The systemd journal stores messages with structured metadata in journal files. That makes service output searchable without first inventing a parser for every application’s text format. Keeping those messages for a useful period is a separate requirement. Storage location, size policy, rotation, and forwarding determine which evidence remains available when an incident is investigated later. ...
Inotify delivers Linux filesystem notifications through an instance with registered watches and a readable event queue. Each record identifies a watch, event mask, and possibly a name and move cookie. This can avoid repeatedly checking unchanged paths, but the stream is not an unlimited audit log. Correct consumers interpret notifications as evidence that relevant state may need to be inspected again. ...
A hard link adds another directory entry referring to an existing file. Neither name is intrinsically the original: both identify the same inode within its filesystem. File content, ownership, permissions, and link count belong to that shared object. A filename is a way to reach it, so changes to directory entries must be distinguished from changes to the file itself. ...
Cron matches calendar fields and launches commands at matching times. It does not inherently wait for an earlier invocation of the same command to finish. If a minute-level task takes seventy seconds, the next minute can launch another copy. Treat trigger policy, concurrency policy, and successful completion as separate responsibilities when operating recurring work. ...
/tmp is a shared namespace of names, not a private scratch area. Any local user can create the name your process is about to use, so a name is not a claim on a file, and hier(7) says the contents may be deleted with no notice. ...
A process group is a kernel-maintained set of processes identified by a process group ID, or PGID. It provides a target for operations such as delivering a signal to a job. Parent-child relationships and group membership are separate: related processes can occupy different groups, and a group’s leader does not need to remain alive for other members to continue belonging to it. ...
A symbolic link is a filesystem object containing a target pathname. Following the link resolves that pathname at the time of the operation; it does not bind the link permanently to the target’s inode. This distinction explains why links can cross filesystem boundaries, become dangling, or start reaching a different object even though their stored target text has not changed. ...
A shell’s ulimit builtin changes resource limits for that shell and the processes it subsequently starts. For file descriptors, RLIMIT_NOFILE has a current soft value and a hard ceiling. Understanding where those values are inherited is essential when a terminal reports a generous limit but an independently managed service still fails to open files or accept connections. ...