diff --git a/ofborg/src/evalchecker.rs b/ofborg/src/evalchecker.rs
index f485d77..096d188 100644
--- a/ofborg/src/evalchecker.rs
+++ b/ofborg/src/evalchecker.rs
@@ -7,16 +7,16 @@ use ofborg::nix;
 
 pub struct EvalChecker {
     name: String,
-    cmd: String,
+    op: nix::Operation,
     args: Vec<String>,
     nix: nix::Nix,
 }
 
 impl EvalChecker {
-    pub fn new(name: &str, cmd: &str, args: Vec<String>, nix: nix::Nix) -> EvalChecker {
+    pub fn new(name: &str, op: nix::Operation, args: Vec<String>, nix: nix::Nix) -> EvalChecker {
         EvalChecker {
             name: name.to_owned(),
-            cmd: cmd.to_owned(),
+            op: op,
             args: args,
             nix: nix,
         }
@@ -27,11 +27,11 @@ impl EvalChecker {
     }
 
     pub fn execute(&self, path: &Path) -> Result<File, File> {
-        self.nix.safely(&self.cmd, path, self.args.clone(), false)
+        self.nix.safely(self.op.clone(), path, self.args.clone(), false)
     }
 
     pub fn cli_cmd(&self) -> String {
-        let mut cli = vec![self.cmd.clone()];
+        let mut cli = vec![self.op.to_string()];
         cli.append(&mut self.args.clone());
         return cli.join(" ");
     }
diff --git a/ofborg/src/nix.rs b/ofborg/src/nix.rs
index 6183c47..aaeac78 100644
--- a/ofborg/src/nix.rs
+++ b/ofborg/src/nix.rs
@@ -1,5 +1,5 @@
 use std::env;
-use std::ffi::OsString;
+use std::fmt;
 use std::fs::File;
 use std::io::Seek;
 use std::io::SeekFrom;
@@ -7,6 +7,37 @@ use std::path::Path;
 use std::process::{Command, Stdio};
 use tempfile::tempfile;
 
+#[derive(Clone, Debug)]
+pub enum Operation {
+    Build,
+    Instantiate,
+    Unknown { program: String },
+}
+
+impl Operation {
+    pub fn new(program: &str) -> Operation {
+        Operation::Unknown { program: program.to_owned() }
+    }
+
+    fn command(&self) -> Command {
+        match *self {
+            Operation::Build => Command::new("nix-build"),
+            Operation::Instantiate => Command::new("nix-instantiate"),
+            Operation::Unknown { ref program } => Command::new(program),
+        }
+    }
+}
+
+impl fmt::Display for Operation {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match *self {
+            Operation::Build => write!(f, "{}", "nix-build"),
+            Operation::Instantiate => write!(f, "{}", "nix-instantiate"),
+            Operation::Unknown { ref program } => write!(f, "{}", program),
+        }
+    }
+}
+
 #[derive(Clone, Debug, PartialEq)]
 pub struct Nix {
     system: String,
@@ -71,17 +102,17 @@ impl Nix {
             attrargs.push(attr);
         }
 
-        return self.safe_command("nix-build", nixpkgs, attrargs);
+        return self.safe_command(Operation::Build, nixpkgs, attrargs);
     }
 
     pub fn safely(
         &self,
-        cmd: &str,
+        op: Operation,
         nixpkgs: &Path,
         args: Vec<String>,
         keep_stdout: bool,
     ) -> Result<File, File> {
-        return self.run(self.safe_command(cmd, nixpkgs, args), keep_stdout);
+        return self.run(self.safe_command(op, nixpkgs, args), keep_stdout);
     }
 
     pub fn run(&self, mut cmd: Command, keep_stdout: bool) -> Result<File, File> {
@@ -113,12 +144,10 @@ impl Nix {
         }
     }
 
-    pub fn safe_command(&self, cmd: &str, nixpkgs: &Path, args: Vec<String>) -> Command {
-        let mut nixpath = OsString::new();
-        nixpath.push("nixpkgs=");
-        nixpath.push(nixpkgs.as_os_str());
+    pub fn safe_command(&self, op: Operation, nixpkgs: &Path, args: Vec<String>) -> Command {
+        let nixpath = format!("nixpkgs={}", nixpkgs.display());
 
-        let mut command = Command::new(cmd);
+        let mut command = op.command();
         command.env_clear();
         command.current_dir(nixpkgs);
         command.env("HOME", "/homeless-shelter");
@@ -276,7 +305,7 @@ mod tests {
 
         let ret: Result<File, File> =
             nix.run(
-                nix.safe_command("./environment.sh", build_path().as_path(), vec![]),
+                nix.safe_command(Operation::new("./environment.sh"), build_path().as_path(), vec![]),
                 true,
             );
 
@@ -298,7 +327,7 @@ mod tests {
 
         let ret: Result<File, File> =
             nix.run(
-                nix.safe_command("./environment.sh", build_path().as_path(), vec![]),
+                nix.safe_command(Operation::new("./environment.sh"), build_path().as_path(), vec![]),
                 true,
             );
 
@@ -320,7 +349,7 @@ mod tests {
         let nix = nix();
 
         let ret: Result<File, File> = nix.run(
-            nix.safe_command("echo", build_path().as_path(), vec![]),
+            nix.safe_command(Operation::new("echo"), build_path().as_path(), vec![]),
             true,
         );
 
@@ -393,7 +422,7 @@ mod tests {
     #[test]
     fn instantiation() {
         let ret: Result<File, File> = nix().safely(
-            "nix-instantiate",
+            Operation::Instantiate,
             passing_eval_path().as_path(),
             vec![],
             true,
diff --git a/ofborg/src/outpathdiff.rs b/ofborg/src/outpathdiff.rs
index f3e56c7..27c2e3b 100644
--- a/ofborg/src/outpathdiff.rs
+++ b/ofborg/src/outpathdiff.rs
@@ -170,7 +170,7 @@ impl OutPaths {
         }
 
         self.nix.safely(
-            "nix-env",
+            nix::Operation::new("nix-env"),
             &self.path,
             vec![
                 String::from("-f"),
diff --git a/ofborg/src/tasks/massrebuilder.rs b/ofborg/src/tasks/massrebuilder.rs
index 389f920..8d29875 100644
--- a/ofborg/src/tasks/massrebuilder.rs
+++ b/ofborg/src/tasks/massrebuilder.rs
@@ -10,7 +10,7 @@ use std::path::Path;
 use std::path::PathBuf;
 use ofborg::checkout;
 use ofborg::message::{massrebuildjob, buildjob};
-use ofborg::nix::Nix;
+use ofborg::nix;
 
 use ofborg::acl::ACL;
 use ofborg::stats;
@@ -25,7 +25,7 @@ use hubcaps;
 
 pub struct MassRebuildWorker<E> {
     cloner: checkout::CachedCloner,
-    nix: Nix,
+    nix: nix::Nix,
     github: hubcaps::Github,
     acl: ACL,
     identity: String,
@@ -36,7 +36,7 @@ pub struct MassRebuildWorker<E> {
 impl<E: stats::SysEvents> MassRebuildWorker<E> {
     pub fn new(
         cloner: checkout::CachedCloner,
-        nix: Nix,
+        nix: nix::Nix,
         github: hubcaps::Github,
         acl: ACL,
         identity: String,
@@ -291,7 +291,7 @@ impl<E: stats::SysEvents> worker::SimpleWorker for MassRebuildWorker<E> {
         let eval_checks = vec![
             EvalChecker::new(
                 "package-list",
-                "nix-env",
+                nix::Operation::new("nix-env"),
                 vec![
                     String::from("--file"),
                     String::from("."),
@@ -304,7 +304,7 @@ impl<E: stats::SysEvents> worker::SimpleWorker for MassRebuildWorker<E> {
 
             EvalChecker::new(
                 "nixos-options",
-                "nix-instantiate",
+                nix::Operation::Instantiate,
                 vec![
                     String::from("./nixos/release.nix"),
                     String::from("-A"),
@@ -315,7 +315,7 @@ impl<E: stats::SysEvents> worker::SimpleWorker for MassRebuildWorker<E> {
 
             EvalChecker::new(
                 "nixos-manual",
-                "nix-instantiate",
+                nix::Operation::Instantiate,
                 vec![
                     String::from("./nixos/release.nix"),
                     String::from("-A"),
@@ -326,7 +326,7 @@ impl<E: stats::SysEvents> worker::SimpleWorker for MassRebuildWorker<E> {
 
             EvalChecker::new(
                 "nixpkgs-manual",
-                "nix-instantiate",
+                nix::Operation::Instantiate,
                 vec![
                     String::from("./pkgs/top-level/release.nix"),
                     String::from("-A"),
@@ -337,7 +337,7 @@ impl<E: stats::SysEvents> worker::SimpleWorker for MassRebuildWorker<E> {
 
             EvalChecker::new(
                 "nixpkgs-tarball",
-                "nix-instantiate",
+                nix::Operation::Instantiate,
                 vec![
                     String::from("./pkgs/top-level/release.nix"),
                     String::from("-A"),
@@ -348,7 +348,7 @@ impl<E: stats::SysEvents> worker::SimpleWorker for MassRebuildWorker<E> {
 
             EvalChecker::new(
                 "nixpkgs-unstable-jobset",
-                "nix-instantiate",
+                nix::Operation::Instantiate,
                 vec![
                     String::from("./pkgs/top-level/release.nix"),
                     String::from("-A"),
@@ -542,7 +542,7 @@ pub enum System {
 
 #[derive(Debug, PartialEq)]
 struct Stdenvs {
-    nix: Nix,
+    nix: nix::Nix,
     co: PathBuf,
 
     linux_stdenv_before: Option<String>,
@@ -553,7 +553,7 @@ struct Stdenvs {
 }
 
 impl Stdenvs {
-    fn new(nix: Nix, co: PathBuf) -> Stdenvs {
+    fn new(nix: nix::Nix, co: PathBuf) -> Stdenvs {
         return Stdenvs {
             nix: nix,
             co: co,
@@ -615,7 +615,7 @@ impl Stdenvs {
 
     fn evalstdenv(&self, system: &str) -> Option<String> {
         let result = self.nix.with_system(system.to_owned()).safely(
-            "nix-instantiate",
+            nix::Operation::Instantiate,
             &self.co,
             vec![
                 String::from("."),
@@ -768,8 +768,7 @@ mod tests {
         let nixpkgs = String::from_utf8(output.stdout)
             .expect("nixpkgs required");
 
-        let nix = Nix::new(String::from("x86_64-linux"), String::from("daemon"), 1200, None);
-
+        let nix = nix::Nix::new(String::from("x86_64-linux"), String::from("daemon"), 1200, None);
         let mut stdenv =
             Stdenvs::new(
                 nix.clone(),