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28.

The fde-setup-request command is used inside the fde-setup hook. It will
return information about what operation for full-disk encryption is
requested and auxiliary data to complete this operation.

The fde-setup hook should do what is requested and then call
"snapctl fde-setup-result" and pass the result data to stdin.

Here is an example for how the fde-setup hook is called initially:
$ snapctl fde-setup-request
{"op":"features"}
$ echo '{"features": []}' | snapctl fde-setup-result

Alternatively the hook could reply with:
$ echo '{"error":"hardware-unsupported"}' | snapctl fde-setup-result

And then it is called again with a request to do the initial key setup:
$ snapctl fde-setup-request
{"op":"initial-setup", "key": "key-to-seal"}
$ echo "{"sealed-key":"$base64_encoded_sealed_key"}" | snapctl fde-setup-result

29.

The fde-setup-result command sets the result data for a fde-setup hook
reading it from stdin.

For example:
When the fde-setup hook is called with "op":"features:
$ echo '{"features": []}' | snapctl fde-setup-result

When the fde-setup hook is called with "op":"initial-setup":
$ echo "{"sealed-key":"$base64_encoded_sealed_key"}" | snapctl fde-setup-result

33.

The get command prints configuration options for the current snap.

$ snapctl get username
frank

If multiple option names are provided, a document is returned:

$ snapctl get username password
{
"username": "frank",
"password": "..."
}

Nested values may be retrieved via a dotted path:

$ snapctl get author.name
frank

Values of interface connection settings may be printed with:

$ snapctl get :myplug usb-vendor
$ snapctl get :myslot path

This will return the named setting from the local interface endpoint,
regardless whether it's a plug or a slot. Returning the setting from the
connected snap's endpoint is also possible by requesting the setting explicitly
with optional --plug and --slot command options:

$ snapctl get :myplug --slot usb-vendor

This requests the "usb-vendor" setting from the slot that is connected to
"myplug".

37.

The install command installs the named snaps on the system.

To install multiple instances of the same snap, append an underscore and a
unique identifier (for each instance) to a snap's name.

With no further options, the snaps are installed tracking the stable channel,
with strict security confinement. All available channels of a snap are listed in
its 'snap info' output.

When --revision is used, a later refresh will typically undo the revision
override, taking the snap back to the current revision of the channel it's
tracking.

Use --name to set the instance name when installing from snap file.

40.

The is-connected command returns success if the given plug or slot of the
calling snap is connected, and failure otherwise.

$ snapctl is-connected plug
$ echo $?
1

Snaps can only query their own plugs and slots - snap name is implicit and
implied by the snapctl execution context.

The --list option lists all connected plugs and slots.

The --pid and --aparmor-label options can be used to determine whether
a plug or slot is connected to the snap identified by the given
process ID or AppArmor label. In this mode, additional failure exit
codes may be returned: 10 if the other snap is not connected but uses
classic confinement, or 11 if the other process is not snap confined.

The --pid and --apparmor-label options may only be used with slots of
interface type "pulseaudio", "audio-record", or "cups-control".

53.

The pack command packs the given snap-dir as a snap and writes the result to
target-dir. If target-dir is omitted, the result is written to current
directory. If both source-dir and target-dir are omitted, the pack command packs
the current directory.

The default file name for a snap can be derived entirely from its snap.yaml, but
in some situations it's simpler for a script to feed the filename in. In those
cases, --filename can be given to override the default. If this filename is
not absolute it will be taken as relative to target-dir.

When used with --check-skeleton, pack only checks whether snap-dir contains
valid snap metadata and raises an error otherwise. Application commands listed
in snap metadata file, but appearing with incorrect permission bits result in an
error. Commands that are missing from snap-dir are listed in diagnostic
messages.

64.

The refresh command prints pending refreshes of the calling snap and can hold
back disruptive refreshes of other snaps, such as refreshes of the kernel or
base snaps that can trigger a restart. This command can be used from the
gate-auto-refresh hook which is only run during auto-refresh.

Snap can query pending refreshes with:
$ snapctl refresh --pending
pending: ready
channel: stable
version: 2
revision: 2
base: false
restart: false

The 'pending' flag can be "ready", "none" or "inhibited". It is set to "none"
when a snap has no pending refreshes. It is set to "ready" when there are
pending refreshes and to ”inhibited” when pending refreshes are being
held back because more or more snap applications are running with the
“refresh app awareness” feature enabled.

The "base" and "restart" flags indicate whether the base snap is going to be
updated and/or if a restart will occur, both of which are disruptive. A base
snap update can temporarily disrupt the starting of applications or hooks from
the snap.

To tell snapd to proceed with pending refreshes:
$ snapctl refresh --pending --proceed

Note, a snap using --proceed cannot assume that the updates will occur as they
might be held back by other snaps.

To hold refresh for up to 90 days for the calling snap:
$ snapctl refresh --pending --hold

65.

The refresh command updates the specified snaps, or all snaps in the system if
none are specified.

With no further options, the snaps are refreshed to the current revision of the
channel they're tracking, preserving their confinement options. All available
channels of a snap are listed in its 'snap info' output.

When --revision is used, a later refresh will typically undo the revision
override.

Hold (--hold) is used to postpone snap refresh updates for all snaps when no
snaps are specified, or for the specified snaps.

When no snaps are specified --hold is only effective on auto-refreshes and will
not block either general refresh requests from 'snap refresh' or specific snap
requests from 'snap refresh target-snap'.

When snaps are specified --hold is effective on both their auto-refreshes
and general refresh requests from 'snap refresh'. However, specific snap
requests from 'snap refresh target-snap' remain unblocked and will proceed.

68.

The remove-quota command removes the given quota group.

Currently, only quota groups with no sub-groups can be removed. In order to
remove a quota group with sub-groups, the sub-groups must first be removed until
there are no sub-groups for the group, then the group itself can be removed.

84.

The set command sets the provided configuration options as requested.

$ snapctl set username=frank password=$PASSWORD

All configuration changes are persisted at once, and only after the hook
returns successfully.

Nested values may be modified via a dotted path:

$ snapctl set author.name=frank

Configuration option may be unset with exclamation mark:
$ snapctl set author!

Plug and slot attributes may be set in the respective prepare and connect hooks
by naming the respective plug or slot:

$ snapctl set :myplug path=/dev/ttyS0