When a daemon misbehaves in production, you need reliable tools to diagnose and resolve issues quickly. Three essential Linux utilities for this purpose are strace, lsof, and journalctl. Each serves a unique role in debugging running daemons. strace allows you to trace the system calls and signals made by a process, giving you insight into what the daemon is doing at the operating system level. lsof lists open files and network sockets held by a process, which is invaluable for understanding resource usage or detecting leaks. journalctl lets you view logs from systemd-managed daemons, providing a timeline of events, errors, and warnings that can help pinpoint problems.

Techniques for tracing stuck or misbehaving processes

When a daemon appears stuck or is not behaving as expected, you can use these tools in combination to uncover the root cause. Start by identifying the process ID (PID) of the daemon, often available in its PID file or by using ps or systemctl status. With the PID in hand, strace can attach to the running process and display real-time system calls, revealing where the daemon may be hanging, such as waiting on I/O or a network response. If you suspect resource exhaustion or file descriptor leaks, lsof will show all files, sockets, and pipes opened by the daemon, helping you spot unusual patterns. Meanwhile, journalctl provides a historical record of log messages, including any errors or warnings emitted by the daemon or systemd, which can offer clues about recent failures or misconfigurations.