# Flags
Flags are named options passed by the user at the command line, like `--silent` or `--output file.txt`.
vecli handles parsing, alias resolution, and delivery to your handler automatically.
## Defining a Flag
Flags are built with `Flag::new()` and attached to a command with `.flag()`:
```rust
Command::new("hello", function_that_handles_hello)
.flag(Flag::new("silent").alias("s").description("Suppress output."))
```
The name is the canonical long form, without the `--` prefix. An alias is the short form, without `-`.
## Reading Flags in a Handler
Flags are available in `ctx.flags`, a `HashMap<String, String>`. Boolean flags have the value `"true"`.
```rust
fn greet(ctx: &CommandContext) {
if ctx.flags.contains_key("silent") {
return;
}
println!("Hello!");
}
```
Value-carrying flags (e.g. `--output file.txt`) store the value as a string:
```rust
let path = ctx.flags.get("output").map(String::as_str).unwrap_or("out.txt");
```
## Aliases
An alias lets users pass `-s` instead of `--silent`. vecli resolves it to the canonical name before
your handler is called, so you always check for `"silent"`, never `"s"`.
```rust
Flag::new("silent").alias("s")
```
> **Note:** Aliases are always boolean. `-s value` will not capture `value` as the flag's value.
## Global Flags
A global flag is available to every command without any extra setup. Define it on the app with `Flag::global()`:
```rust
App::new("mytool")
.flag(Flag::global("verbose").alias("v").description("Enable verbose output."))
```
It shows up in every handler's `ctx.flags` automatically. Making a regular flag inside `App::flag()` means it's only available to the main entry (`.main(entry)`), if defined.
## Strict Mode
By default, unknown flags produce a warning and execution continues. Enable strict mode on a command
to treat unknown flags as a hard error instead:
```rust
Command::new("add", add)
.strict_flags(true)
```
Strict mode can also be set at the app level via `App::strict_flags(true)`, which applies to app-level
flag parsing before any command is dispatched.
---
While making commands, you may have noticed that the function takes this peculiar argument: `&CommandContext`. What is that? Know more about the [CommandContext](./context.md) as our next stop.