systemctl: is-enabled output table and mask command

────────────┼───────────┤ │ "alias" │ The name is an alias (symlink to another unit file). │ 0 │ ├───────────────────┼──────────────────────────────────────────────────────────┼───────────┤ │ "masked" │ Completely disabled, so that any start operation on it │ │ ├───────────────────┤ fails (permanently in /etc/systemd/system/ or │ > 0 │ │ "masked-runtime" │ transiently in /run/systemd/systemd/). │ │ ├───────────────────┼──────────────────────────────────────────────────────────┼───────────┤ │ "static" │ The unit file is not enabled, and has no provisions for │ 0 │ │ │ enabling in the [Install] unit file section. │ │ ├───────────────────┼──────────────────────────────────────────────────────────┼───────────┤ │ "indirect" │ The unit file itself is not enabled, but it has a │ 0 │ │ │ non-empty Also= setting in the [Install] unit file │ │ │ │ section, listing other unit files that might be enabled, │ │ │ │ or it has an alias under a different name through a │ │ │ │ symlink that is not specified in Also=. For template │ │ │ │ unit files, an instance different than the one specified │ │ │ │ in DefaultInstance= is enabled. │ │ ├───────────────────┼──────────────────────────────────────────────────────────┼───────────┤ │ "disabled" │ The unit file is not enabled, but contains an [Install] │ > 0 │ │ │ section with installation instructions. │ │ ├───────────────────┼──────────────────────────────────────────────────────────┼───────────┤ │ "generated" │ The unit file was generated dynamically via a generator │ 0 │ │ │ tool. See systemd.generator(7). Generated unit files may │ │ │ │ not be enabled, they are enabled implicitly by their │ │ │ │ generator. │ │ ├───────────────────┼──────────────────────────────────────────────────────────┼───────────┤ │ "transient" │ The unit file has been created dynamically with the │ 0 │ │ │ runtime API. Transient units may not be enabled. │ │ ├───────────────────┼──────────────────────────────────────────────────────────┼───────────┤ │ "bad" │ The unit file is invalid or another error occurred. Note │ > 0 │ │ │ that is-enabled will not actually return this state, but │ │ │ │ print an error message instead. However the unit file │ │ │ │ listing printed by list-unit-files might show it. │ │ ├───────────────────┼──────────────────────────────────────────────────────────┼───────────┤ │ "not-found" │ The unit file doesn't exist. │ 4 │ └───────────────────┴──────────────────────────────────────────────────────────┴───────────┘ mask UNIT... Mask one or more units, as specified on the command line. This will link these unit files to /dev/null, making it impossible to start them. This is a stronger version of disable, since it prohibits all kinds of activation of the unit, including enablement and manual activation. Use this option

This section details the output of the `is-enabled` command in systemctl, describing the possible states of a unit file and their corresponding exit codes. It covers states like "alias", "masked", "static", "indirect", "disabled", "generated", "transient", "bad", and "not-found". It also introduces the `mask` command, which disables a unit file by linking it to /dev/null, preventing any activation.