du, ls: add integration tests for block size env var behavior

tests/by-util/test_du.rs

@@ -371,6 +371,74 @@ fn test_du_invalid_binary_size() {
         .stderr_only("du: invalid suffix in --threshold argument '0b123'\n");
 }
 
+#[test]
+fn test_du_invalid_env_block_size_stops_lookup() {
+    // When DU_BLOCK_SIZE is set but invalid, it should stop the lookup
+    // and use the default block size — not fall through to BLOCK_SIZE.
+    let ts = TestScenario::new(util_name!());
+    let at = &ts.fixtures;
+    let dir = "a";
+
+    at.mkdir(dir);
+    at.write(&format!("{dir}/file"), "some content");
+
+    let default_output = ts
+        .ucmd()
+        .arg(dir)
+        .arg("--block-size=1024")
+        .succeeds()
+        .stdout_move_str();
+
+    // Invalid DU_BLOCK_SIZE should use default (1024), not BLOCK_SIZE=1
+    let result = ts
+        .ucmd()
+        .arg(dir)
+        .env("DU_BLOCK_SIZE", "invalid")
+        .env("BLOCK_SIZE", "1")
+        .succeeds()
+        .stdout_move_str();
+
+    assert_eq!(default_output, result);
+
+    // Empty DU_BLOCK_SIZE should also use default
+    let result = ts
+        .ucmd()
+        .arg(dir)
+        .env("DU_BLOCK_SIZE", "")
+        .env("BLOCK_SIZE", "1")
+        .succeeds()
+        .stdout_move_str();
+
+    assert_eq!(default_output, result);
+}
+
+#[test]
+fn test_du_posixly_correct_default() {
+    // With POSIXLY_CORRECT and no block size env vars, default is 512
+    let ts = TestScenario::new(util_name!());
+    let at = &ts.fixtures;
+    let dir = "a";
+
+    at.mkdir(dir);
+    at.write(&format!("{dir}/file"), "some content");
+
+    let expected = ts
+        .ucmd()
+        .arg(dir)
+        .arg("--block-size=512")
+        .succeeds()
+        .stdout_move_str();
+
+    let result = ts
+        .ucmd()
+        .arg(dir)
+        .env("POSIXLY_CORRECT", "1")
+        .succeeds()
+        .stdout_move_str();
+
+    assert_eq!(expected, result);
+}
+
 #[test]
 fn test_du_binary_edge_cases() {
     let ts = TestScenario::new(util_name!());

tests/by-util/test_ls.rs

@@ -5781,6 +5781,53 @@ fn test_ls_block_size_override() {
         .stdout_contains_line("total 8");
 }
 
+#[cfg(unix)]
+#[test]
+#[cfg(not(target_os = "openbsd"))]
+fn test_ls_block_size_si_file_size() {
+    // Verify --si and --block-size interaction for file size display in -l
+    let scene = TestScenario::new(util_name!());
+    let at = &scene.fixtures;
+
+    at.write_bytes("file", &[0u8; 1024]);
+
+    // --si last: file size shown as human-readable SI
+    scene
+        .ucmd()
+        .arg("-l")
+        .arg("--block-size=512")
+        .arg("--si")
+        .succeeds()
+        .stdout_contains("1.1k");
+
+    // --block-size last: file size shown in 512-byte blocks
+    scene
+        .ucmd()
+        .arg("-l")
+        .arg("--si")
+        .arg("--block-size=512")
+        .succeeds()
+        .stdout_contains(" 2 ");
+
+    // --human-readable last: file size shown as human-readable IEC
+    scene
+        .ucmd()
+        .arg("-l")
+        .arg("--block-size=512")
+        .arg("--human-readable")
+        .succeeds()
+        .stdout_contains("1.0K");
+
+    // --block-size last: file size shown in 512-byte blocks
+    scene
+        .ucmd()
+        .arg("-l")
+        .arg("--human-readable")
+        .arg("--block-size=512")
+        .succeeds()
+        .stdout_contains(" 2 ");
+}
+
 #[test]
 fn test_ls_block_size_override_self() {
     new_ucmd!()