Function default

default(arg: string): string
Convert a string into a POSIX sh-safe single argument literal.

Escapes a JavaScript string so it can be safely used as a single argument in POSIX-compatible shells (sh, bash, zsh, dash, etc.). The result, when parsed by the shell, produces exactly one argument with the original content.

Escaping Strategy
- Safe strings are returned as-is: Plain alphanumeric, Japanese, emoji, and other "harmless" Unicode characters pass through unchanged.
- Unsafe strings are single-quoted: Wraps in '...' with internal ' escaped as '\'' (close, escaped quote, reopen).
When Quoting is Required
- Empty string (would disappear without quotes)
- Contains Unicode whitespace (space, tab, newline, NBSP, ideographic space, etc.)
- Contains ASCII control characters (U+0001–U+001F, U+007F)
- Contains shell meta-characters: ' " \ $ \ | & ; < > ( ) * ? [ ] { } ! #`
- Starts with ~ (prevents tilde expansion)
Limitations
- Single argument only: Does not build or format entire command lines.
- POSIX shells only: Not for Windows cmd.exe or PowerShell.
- Shell parsing safety only: Does not protect against program-level option interpretation (e.g., --dangerous-flag).
Parameters
- arg: string
  The string to escape for shell use.
Returns string
The escaped string, safe to use as a single shell argument.
Throws
If arg is not a string.

Throws
If arg contains NUL (\u0000), which POSIX arguments cannot include.
Example
```
// Safe strings pass through unchanged
shellEscapeArg("abc")       // "abc"
shellEscapeArg("日本語")     // "日本語"
```
Example
```
// Strings with spaces or special chars are quoted
shellEscapeArg("a b")       // "'a b'"
shellEscapeArg("$HOME")     // "'$HOME'"
shellEscapeArg("*.txt")     // "'*.txt'"
```
Example
```
// Single quotes are escaped with '\''
shellEscapeArg("O'Reilly")  // "'O'\\''Reilly'"
```
Example
```
// Empty string becomes ''
shellEscapeArg("")          // "''"
```
Example
```
// Leading tilde is quoted to prevent expansion
shellEscapeArg("~user")     // "'~user'"
```
- Defined in index.ts:58