lf343, UNIXBasics: Why does this not work!? How to find and fix faults in Linux applications.

lf343, UNIXBasics: Why does this not work!? How to find and fix faults in Linux applications.

Logs
The most obvious and simplest thing you can do is to look at file in /var/log/... What you find in those files and what the names of those logs files are is configurable. /var/log/messages is usually the file you want to look at. Bigger applications may have their own log directories (/var/log/httpd/ /var/log/exim ...).
Most distributions use syslog as system logger and its behavior is controlled via the configuration file /etc/syslog.conf The syntax of this file is documented in "man syslog.conf".

Logging works such that the designer of an program can add a syslog line to his code. This is much like a printf except that it writes to the system log. In this statement you specify a priority and a facility to classify the message:

#include

void openlog(const char *ident, int option, int facility);
void syslog(int priority, const char *format, ...);
void closelog(void);

facility classifies the type of application sending the message.
priority determines the importance of the message. Possible
values in order of importance are:

LOG_EMERG
LOG_ALERT
LOG_CRIT
LOG_ERR
LOG_WARNING
LOG_NOTICE
LOG_INFO
LOG_DEBUG

With this C-interface any application written in C can write to the system log. Other languages do have similar APIs. Even shell scripts can write to the log with the command:

logger -p err "this text goes to /var/log/messages"