Terminology

• PID - Process ID; the numeric identifier for a running process.
• PPID - Parent Process ID; the process that started another process.
• Daemon - A long-running background process.
• Init - The first userspace process started at boot; on most modern systems this is systemd.
• Unit file - A systemd config file describing a service, socket, timer, or other managed object.
• journald - The system service that collects and stores structured logs.
• stdout / stderr - Standard output and standard error; many services write logs here and let systemd collect them.
• Signal - A message sent to a process, such as TERM, KILL, or HUP.
• Exit code - The numeric status a process returns when it ends.
• PATH - The environment variable that controls where the shell looks for commands.
• Permission bits - The read, write, and execute flags on files and directories.
• Owner - The user that owns a file.
• Group - The group associated with a file or process.
• Root - The superuser account with full privileges.
• Socket - An endpoint a program uses for network or local IPC communication.
• Listener - A process waiting on a port or socket for incoming connections.
• Mount - Attaching a filesystem at a path in the directory tree.
• Inode - Filesystem metadata for a file; you can run out of inodes even if bytes remain.
• Package manager - The tool that installs, upgrades, and removes software packages.
• DNS - The system that maps names to IP addresses.
• Loopback - The local host network interface, usually 127.0.0.1.
• TTY - A terminal device or session.