Logs are not appearing in the default /var/log/messages file on an SRX device.

First, verify whether your syslog configuration already includes the messages file. If it doesn’t, enter configuration mode and add:

set system syslog file messages any any

This command directs all facilities at all severity levels into /var/log/messages.

To restrict logging to specific severity levels, you can filter by replacing the second any with your desired level. 

root@SRX# set system syslog file messages any ?   

Possible completions:

 alert        Conditions that should be corrected immediately

 any         All levels

 critical       Critical conditions

 emergency      Panic conditions

 error        Error conditions

 info         Informational messages

 none         No messages

 notice        Conditions that should be handled specially

 warning       Warning messages