Start, stop, restart, enable, disable, and inspect systemd-managed services, and understand how service state relates to boot-time behavior.
systemctl startsystemctl stopsystemctl statussystemctl enablesystemctl disablesystemd is the init system and service manager on RHEL — the first process the kernel starts (PID 1), responsible for bringing the rest of the system up and managing every service afterward. Everything systemd manages is represented as a unit.
| Unit Type | Extension | Manages |
|---|---|---|
| Service | .service | A background process / daemon |
| Socket | .socket | A network or IPC socket, often used to start a service on demand |
| Target | .target | A group of units representing a system state (like a boot milestone) |
| Timer | .timer | A scheduled trigger, similar in purpose to cron |
| Mount | .mount | A filesystem mount point |
sshd to networking to logging — is a systemd unit under the hood. systemctl is the one tool you'll reach for constantly, across nearly every administrative task.
# Check whether a service is currently running, and recent log lines systemctl status sshd # Start a service right now (does not affect boot behavior) sudo systemctl start sshd # Stop a running service sudo systemctl stop sshd # Restart a service (stop, then start) sudo systemctl restart sshd # Reload configuration WITHOUT dropping active connections, if supported sudo systemctl reload sshd # Reload if supported, otherwise fall back to a full restart sudo systemctl reload-or-restart sshd
reload is gentler when available — it applies configuration changes without interrupting active connections (important for something like sshd with sessions already open). Not every service supports it; restart always works but briefly drops the service entirely.
Whether a service is currently active and whether it's enabled to start at boot are two entirely separate settings.
# Enable a service to start automatically at boot (doesn't start it now) sudo systemctl enable sshd # Enable AND start it immediately, in one command sudo systemctl enable --now sshd # Disable a service from starting at boot (doesn't stop it now) sudo systemctl disable sshd # Check whether a service is enabled systemctl is-enabled sshd # Check whether a service is currently active systemctl is-active sshd
Masking goes a step further than disabling: it links the unit file to /dev/null, preventing the service from being started at all — even manually, even as a dependency of another unit.
# Prevent a service from being started by any means sudo systemctl mask telnet.socket # Reverse it sudo systemctl unmask telnet.socket
# List all currently loaded units systemctl list-units # List all service units, including inactive ones systemctl list-units --type=service --all # Show the raw contents of a unit file systemctl cat sshd # Show detailed properties of a unit systemctl show sshd # List every failed unit — a fast health check systemctl --failed
systemctl --failed is often the fastest way to see whether a specific service is the culprit before digging further.
systemd's logging component, the journal, captures output from every unit. A full treatment comes in a later chapter, but a couple of commands are worth knowing now while troubleshooting service state.
# Show recent log entries for a specific unit journalctl -u sshd # Follow a unit's log entries live journalctl -u sshd -f
.service)journalctlhttpd to start at boot AND starts it immediately.systemctl restart and systemctl reload?systemctl disable not be enough to guarantee a service never starts, and what command would guarantee it?sshd service.