The systemctl command provides a subset of the functionality of Systemd's systemctl; specifically, most of the service-related commands.
systemctl --help
systemctl [options] version
systemctl [options] daemon-reload
systemctl [options] <command> service [...]
The following options are supported:
The --debug option causes certain operations to produce additional output useful to developers.
Note that this option is not supported by the real systemctl.
The --help option causes systemctl to print a brief usage message and immediately exit.
When invoked with --root=path, systemctl will perform all operations relative to the specified path instead of the filesystem root.
Note that this is not suitable for preparing a chroot environment or similar as all symlinks created by systemctl have absolute targets.
The --quiet option suppresses output from is-active, is-enabled, and status.
The --verbose option causes certain operations to produce additional output. Most notably, any operation that invokes a service's init script will suppress the script's output unless the --verbose option was specified.
Note that this option is not supported by the real systemctl.
The --version option causes systemctl to print a fictive version number in a format similar to the one used by the real systemctl and immediately exit.
Every command that operates on a service will print an error message and return a non-zero exit code if the service does not exist, i.e. there is no file with that name in /etc/init.d.
Every command that operates on a service can operate on multiple services. The output will be the same as if the command was executed for each service individually. The exit code varies from one command to another and is not always what you expect.
The following commands are currently supported:
The daemon-reload command does nothing.
The status command prints information about a service in human-readable form and returns an exit code which conforms vaguely to the Linux Standard Base conventions for init scripts. It returns a non-zero exit code if it fails to invoke the init script or the init script returns a non-zero exit code, and zero otherwise.
When operating on multiple services, the status command returns a non-zero exit code if any of the individual operations fail, and zero otherwise.
The is-enabled command print a single word indicating whether a service is enabled. It returns a non-zero exit code if a filesystem operation fails, if it fails to invoke the init script, if the init script fails, or if the service is disabled, and zero otherwise.
The is-active commands print a single word indicating whether a service is active (i.e. running). It returns a non-zero exit code if a filesystem operation fails, if it fails to invoke the init script, if the init script fails, or if the service is stopped or dead.
When operating on multiple services, the is-enabled and is-active commands return a non-zero exit code if all of the individual operations fail, and zero otherwise.
The enable command creates symlinks to the service's init script in the rc directories for runlevels 2 through 5. It also deletes any existing symlinks, to avoid duplicates.
The disable command deletes any symlinks to the service's init script that it finds in the rc directories for runlevels 2 through 5.
Both commands return a non-zero exit code if a filesystem operation fails, and zero otherwise. When operating on multiple services, they return a non-zero exit code if any of the individual operations fail, and zero otherwise.
The start command starts a service by invoking its init script if it is not already running, and does nothing if it is.
The stop command stops a service by invoking its init script if it is running, and does nothing if it is not.
The restart command restarts a service if it is running, or starts it if it is not.
The try-restart command restarts a service by invoking its init script if it is running, and does nothing if it is not.
The start, stop, and try-restart commands start by performing the equivalent of a status command, and return a non-zero exit code if they fail to invoke the init script or interpret the result. If they determine that no further action is necessary, they immediately return an exit code of zero. If not, they attempt to invoke the requisite command and return a non-zero exit code if they fail to invoke the init script or the init script returns a non-zero exit code, and zero otherwise.
The restart command returns a non-zero exit code if it fails to invoke the init script or the init script returns a non-zero exit code, and zero otherwise.
When operating on multiple services, all four commands return a non-zero exit code if any of the individual operations fail, and zero otherwise.
The reload command reloads a service by invoking its init script.
The reload-or-restart command reloads a service by invoking its init script, and if that fails, restarts it instead. If the service is not running, it will be started.
The reload-or-try-restart command is a compatibility alias for try-reload-or-restart.
The try-reload-or-restart command reloads a service by invoking its init script if it is running, and does nothing if it is not. If the service is running but the reload fails, it restarts the service instead.
The reload-or-restart returns a non-zero exit code if it fails to invoke the init script or its the final invocation returns a non-zero exit code, and zero otherwise.
The try-reload-or-restart command first performs the equivalent of a status command, and returns a non-zero exit code if it fails to invoke the init script or the init script returns a non-zero exit code. Otherwise, it performs the equivalent of a reload-or-restart command, with the same results.
When operating on multiple services, all four commands return a non-zero exit code if any of the individual operations fail, and zero otherwise.
The show command prints each listed service's service file to standard output. It returns a non-zero exit code if any of the service files can't be found or contains a syntax error, and zero otherwise.
For the purposes of enabling and disabling services, the definition of a link to the service's init script is a link whose target has the same device and inode numbers as the init script. This can lead to surprising results if multiple services are symlinked to a single script that uses $0 to differentiate between them. Don't do that.
The reason for comparing targets rather than names is that other management tools such as chkconfig may create links with names that include additional information, such as a numerical prefix.
Commands that directly or indirectly query the service's current status (is-active or try-reload-or-restart, for instance) assume that the init script conforms to the Linux Standard Base Core Specification. There is no shortage of real-world examples of scripts that don't. At the time of writing, one such example is the Oracle VirtualBox guest additions service: its status command returns 0 regardless of whether the service is running.