diff --git a/src/write_coeff.cpp b/src/write_coeff.cpp
index f71cab0fdd..9dfdeb5141 100644
--- a/src/write_coeff.cpp
+++ b/src/write_coeff.cpp
@@ -41,7 +41,7 @@ void WriteCoeff::command(int narg, char **arg)
   if (domain->box_exist == 0)
     error->all(FLERR, "Write_coeff command before simulation box is defined");
 
-  if (narg != 1) error->all(FLERR, "Illegal write_coeff command");
+  if (narg != 1) utils::missing_cmd_args(FLERR, "write_coeff", error);
 
   char *file = utils::strdup(fmt::format("{}.tmp", arg[0]));
 
@@ -167,6 +167,5 @@ void WriteCoeff::command(int narg, char **arg)
     fclose(two);
     platform::unlink(file);
   }
-
   delete[] file;
 }
diff --git a/src/write_data.cpp b/src/write_data.cpp
index 447399c65f..75182c869f 100644
--- a/src/write_data.cpp
+++ b/src/write_data.cpp
@@ -57,7 +57,7 @@ void WriteData::command(int narg, char **arg)
   if (domain->box_exist == 0)
     error->all(FLERR,"Write_data command before simulation box is defined");
 
-  if (narg < 1) error->all(FLERR,"Illegal write_data command");
+  if (narg < 1) utils::missing_cmd_args(FLERR, "write_data", error);
 
   // if filename contains a "*", replace with current timestep
 
@@ -79,10 +79,10 @@ void WriteData::command(int narg, char **arg)
   int iarg = 1;
   while (iarg < narg) {
     if (strcmp(arg[iarg],"pair") == 0) {
-      if (iarg+2 > narg) error->all(FLERR,"Illegal write_data command");
+      if (iarg+2 > narg) utils::missing_cmd_args(FLERR, "write_data pair", error);
       if (strcmp(arg[iarg+1],"ii") == 0) pairflag = II;
       else if (strcmp(arg[iarg+1],"ij") == 0) pairflag = IJ;
-      else error->all(FLERR,"Illegal write_data command");
+      else error->all(FLERR,"Unknown write_data pair option: {}", arg[iarg+1]);
       iarg += 2;
     } else if (strcmp(arg[iarg],"noinit") == 0) {
       noinit = 1;
@@ -97,12 +97,12 @@ void WriteData::command(int narg, char **arg)
       lmapflag = 0;
       iarg++;
     } else if (strcmp(arg[iarg],"types") == 0) {
-      if (iarg+2 > narg) error->all(FLERR,"Illegal write_data command");
+      if (iarg+2 > narg) utils::missing_cmd_args(FLERR, "write_data types", error);
       if (strcmp(arg[iarg+1],"numeric") == 0) atom->types_style = Atom::NUMERIC;
       else if (strcmp(arg[iarg+1],"labels") == 0) atom->types_style = Atom::LABELS;
-      else error->all(FLERR,"Illegal write_data command");
+      else error->all(FLERR,"Unknown write_data types option: {}", arg[iarg+1]);
       iarg += 2;
-    } else error->all(FLERR,"Illegal write_data command");
+    } else error->all(FLERR,"Unknown write_data keyword: {}", arg[iarg]);
   }
 
   // init entire system since comm->exchange is done
diff --git a/src/write_dump.cpp b/src/write_dump.cpp
index e1a3a6b74d..a7f143aa43 100644
--- a/src/write_dump.cpp
+++ b/src/write_dump.cpp
@@ -33,7 +33,7 @@ using namespace LAMMPS_NS;
 
 void WriteDump::command(int narg, char **arg)
 {
-  if (narg < 3) error->all(FLERR, "Illegal write_dump command");
+  if (narg < 3) utils::missing_cmd_args(FLERR, "write_dump", error);
 
   // modindex = index in args of "modify" keyword
   // will be narg if "modify" is not present
diff --git a/unittest/formats/test_file_operations.cpp b/unittest/formats/test_file_operations.cpp
index d0a6ef873c..ef954b51f3 100644
--- a/unittest/formats/test_file_operations.cpp
+++ b/unittest/formats/test_file_operations.cpp
@@ -439,9 +439,17 @@ TEST_F(FileOperationsTest, write_data)
     ASSERT_FILE_EXISTS("charge.data");
     ASSERT_EQ(count_lines("charge.data"), 30);
 
-    TEST_FAILURE(".*ERROR: Illegal write_data command.*", command("write_data"););
-    TEST_FAILURE(".*ERROR: Illegal write_data command.*", command("write_data test.data xxxx"););
-    TEST_FAILURE(".*ERROR: Illegal write_data command.*", command("write_data test.data pair xx"););
+    TEST_FAILURE(".*ERROR: Illegal write_data command: missing argument.*", command("write_data"););
+    TEST_FAILURE(".*ERROR: Unknown write_data keyword: xxxx.*",
+                 command("write_data test.data xxxx"););
+    TEST_FAILURE(".*ERROR: Illegal write_data pair command: missing argument.*",
+                 command("write_data test.data pair"););
+    TEST_FAILURE(".*ERROR: Unknown write_data pair option: xx.*",
+                 command("write_data test.data pair xx"););
+    TEST_FAILURE(".*ERROR: Illegal write_data types command: missing argument.*",
+                 command("write_data test.data types"););
+    TEST_FAILURE(".*ERROR: Unknown write_data types option: xx.*",
+                 command("write_data test.data types xx"););
     TEST_FAILURE(".*ERROR on proc 0: Cannot open data file some_crazy_dir/test.data:"
                  " No such file or directory.*",
                  command("write_data some_crazy_dir/test.data"););