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DC/OS CLI Style Guide

Intro

The DC/OS CLI is the main tool used by developers and operators to interact with DC/OS cluster and services running on the clusters. The teams developing features and CLI plugins should build the CLI per this style guide so that we offer a consistent and usable experience to users of DC/OS.

Naming Commands

  • Generally top level commands are single nouns e.g. dcos job, dcos service
  • Top level commands are followed by verbs e.g. dcos job create, dcos cluster rename
  • Command names should always be a single, lowercase word without spaces, hyphens, underscores, or other word delimiters
  • If there is no obvious way to avoid having multiple words, separate with kebab-case e.g. dcos auth list-providers

Inputs

Actions

  • If no action provided then default to showing usage help output
  • Try to be consistent with other subcommands; only break convention if necessary
    • Create - Creating a new object aka New, Add
    • Delete - Deleting an object from the system aka Destroy
    • Add - Adding an object that already exists but not creating a new object e.g. adding an existing user to a group
    • Remove - Removing from an object but not deleting e.g. removing a user from a group
    • Show - Show a description or definition aka Describe, Get
    • List - List all objects

Arguments/Options

CLI arguments should follow the GNU standards as much as possible.

Commonly used options are:

  • --help and -h should always output one level of help
  • --json outputs JSON
  • --force and -f to force
  • --quiet and -q to silence standard output
  • -v and -vv for verbose outputs on stderr ("verbose" or "very verbose")
  • --lines=N to restrict amount of results

Please refer to this table for other common option names.

Prompts

Destructive Action Confirmations

If a user tries to delete (or destroy) an object we should ask them to confirm.

$ dcos backup delete <id>
Are you sure you want to delete this backup? [y/n]

Commands should support passing confirmation via --yes and -y options to satisfy scriptability needs.

$ dcos backup delete <id> --yes

Outputs

JSON

  • Commands should be able to accept the --json flag and output in JSON format
  • JSON should be formatted with multi-line and indentation i.e. not one line

Tables

  • Column headings are UPPERCASE
  • Table cells lower case
  • Strings left aligned
  • Integers right aligned
  • First column should be the primary identifier e.g. name or ID
  • Column heading alignment should match alignment of data
  • Default sorting to the primary column; typically A-Z or date/time
  • Do not sort by ID, this is usually useless
  • 2 spaces between each column
$ dcos <command> list
NAME   HOST      TASKS  STATE    CPU  ID
kafka  10.0.2.3      2	RUNNING  1.1  123-456-789
kafka2 10.0.2.3     99	RUNNING  2.0  123-456-789

stdout/stderr

CLI main output, such as an ID, JSON or table outputs, should be written to stdout.

Errors, deprecation notices, informational and debug messages should be written to stderr.

When in doubt, one should wonder: "Would it make sense to pipe this output to another command?". If so, it probably belongs to stdout, otherwise stderr.

Help

Only output help for a single level to reduce cognitive load. For example:

$ dcos marathon -h

Should output help for “marathon” but not levels below that.

$ dcos marathon task -h

Should output help for “marathon task” but not levels above or below that.

Help Formatting

Output help in the following format:

$ command -h
Description:
    <description>

Usage:
    dcos <command> <action>

Examples:
    An optional section of example(s) on how to run the command.

Commands:
    <command>
        <commandDescription>
    <command>
        <commandDescription>
    …

Options:
    --<option>
        <optionDescription>
    --<option>
        <optionDescription>
    …

Descriptions for commands or options should be an imperative and start with an upper case.

Error Messages

In general on success there is no output, this follows a UNIX good practice ("No News Is Good News"). However on errors the CLI must always display an informative message, it shouldn’t be too low-level.

$ dcos cluster setup https://not-reachable.com
Couldn’t reach https://not-reachable.com, is it a DC/OS cluster master node?

If users want the low level error messages, they’d need to run the command with the verbose option:

$ dcos -v cluster setup https://not-reachable.com
[ERR] couldn’t download CA certificates : dial tcp: lookup not-reachable.com: no such host
Couldn’t reach https://not-reachable.com, is it a DC/OS cluster master node?

Do and Don't Examples

✅ Do

$ dcos <command> <subcommand>

🚫 Don't

$ dcos <command> --<subcommand>

✅ Do

$ dcos <command> -h
<Description>
<Usage>
<Examples>
<Commands>
<Options>

🚫 Don't

$ dcos <command> -h
<Usage>
<Options>
<Commands>

✅ Do

$ dcos cluster setup https://not-reachable.com
Couldn’t reach https://not-reachable.com, is it a DC/OS cluster master node?

🚫 Don't

$ dcos cluster setup https://not-reachable.com
Error: couldn’t download CA certificates : dial tcp: lookup not-reachable.com on 127.0.0.53:53: no such host