Hooks

Hooks allow you to run a code snippet at some predefined situations. They are only available in the interactive mode (REPL), they do not work if you run Nushell with a script (nu script.nu) or command (nu -c "print foo") argument.

Currently, we support these types of hooks:

pre_prompt : Triggered before the prompt is drawn
pre_execution : Triggered before the line input starts executing
env_change : Triggered when an environment variable changes
display_output : A block that the output is passed to
command_not_found : Triggered when a command is not found

To make it clearer, we can break down Nushell's execution cycle. The steps to evaluate one line in the REPL mode are as follows:

Check for pre_prompt hooks and run them
Check for env_change hooks and run them
Display prompt and wait for user input
After user typed something and pressed "Enter": Check for pre_execution hooks and run them
Parse and evaluate user input
If a command is not found: Run the command_not_found hook. If it returns a string, show it.
If display_output is defined, use it to print command output
Return to 1.

Basic Hooks

To enable hooks, define them in your config:

$env.config.hooks = {
    pre_prompt: [{ print "pre prompt hook" }]
    pre_execution: [{ print "pre exec hook" }]
    env_change: {
        PWD: [{|before, after| print $"changing directory from ($before) to ($after)" }]
    }
}

Try putting the above into your config, running Nushell and moving around your filesystem. When you change a directory, the PWD environment variable changes and the change triggers the hook with the previous and the current values stored in before and after variables, respectively.

Instead of defining just a single hook per trigger, it is possible to define a list of hooks which will run in sequence:

$env.config.hooks = {
    pre_prompt: [
        { print "pre prompt hook" }
        { print "pre prompt hook2" }
    ]
    pre_execution: [
        { print "pre exec hook" }
        { print "pre exec hook2" }
    ]
    env_change: {
        PWD: [
            {|before, after| print $"changing directory from ($before) to ($after)" }
            {|before, after| print $"changing directory from ($before) to ($after) 2" }
        ]
    }
}

Instead of replacing all hooks, you can append a new hook to existing configuration:

$env.config.hooks.pre_execution = $env.config.hooks.pre_execution | append { print "pre exec hook3" }

Changing Environment

One feature of the hooks is that they preserve the environment. Environment variables defined inside the hook block will be preserved in a similar way as def --env. You can test it with the following example:

$env.config = ($env.config | upsert hooks {
    pre_prompt: { $env.SPAM = "eggs" }
})

$env.SPAM
# => eggs

The hook blocks otherwise follow the general scoping rules, i.e., commands, aliases, etc. defined within the block will be thrown away once the block ends.

`pre_execution` Hooks

pre_execution hooks can inspect the to-be-executed command through the commandline command.

For example, to print the command being executed:

$env.config = (
    $env.config
    | upsert hooks.pre_execution [ {||
        $env.repl_commandline = (commandline)
        print $"Command: ($env.repl_commandline)"
    } ]
)

print (1 + 3)
# => Command: print (1 + 3)
# => 4

Conditional Hooks

One thing you might be tempted to do is to activate an environment whenever you enter a directory:

$env.config = ($env.config | upsert hooks {
    env_change: {
        PWD: [
            {|before, after|
                if $after == /some/path/to/directory {
                    load-env { SPAM: eggs }
                }
            }
        ]
    }
})

This won't work because the environment will be active only within the if block. In this case, you could easily rewrite it as load-env (if $after == ... { ... } else { {} }) but this pattern is fairly common and later we'll see that not all cases can be rewritten like this.

To deal with the above problem, we introduce another way to define a hook - a record:

$env.config = ($env.config | upsert hooks {
    env_change: {
        PWD: [
            {
                condition: {|before, after| $after == /some/path/to/directory }
                code: {|before, after| load-env { SPAM: eggs } }
            }
        ]
    }
})

When the hook triggers, it evaluates the condition block. If it returns true, the code block will be evaluated. If it returns false, nothing will happen. If it returns something else, an error will be thrown. The condition field can also be omitted altogether in which case the hook will always evaluate.

The pre_prompt and pre_execution hook types also support the conditional hooks but they don't accept the before and after parameters.

Hooks as Strings

So far a hook was defined as a block that preserves only the environment, but nothing else. To be able to define commands or aliases, it is possible to define the code field as a string. You can think of it as if you typed the string into the REPL and hit Enter. So, the hook from the previous section can be also written as

$env.config = ($env.config | upsert hooks {
    pre_prompt: '$env.SPAM = "eggs"'
})

$env.SPAM
# => eggs

This feature can be used, for example, to conditionally bring in definitions based on the current directory:

$env.config = ($env.config | upsert hooks {
    env_change: {
        PWD: [
            {
                condition: {|_, after| $after == /some/path/to/directory }
                code: 'def foo [] { print "foo" }'
            }
            {
                condition: {|before, _| $before == /some/path/to/directory }
                code: 'hide foo'
            }
        ]
    }
})

When defining a hook as a string, the $before and $after variables are set to the previous and current environment variable value, respectively, similarly to the previous examples:

$env.config = ($env.config | upsert hooks {
    env_change: {
        PWD: {
            code: 'print $"changing directory from ($before) to ($after)"'
        }
    }
}

Examples

Adding a Single Hook to Existing Config

An example for PWD env change hook:

$env.config = ($env.config | upsert hooks.env_change.PWD {|config|
    let val = ($config | get -i hooks.env_change.PWD)

    if $val == null {
        $val | append {|before, after| print $"changing directory from ($before) to ($after)" }
    } else {
        [
            {|before, after| print $"changing directory from ($before) to ($after)" }
        ]
    }
})

Automatically Activating an Environment when Entering a Directory

This one looks for test-env.nu in a directory

$env.config = ($env.config | upsert hooks.env_change.PWD {
    [
        {
            condition: {|_, after|
                ($after == '/path/to/target/dir'
                    and ($after | path join test-env.nu | path exists))
            }
            code: "overlay use test-env.nu"
        }
        {
            condition: {|before, after|
                ('/path/to/target/dir' not-in $after
                    and '/path/to/target/dir' in ($before | default "")
                    and 'test-env' in (overlay list))
            }
            code: "overlay hide test-env --keep-env [ PWD ]"
        }
    ]
})

Filtering or Diverting Command Output

You can use the display_output hook to redirect the output of commands. You should define a block that works on all value types. The output of external commands is not filtered through display_output.

This hook can display the output in a separate window, perhaps as rich HTML text. Here is the basic idea of how to do that:

$env.config = ($env.config | upsert hooks {
    display_output: { to html --partial --no-color | save --raw /tmp/nu-output.html }
})

You can view the result by opening file:///tmp/nu-output.html in a web browser. Of course this isn't very convenient unless you use a browser that automatically reloads when the file changes. Instead of the save command, you would normally customize this to send the HTML output to a desired window.

Changing how Output is Displayed

You can change to default behavior of how output is displayed by using the display_output hook. Here is an example that changes the default display behavior to show a table 1 layer deep if the terminal is wide enough, or collapse otherwise:

$env.config = ($env.config | upsert hooks {
    display_output: {if (term size).columns >= 100 { table -ed 1 } else { table }}
})

`command_not_found` Hook in Arch Linux

The following hook uses the pkgfile command, to find which packages commands belong to in Arch Linux.

$env.config = {
    ...other config...

    hooks: {
        ...other hooks...

        command_not_found: {
            |cmd_name| (
                try {
                    let pkgs = (pkgfile --binaries --verbose $cmd_name)
                    if ($pkgs | is-empty) {
                        return null
                    }
                    (
                        $"(ansi $env.config.color_config.shape_external)($cmd_name)(ansi reset) " +
                        $"may be found in the following packages:\n($pkgs)"
                    )
                }
            )
        }
    }
}

`command_not_found` Hook in NixOS

NixOS comes with the command command-not-found. We only need to plug it in the nushell hook:

$env.config.hooks.command_not_found = {
  |command_name|
  print (command-not-found $command_name | str trim)
}

`command_not_found` Hook in Windows

The following hook uses the ftype command, to find program paths in Windows that might be relevant to the user for alias-ing.

$env.config = {
    ...other config...

    hooks: {
        ...other hooks...

        command_not_found: {
            |cmd_name| (
                try {
                    let attrs = (
                        ftype | find $cmd_name | to text | lines | reduce -f [] { |line, acc|
                            $line | parse "{type}={path}" | append $acc
                        } | group-by path | transpose key value | each { |row|
                            { path: $row.key, types: ($row.value | get type | str join ", ") }
                        }
                    )
                    let len = ($attrs | length)

                    if $len == 0 {
                        return null
                    } else {
                        return ($attrs | table --collapse)
                    }
                }
            )
        }
    }
}