Trait DynamicVariable

pub trait DynamicVariable {
    // Required methods
    fn get(&mut self) -> Option<CString>;
    fn set(&mut self, value: &CStr);
}

The DynamicVariable provides the implementation to create dynamic variables.

get is called when the value of the shell variable is required. set is called when a value is assigned to a variable.

Use variables::bind to create a dynamic variable with an instance of a type implementing DynamicVariable.

Deleting Dynamic Variables

A dynamic variable can be deleted with unset. However, the instance bound to it is not dropped, since bash does not notify when a variable is removed.

If the builtin is removed (enable -d <name>), dynamic variables are removed before unloading the shared object.

Example

To create a counter using DynamicVariable, first we need to implement a Counter type similar to this:

use std::ffi::{CStr, CString};
use bash_builtins::error;
use bash_builtins::variables::DynamicVariable;

struct Counter(isize);

impl DynamicVariable for Counter {
    fn get(&mut self) -> Option<CString> {
        let value = CString::new(format!("{}", self.0)).ok();
        self.0 += 1;
        value
    }

    fn set(&mut self, value: &CStr) {
        self.0 = match value.to_str().map(str::parse) {
            Ok(Ok(n)) => n,
            _ => {
                error!("invalid value: {:?}", value);
                return;
            }
        }
    }
}

Then, dynamic variables with any name can be created using a function like this:

use bash_builtins::variables::{bind, VariableError};

fn create_counter(name: &str) -> Result<(), VariableError> {
    bind(name, Counter(0))
}

The varcounter example implements this functionality:

$ cargo build --release --examples

$ enable -f target/release/examples/libvarcounter.so varcounter

$ varcounter FOO BAR

$ echo $FOO
0

$ echo $FOO
1

$ echo $FOO
2

$ echo $FOO $BAR $BAR
3 0 1

$ FOO=1000

$ echo $FOO
1000

$ echo $FOO
1001

Required Methods

fn get(&mut self) -> Option<CString>

Returns the value for the shell variable.

If it returns None, the variable will be empty.

fn set(&mut self, value: &CStr)

Called when a string is assigned to the shell variable.

Implementors