diff --git a/os/build.py b/os/build.py
index 5d47da7..23e6127 100755
--- a/os/build.py
+++ b/os/build.py
@@ -66,6 +66,9 @@ def run(args, check=True, **kwargs):
     print(f"> {' '.join(args)}")
     subprocess.run(args, check=check, **kwargs)
 
+def mkdir(dirname):
+    pathlib.Path(dirname).mkdir(parents=True, exist_ok=True)
+
 def edit_file(filename, fn):
     with open(filename) as f:
         content = f.read()
@@ -87,7 +90,6 @@ def main():
     args = parser.parse_args()
 
     script_dir = pathlib.Path(__file__).parent.resolve()
-    chroot_includes = pathlib.Path("config/includes.chroot")
 
     with open(script_dir / "config/config.toml", "rb") as f:
         config = tomllib.load(f)
@@ -97,7 +99,7 @@ def main():
     run("rm -rf .build config udeb-build".split())
 
     # Download files.
-    run("mkdir -p downloads".split())
+    mkdir("downloads")
     for download in DOWNLOADS:
         filename = f'downloads/{download["name"]}'
         try:
@@ -144,7 +146,7 @@ def main():
             lambda s: s.replace("@install_admin_password@", config["install_admin_password"]))
 
         # Copy inventory file.
-        run("mkdir -p config/includes.binary/install".split())
+        mkdir("config/includes.binary/install")
         run(["cp", f"{script_dir}/config/installer-inventory.txt", "config/includes.binary/install/inventory.txt"])
 
         # Insert build date in login screen logo.
@@ -154,7 +156,7 @@ def main():
         # Build and install custom udeb packages for installer.
         run(["cp", "-rT", f"{script_dir}/installer-udeb", "udeb-build"])
         run("dpkg-buildpackage --build=all".split(), cwd="udeb-build/inventory-hostname")
-        run("mkdir -p config/packages.binary".split())
+        mkdir("config/packages.binary")
         run("cp udeb-build/inventory-hostname_0_all.udeb config/packages.binary/".split())
 
         # Copy the source lists. The installer deletes everything in sources.list.d,
@@ -167,7 +169,7 @@ def main():
             lambda s: s.replace("@contestant_root_password@", config["contestant_root_password"]))
 
         # Copy authorized_keys.
-        run("mkdir -p config/includes.chroot/root/.ssh".split())
+        mkdir("config/includes.chroot/root/.ssh")
         run(["cp", f"{script_dir}/config/contestant_authorized_keys", "config/includes.chroot/root/.ssh/authorized_keys"])
 
     # Configure boot options.
@@ -194,17 +196,17 @@ def main():
         f"\n"
         for i, option in enumerate(VARIANT_BOOT_OPTIONS[args.variant])
     )
-    pathlib.Path("config/bootloaders/syslinux_common").mkdir(parents=True, exist_ok=True)
+    mkdir("config/bootloaders/syslinux_common")
     with open("config/bootloaders/syslinux_common/live.cfg.in", "w") as f:
         f.write(syslinux_boot_options)
 
     # Install soi header.
-    (chroot_includes / "usr/local/include").mkdir(parents=True, exist_ok=True)
-    run(["tar", "--overwrite", "-xf", f"downloads/soi-header.tar.gz", "-C", f"{chroot_includes}/usr/local/include", "--strip-components=2", "soi-header/include/"])
+    mkdir("config/includes.chroot/usr/local/include")
+    run(["tar", "--overwrite", "-xf", "downloads/soi-header.tar.gz", "-C", "config/includes.chroot/usr/local/include", "--strip-components=2", "soi-header/include/"])
 
     # Install codeblocks template.
-    (chroot_includes / "usr/share/codeblocks/templates/wizard").mkdir(parents=True, exist_ok=True)
-    run(["unzip", "-o", f"downloads/soi_template_codeblocks_ubuntu.zip", "-d", f"{chroot_includes}/usr/share/codeblocks/templates/wizard/"])
+    mkdir("config/includes.chroot/usr/share/codeblocks/templates/wizard")
+    run(["unzip", "-o", "downloads/soi_template_codeblocks_ubuntu.zip", "-d", "config/includes.chroot/usr/share/codeblocks/templates/wizard/"])
 
     # Start the build.
     run("lb build".split())