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Client spec: harmonize formatting.
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# tldr-pages client specification
tldr-pages client specification
===============================

- **Current Specification Version:** 1.2
**Current Specification Version:** 1.2

This document contains the official specification for tldr-pages clients. It is _not_ a specification of the format of the pages themselves - only a specification of how a user should be able to interface with an official client. For a list of previous versions of the specification, see the [changelog section](#Changelog) below.

The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC 2119](https://tools.ietf.org/html/rfc2119).


## Terminology
Terminology
-----------

In order to aid the understanding of this specification document, a number of terms will be defined in this section.

### Page

tldr-pages consists of multiple _pages_ - each of which describes a specific command.

### Platform

Pages are grouped by platform. A platform is an operating system. For example, `windows`, `linux`, `osx`. The special platform `common` contains pages for commands that work identically across more than 1 platform.

If a page is common across multiple platforms, but slightly differently on a given platform, then the page is still stored in `common`, but a copy tailored for the differing platform is placed in that platform's specific folder.

For example, if the command `foo` is common to `mac`, `windows`, and `linux` but functions differently on `windows`, then the main page will be stored in `common`, and a copy will be placed in `windows` that's altered to match the different functionality.


## Command-Line Interface
Command-Line Interface
----------------------

This section describes the standardised command-line interface (CLI) for clients implementing one. Clients that do not provide a CLI can ignore this section.

### Arguments

A number of arguments MUST be supported (unless otherwise specified) if a CLI is implemented:

Argument | Required? | Meaning
--------------------|-----------|--------------------
`-v`, `--version` | Yes | Shows the current version of the client, and the version of this specification that it implements.
`-p`, `--platform` | Yes | Specifies the platform to be used to perform the action (either listing or searching). If this option is specified, the selected platform MUST be checked first instead of the current platform as described below.
`-u`, `--update` | Conditional | Updates the offline cache of pages. MUST be implemented if cache is supported.
`-l`, `--list` | No | Lists all the pages in the current platform to the standard output. If the special platform `all` is specified a list of all pages in all platforms MUST be displayed.
`-L, --language` | No | Specifies the preferred language for the page returned. Overrides other language detection mechanisms. See the [language section](#language) for more information.
Argument | Required? | Meaning
-------------------|-------------|----------
`-v`, `--version` | Yes | Shows the current version of the client, and the version of this specification that it implements.
`-p`, `--platform` | Yes | Specifies the platform to be used to perform the action (either listing or searching). If this option is specified, the selected platform MUST be checked first instead of the current platform as described below.
`-u`, `--update` | Conditional | Updates the offline cache of pages. MUST be implemented if cache is supported.
`-l`, `--list` | No | Lists all the pages in the current platform to the standard output. If the special platform `all` is specified a list of all pages in all platforms MUST be displayed.
`-L`, `--language` | No | Specifies the preferred language for the page returned. Overrides other language detection mechanisms. See the [language section](#language) for more information.

Clients MAY NOT implement the long form of an argument (`--update`, `--version`, `--list`, `--platform`).

Expand All @@ -50,6 +58,7 @@ tldr -l
```

### Page Names

The first argument that does not start with a dash (`-`) MUST be considered the page name.

In addition, page names MAY contain spaces (e.g. `git status`) - such page names MUST be transparently concatenated with dashes (`-`). For example, the page name:
Expand All @@ -58,7 +67,7 @@ In addition, page names MAY contain spaces (e.g. `git status`) - such page names
git checkout
```

...becomes this:
becomes this:

```
git-checkout
Expand All @@ -74,30 +83,33 @@ tldr --platform osx bash
```


## Directory Structure
Directory Structure
-------------------

This section documents the directory structure that contains the pages themselves.

The master version of every page is stored inside (but not directly) the `pages` directory. Inside this directory, there is a folder for each platform - for example `windows`, `linux`, and the special `common` platform:

- `pages/`
+ `common/`
+ `linux/`
+ `windows/`
+ `osx/`
+ .....etc.
- `common/`
- `linux/`
- `windows/`
- `osx/`
- ...etc.

Additional platforms MAY be added in the future. Clients MAY NOT support new platforms (though such support is RECOMMENDED), but MUST NOT break if additional platforms are added.

The pages themselves sit inside the appropriate platform folder, with the extension `.md`. Here are some example mappings:
The pages themselves reside inside the appropriate platform folder, with the extension `.md`. Here are some example mappings:

Command name | Mapped name | Filename
----------------|-----------------|-------------------
`7za` | `7za` | `7za.md`
`git checkout` | `git-checkout` | `git-checkout.md`
`git checkout` | `git-checkout` | `git-checkout.md`
`tar` | `tar` | `tar.md`


### Translations

Other directories sit alongside the main `pages` directory, and contain translations of the master versions of every page - though pages MAY NOT have a translation available for a given language yet. Furthermore, a given language MAY NOT have a folder yet either. The format of these directories is `pages.<locale>`, where `<locale>` is a [POSIX Locale Name](https://www.gnu.org/software/gettext/manual/html_node/Locale-Names.html#Locale-Names) in the form of `<language>_<country>`, where:

- `<language>` is the shortest [ISO 639](https://en.wikipedia.org/wiki/ISO_639) language code for the chosen language (see [here](https://en.wikipedia.org/wiki/List_of_ISO_639-2_codes) for a complete list).
Expand All @@ -112,11 +124,15 @@ Some examples:
The structure inside these translation folders is identical to that of the main `pages` folder.


## Page Structure
Page Structure
--------------

Although this specification is about the interface that clients must provide, it is also worth noting that pages are written in standard [CommonMark](https://commonmark.org/), which the exception of the non-standard `{{` and `}}` syntax, which surrounds values in an example that users may edit. Clients MUST NOT break if the page format is changed within the _CommonMark_ specification.


## Page Resolution
Page Resolution
---------------

This section defines the algorithm by which a client can decide which page a user has requested.

After transparently replacing spaces (` `) with dashes (`-`), clients have several decisions to make:
Expand All @@ -125,6 +141,7 @@ After transparently replacing spaces (` `) with dashes (`-`), clients have sever
- The platform to display a page from

### Platform

Clients MUST default to displaying tldr-page from the platform upon which the client is running. For example, a client running on _Windows 10_ will default to displaying pages from the `windows` platform. Clients MAY provide a user-configurable option to override this behaviour, however.

If a page is not available for the host platform, clients MUST fallback to the special `common` platform.
Expand All @@ -142,20 +159,24 @@ Steps #3 and #4 may be done in either order.

It is possible that due to this page resolution logic, the client may show a page which does not belong to the host platform because a page can reside in `common`, and not be present on the host platform. Clients must not assume that a given command is always executable on the host platform.

#### If a page isn't found
#### If a page is not found

If a page cannot be found in _any_ platform, then it is RECOMMENDED that clients display an error message with a link to create a new issue against the `tldr-pages/tldr` GitHub repository. Said link might take the following form:

```url
https://github.com/tldr-pages/tldr/issues/new?title=page%20request:%20{command_name}
```

....where `{command_name}` is the name of the command that was not found.
where `{command_name}` is the name of the command that was not found.

#### If multiple versions of a page were found

If multiple versions of a page were found for different platforms, then a client MAY choose to display a notice to the user notifying them of this.


## Language
Language
--------

Pages can be written in multiple languages. If a client has access to environment variables, several standard ones exist to specify the language in which a client should operate. If not, then clients MUST make reasonable assumptions based on the information provided by the environment in which they operate (e.g. consulting `navigator.languages` in a browser, etc.). If possible, it is RECOMMENDED to also make language configurable, as to not only rely on the environment. Clients SHOULD therefore offer options to configure or override the language using configuration files or command line arguments (like `-L, --language` suggested in the [arguments section](#Arguments) above).

The [`LANG` environment variable](https://www.gnu.org/software/gettext/manual/html_node/Locale-Environment-Variables.html), if present, MUST be used to determine the language of pages to display.
Expand All @@ -164,20 +185,27 @@ The [`LANGUAGE` environment variable](https://www.gnu.org/software/gettext/manua

The [`LC_MESSAGES` environment variable](https://www.gnu.org/software/gettext/manual/html_node/Locale-Environment-Variables.html) MAY be present. If the client itself is localized and this environment variable is present, it MUST use its value in order to determine the language in which interface text is shown (separately from the language used for pages). In absence of `LC_MESSAGES`, then `LANG` and `LANGUAGE` MUST be used for this purpose instead.

## Other Considerations

Other Considerations
--------------------

This section contains a number of other items that don't neatly fit into any of the sections defined above.

If appropriate, it is RECOMMENDED that clients implement a cache of pages. If implemented, clients MUST download the _entire_ archive from **https://github.com/tldr-pages/tldr**.

Additionally, clients MAY automatically update the cache on a regular basis.


## Changelog
Changelog
---------

- [v1.2, July 3rd 2019](https://github.com/tldr-pages/tldr/blob/master/CLIENT-SPECIFICATION.md) (#3168)
- Addition of a new `-L, --language` recommended command-line argument.
- Rewording of the language section also encouraging the use of configuration files for language.
- Shift from BCP-47 to POSIX style locale tags, with consequent **deprecation of previous versions of the spec**.

- [v1.1, April 1st 2019](https://github.com/tldr-pages/tldr/blob/fbdc06b7425f92cc0d4fc9a5cfc5860ef017251e/CLIENT-SPECIFICATION.md) (deprecated) (#2859)
- Clarified platform section.

- [v1.0, January 23rd 2019](https://github.com/tldr-pages/tldr/blob/f5be8a2614a455288f26e42953efeb8cb3bc50b0/CLIENT-SPECIFICATION.md) (deprecated) (#2706)
- Initial release.

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