Note that `alias uuidgen = uuidgen | tr A-F a-f` (to make uuidgen on mac behave like linux) won't work.
The solution is to define a command without parameters that calls the system program `uuidgen` via `^`.

```nu
def uuidgen [] { ^uuidgen | tr A-F a-f }
```

See more in the [custom commands](custom_commands.md) section of this book.

Or a more idiomatic example with nushell internal commands

```nu
def lsg [] { ls | sort-by type name -i | grid -c | str trim }
```

displaying all listed files and folders in a grid.

## Replacing Existing Commands Using Aliases

::: warning Caution!
When replacing commands it is best to "back up" the command first and avoid recursion error.
:::

How to back up a command like `ls`:

```nu
alias core-ls = ls    # This will create a new alias core-ls for ls
```

Now you can use `core-ls` as `ls` in your nu-programming. You will see further down how to use `core-ls`.

The reason you need to use alias is because, unlike `def`, aliases are position-dependent. So, you need to "back up" the old command first with an alias, before re-defining it.
If you do not backup the command and you replace the command using `def` you get a recursion error.

```nu
def ls [] { ls }; ls    # Do *NOT* do this! This will throw a recursion error

#output:
#Error: nu::shell::recursion_limit_reached
#
#  × Recursion limit (50) reached
#     ╭─[C:\Users\zolodev\AppData\Roaming\nushell\config.nu:807:1]
# 807 │
# 808 │ def ls [] { ls }; ls
#     ·           ───┬──
#     ·              ╰── This called itself too many times
#     ╰────
```

The recommended way to replace an existing command is to shadow the command.
Here is an example shadowing the `ls` command.

```nu
# alias the built-in ls command to ls-builtins
alias ls-builtin = ls

# List the filenames, sizes, and modification times of items in a directory.
def ls [
    --all (-a),         # Show hidden files
    --long (-l),        # Get all available columns for each entry (slower; columns are platform-dependent)
    --short-names (-s), # Only print the file names, and not the path
    --full-paths (-f),  # display paths as absolute paths
    --du (-d),          # Display the apparent directory size ("disk usage") in place of the directory metadata size
    --directory (-D),   # List the specified directory itself instead of its contents
    --mime-type (-m),   # Show mime-type in type column instead of 'file' (based on filenames only; files' contents are not examined)
    --threads (-t),     # Use multiple threads to list contents. Output will be non-deterministic.
    ...pattern: glob,   # The glob pattern to use.
]: [ nothing -> table ] {
    let pattern = if ($pattern | is-empty) { [ '.' ] } else { $pattern }
    (ls-builtin
        --all=$all
        --long=$long
        --short-names=$short_names
        --full-paths=$full_paths
        --du=$du
        --directory=$directory
        --mime-type=$mime_type
        --threads=$threads
        ...$pattern
    ) | sort-by type name -i
}
```