Shell-safe characters

CharSet+ShellSafe.swift

// known-good: Xcode 8.3.3
/*

This shell script outputs which characters are special and need escaping in bash or sh:

#!/bin/bash
special=$'`!@#$%^&*()-_+={}|[]\\;\':",.<>?/ '
for ((i=0; i < ${#special}; i++)); do
    char="${special:i:1}"
    printf -v q_char '%q' "$char"
    if [[ "$char" != "$q_char" ]]; then
        printf 'Yes - character %s needs to be escaped\n' "$char"
    else
        printf 'No - character %s does not need to be escaped\n' "$char"
    fi
done | sort

*/

let shellSafe = CharacterSet(charactersIn: "! #$&'()*,;<>?[\\]^`{|}\"").inverted

let urlPathSafe = CharacterSet.urlPathAllowed
let urlFragSafe = CharacterSet.urlFragmentAllowed
let urlQuerySafe = CharacterSet.urlQueryAllowed
let urlSafe = urlPathSafe.intersection(urlFragSafe).intersection(urlQuerySafe)

let filesystemUnsafe = CharacterSet(charactersIn: "/:")
let filesystemSafe = filesystemUnsafe.inverted

let safeEverywhere = urlSafe.intersection(filesystemSafe)

// conclusion: there are no safe-everywhere paired delimiters. ()<>[]{} are all unsafe.
// conclusion: () is URL and fs-safe, but not shell safe.