![]() ![]() Tail: cannot watch ‘/var/log/messages’: No space left on device As a result, if you’re monitoring lots of files on Linux, you may run into the following message: A number of familiar tools, like tail and its -f option, use this API to continuously output the contents of files as they grow. The inotify API is used for real-time log monitoring in Linux, and allows applications to build watch lists that detect when specific events ( watches) occur for itemized files and directories. If log messages are written to stdout, you can use I/O redirection to append to the log file with the “ >” operator, like so: $ app01 -debug > debug.log 3. Thankfully, you don’t need to modify the application to work around this. This is due to the application not opening the log file in append mode, which means that when it writes to the truncated log file, it continues writing at the last file position instead of starting from the beginning. If you configure logrotate to truncate log files with the copytruncate directive, applications writing to that file can end up filling it with zeros. However, some applications do not handle truncation properly. Tools such as logrotate can automatically make copies of logs once they reach a specific size, and either create a new log file or truncate the existing one. ![]() ![]() Log rotation prevents log files from growing too large and keeps their size manageable. And it’s not just permissions to access individual files that logging tools need-often they also need access to write to the directories containing those logs when creating new files. To fix this misstep, make sure the user running the daemon has permission to access all logs. Occasionally, however, this practice can cause issues with log permissions when the daemon’s user ID cannot access its own logs. It’s common practice to run daemons as non-root users to prevent the class of security issue known as privilege escalation. Whether you’re an old hand or a relative newcomer, here are seven tips on how to monitor log files in Linux that you may have overlooked. But no matter how familiar you are with Linux log monitoring, even gurus of the command line can learn new tricks. Logs provide a window into understanding the health of your systems, and they’re the first place to look when things aren’t working. Monitoring application and Linux system logs is a skill that every seasoned SysAdmin has down cold. ![]()
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