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# Command-Line Arguments
Inferno has a dedicated module for processing commandline flags and arguments, [arg(2)](http://man.cat-v.org/inferno/2/arg).
## Source
### args.b:17,22
The module `Arg` is loaded and initialized. As per the manual, the `arg->init()` function must be called before any other functions can be called from `Arg`.
The usage message is also set pre-emptively for use later, if necessary.
### args.b:24,37
There are two flag arguments, `r` and `c` which set reversal of arguments and the list item indicator mark, respectively.
The flag `r` is a binary option toggle, the flag's presence as an argument is sufficient to change the value of `rev`.
The flag `c` takes an argument. The `arg->earg()` function is used to pop the flag's argument out of the list. Specifically with regards to `earg()` rather than `arg()` as per the manual, `earg()` will call `arg->usage()` if the relevant argument does not exist.
Note: After the processing of all flags, `argv` is re-set from the value contained within `arg` to remove the elements (if any) utilized by flags and their arguments.
### args.b:41,51
This section utilizes the `rev` variable to (naively) reverse the list `argv`. After the `rev` check, the list `argv` is printed in order from beginning to end with each element being printed on a new line with the `mark` variable being prefixed to the list element currently at the head of the `argv` list each iteration during printout.
## Demo
; limbo args.b
; args -h
usage: args [-r] [-c mark] words...
; args a b c d
Argc after flags: 4
→ a
→ b
→ c
→ d
; args -r a b c d
Argc after flags: 4
→ d
→ c
→ b
→ a
; args -c 'quack: ' a b c d
Argc after flags: 4
quack: a
quack: b
quack: c
quack: d
; args -r -c '-> ' d c b a
Argc after flags: 4
-> a
-> b
-> c
-> d
;
## Exercises
- Can you trick `earg()` into accepting a nil value?
- How would you pull an `int` out of `earg()`?
- How would you trip the `usage()` message call-able from `earg()`?
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