How can i print message at console

So, i've added some codes in order to show somewhat like debug-log at existing program's code.

(existing codes)
printf("wan port is down.");

like this.
I'm using UART to see and control the OpenWRT, and i can't see any messages shown at my console.

if i manually type the command to execute that program, i can see it.
also if i add & at the end, i still can see it.
in order to type some commands while that program is running, i should be typing with &

this program is executed by the cron, just after booting.
inside cron file, i can see /usr/sbin/[program] & to run this program.
note that this program is 24/7 alive while my router is powered on.

there are no other files or programs that executes this program.
the only thing that runs this program is cron, and this uses /usr/sbin/[program] & to run it.

How come i cannot see any messages at my console?
and how come i can see the message when i manually type /usr/sbin/[program] & command?

i also tried changing my codes to

system("echo wan port is down.");

instead if using printf. Same result when i was using printf.

what can i do?

Just use echo? The cron job is just run with ash, so it's like typing it into stdin, or logger perhaps since it would run in it's own thread?


So you mean i need to use logger instead of using echo right?
i'll try like this.

system("logger -s wan port is down");

thanks for the help

not working. i still need to manually type the command to execute this program to show at my console.

If you are running a cronjob, it won't show anything on the console. The console is it's own terminal space. The exception to this would be kernel messages, i believe. If your script toggled the ethernet devices, you'll see the Ethernet control messages

For an automated cronjob you should use logger to send whatever it is you are trying to see to the system logs. If you are trying to debug, typing it into the console will give you the output, allowing you to perfect your syntax (although for mullti-line items, consider a bash script and just call the script via cron)


So, you are saying...
In order to get my program's log which is executed by cron, i need to add

system("logger -s messages");

at the code, and i need to check the log file, right?
i searched this and i figured out that those logs are located at var/log/message
i cannot find it. my var is a symlink to tmp directory. So i searched at tmp/log/, but i cannot see anything.

can you tell me where those logs will be saved?


So i need to manually set the path where my logs will be saved.
Thanks a lot!!!

  1. from C code you should use directly the syslog(3) function which allows formatting like printf.
  2. from shell script you should use logger -p priority message
  3. everything you print with printf() goes tho the standard output of the program, NOT specifically to the console! So it depends on how you managed your standard output. If you redirect the standard output to point to /dev/console (or whatever device your console is connected to, mine is /dev/ttyS0) it would have worked.
  4. you also could put in the crontab something like your_prog | logger -p info and use printf() in your program, since like this stdout is redirected to the logger.
  5. to get the last log lines, use the logread program.

@jpt @Grommish
if i start this program not from cron, but from init.d, still printf or echo will not show a message to my console?

I guess it won't, hope i can check this by myself, but i cannot check my router now.

Actual purpose of this printf or echo is not for logging. it's just to indicate developers to know easily the port is down or up while we are looking at the console. so we don't have to physically look at the router.

Ok, now that I know you are trying to log Interface changes, have you looked at /etc/hotplug.d/iface?

Also, Ethernet Interface changes will show up in dmesg (and therefor, you should see it on the console):

[   55.530104] br-lan: port 1(eth1) entered blocking state
[   55.535353] br-lan: port 1(eth1) entered disabled state
[   55.540852] device eth1 entered promiscuous mode
[   55.558625] br-lan: port 2(eth2) entered blocking state
[   55.563890] br-lan: port 2(eth2) entered disabled state
[   55.569406] device eth2 entered promiscuous mode
[   55.574131] br-lan: port 2(eth2) entered blocking state
[   55.579404] br-lan: port 2(eth2) entered forwarding state
[   55.584938] IPv6: ADDRCONF(NETDEV_CHANGE): br-lan: link becomes ready
[   55.625879] eth0: 1000 Mbps Full duplex, port 0, queue 0
[   55.631576] IPv6: ADDRCONF(NETDEV_CHANGE): eth0: link becomes ready
[   56.607746] br-lan: port 2(eth2) entered disabled state
1 Like

here is the way how i changed my code.

(C codes)
system(echo test_message > /dev/console);

Therefore, i can see the test_message at my console.

Still, thanks a lot for the help guys. I learned a lot from that.
@Grommish @jpt

1 Like

This topic was automatically closed 10 days after the last reply. New replies are no longer allowed.