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_The following answer was given by SE user [Oh My Goodness](https://codereview.stackexchange.com/users/131732). The original source can be found [here](https://codereview.stackexchange.com/a/210264/98306)._

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* Instead of `cat "$x" | command` or `echo "$x" | command`, use `command <$x` (vs `cat`) or `command <<<$x` (vs `echo`): it saves a fork and removes the need to quote.
* Instead of `if [ x -lt y ]` use `if [[ x -lt y ]]`: it saves a fork (`[[` is a bash builtin; `help test` for details) and adds some functionality.
* Functions return their last exit value already so `contains()` can be shortened to the following (whether you prefer this is up to you):

```
    contains() {
        test "${1#*$2}" != "$1"
    }
```

* Use bash defaulting mechanism instead of `if [[ -z`, as in `CONF=${2:-./steps.json}`.
* Use `for ((i=0; i<$LIMIT; i++))` instead of `i=0; while ...`.
* Test the exit values of things that shouldn't fail, as in `mkdir -p "$DESTROOT" || exit 1`. **Any invocation of `cd` or `pushd` should be checked for success, always!** A general purpose `DIE()` function can replace the naked exit and take an error message as an argument. If nothing should fail, `set -e` or `trap DIE ERR` (the first argument is a function name) does this globally.
* Constructions like `jq -r ".["$i"].files | length")` and `echo "    ""$FSRC"` are kind of weird and the inner double quotes probably should be removed.
* In a language where every variable is a global, it's a good habit to use fewer variables. For example, `RES=$(foo); LOOP=$( echo "$RES" | ...)` can just be `LOOP=$( foo | ...)`.
* Your get-conf pattern should be in a function like:

```
    get_conf() {
        jq -r $1<<<$CONF
    }
```

* Pruning code paths is important in an interpreted language. Since the wildcard copy method works for regular copies too, just use that one unconditionally and remove `if contains ... "\*"`.
* You don't need to escape wildcards like `*` in double quotes. When in doubt about what will be interpolated, use single quotes. Quoting in bash can be very complex and take a long time to learn; an advanced understanding of it will help to avoid common bugs.
* Since you are using commands that aren't standard, it's a good idea to set `PATH` in the script, or as an optional config directive, and to check that they're there before you begin, as in the following example:

```
    require() {
        for cmd in "$@"; do
            type $cmd >/dev/null || exit 1
        done
    }

    # Code here...

    require jq udisksctl
```

* Read `CONF` just once, into a variable: `conf=$(<$CONF)`, and query that. Then you can edit the config while the script runs.