diff --git a/shell.nix b/shell.nix
index 3482e5e..01d7b61 100644
--- a/shell.nix
+++ b/shell.nix
@@ -1,2 +1,2 @@
 { system ? builtins.currentSystem }:
-(import ./. { inherit system; }).treefmt
+(import ./. { inherit system; }).devShell
diff --git a/src/command/mod.rs b/src/command/mod.rs
index 4c55827..7bad92d 100644
--- a/src/command/mod.rs
+++ b/src/command/mod.rs
@@ -53,7 +53,7 @@ pub struct Cli {
     /// Run as if treefmt was started in <work-dir> instead of the current working directory.
     pub work_dir: PathBuf,
 
-    #[structopt(long = "tree-root")]
+    #[structopt(long = "tree-root", env = "PRJ_ROOT")]
     /// Set the path to the tree root directory. Defaults to the folder holding the treefmt.toml file.
     pub tree_root: Option<PathBuf>,