| # Externs | |
| Calling external commands is a fundamental part of using Nushell as a shell (and often using Nushell as a language). There's a problem, though: Nushell can't help with finding errors in the call, completions, or syntax highlighting with external commands. | |
| This is where `extern` comes in. The `extern` keyword allows you to write a full signature for the command that lives outside of Nushell so that you get all the benefits above. If you take a look at the default config, you'll notice that there are a few extern calls in there. Here's one of them: | |
| ```nu | |
| export extern "git push" [ | |
| remote?: string@"nu-complete git remotes", # the name of the remote | |
| refspec?: string@"nu-complete git branches" # the branch / refspec | |
| --verbose(-v) # be more verbose | |
| --quiet(-q) # be more quiet | |
| --repo: string # repository | |
| --all # push all refs | |
| --mirror # mirror all refs | |
| --delete(-d) # delete refs | |
| --tags # push tags (can't be used with --all or --mirror) | |
| --dry-run(-n) # dry run | |
| --porcelain # machine-readable output | |
| --force(-f) # force updates | |
| --force-with-lease: string # require old value of ref to be at this value | |
| --recurse-submodules: string # control recursive pushing of submodules | |
| --thin # use thin pack | |
| --receive-pack: string # receive pack program | |
| --exec: string # receive pack program | |
| --set-upstream(-u) # set upstream for git pull/status | |
| --progress # force progress reporting | |
| --prune # prune locally removed refs | |
| --no-verify # bypass pre-push hook | |
| --follow-tags # push missing but relevant tags | |
| --signed: string # GPG sign the push | |
| --atomic # request atomic transaction on remote side | |
| --push-option(-o): string # option to transmit | |
| --ipv4(-4) # use IPv4 addresses only | |
| --ipv6(-6) # use IPv6 addresses only | |
| ] | |
| ``` | |
| You'll notice this gives you all the same descriptive syntax that internal commands do, letting you describe flags, short flags, positional parameters, types, and more. | |
| ::: warning Note | |
| A Nushell comment that continues on the same line for argument documentation purposes requires a space before the ` #` pound sign. | |
| ::: | |
| ## Types and custom completions | |
| In the above example, you'll notice some types are followed by `@` followed by the name of a command. We talk more about [custom completions](custom_completions.md) in their own section. | |
| Both the type (or shape) of the argument and the custom completion tell Nushell about how to complete values for that flag or position. For example, setting a shape to `path` allows Nushell to complete the value to a filepath for you. Using the `@` with a custom completion overrides this default behavior, letting the custom completion give you full completion list. | |
| ## Format specifiers | |
| Positional parameters can be made optional with a `?` (as seen above) the remaining parameters can be matched with `...` before the parameter name, which will return a list of arguments. | |
| ```nu | |
| export extern "git add" [ | |
| ...pathspecs: glob | |
| # … | |
| ] | |
| ``` | |
| ## Limitations | |
| There are a few limitations to the current `extern` syntax. In Nushell, flags and positional arguments are very flexible: flags can precede positional arguments, flags can be mixed into positional arguments, and flags can follow positional arguments. Many external commands are not this flexible. There is not yet a way to require a particular ordering of flags and positional arguments to the style required by the external. | |
| The second limitation is that some externals require flags to be passed using `=` to separate the flag and the value. In Nushell, the `=` is a convenient optional syntax and there's currently no way to require its use. | |