAY=:0.0
export XAUTHORITY=/home/vlad/.Xauthority 

#Runs main Alarm check C program. 
/home/vlad/MIA/Alarm/Alarm_Main

system() not working in C on cron job

2 Answers2