diff --git a/src/compute_msd.cpp b/src/compute_msd.cpp
index 7ecb89b4c0..90d8f47b7a 100644
--- a/src/compute_msd.cpp
+++ b/src/compute_msd.cpp
@@ -27,8 +27,7 @@ using namespace LAMMPS_NS;
 
 /* ---------------------------------------------------------------------- */
 
-ComputeMSD::ComputeMSD(LAMMPS *lmp, int narg, char **arg)
-  : Compute(lmp, narg, arg), id_fix(nullptr)
+ComputeMSD::ComputeMSD(LAMMPS *lmp, int narg, char **arg) : Compute(lmp, narg, arg), id_fix(nullptr)
 {
   if (narg < 3) error->all(FLERR, "Illegal compute msd command");
 
@@ -46,15 +45,15 @@ ComputeMSD::ComputeMSD(LAMMPS *lmp, int narg, char **arg)
   int iarg = 3;
   while (iarg < narg) {
     if (strcmp(arg[iarg], "com") == 0) {
-      if (iarg + 2 > narg) error->all(FLERR, "Illegal compute msd command");
+      if (iarg + 2 > narg) utils::missing_cmd_args(FLERR, "compute msd com", error);
       comflag = utils::logical(FLERR, arg[iarg + 1], false, lmp);
       iarg += 2;
     } else if (strcmp(arg[iarg], "average") == 0) {
-      if (iarg + 2 > narg) error->all(FLERR, "Illegal compute msd command");
+      if (iarg + 2 > narg) utils::missing_cmd_args(FLERR, "compute msd average", error);
       avflag = utils::logical(FLERR, arg[iarg + 1], false, lmp);
       iarg += 2;
     } else
-      error->all(FLERR, "Illegal compute msd command");
+      error->all(FLERR, "Unknown compute msd keyword: {}", arg[iarg]);
   }
 
   if (group->dynamic[igroup])
diff --git a/src/fix_indent.cpp b/src/fix_indent.cpp
index 3f99d14a8c..766ec88d8e 100644
--- a/src/fix_indent.cpp
+++ b/src/fix_indent.cpp
@@ -43,7 +43,7 @@ FixIndent::FixIndent(LAMMPS *lmp, int narg, char **arg) :
   Fix(lmp, narg, arg),
   xstr(nullptr), ystr(nullptr), zstr(nullptr), rstr(nullptr), pstr(nullptr)
 {
-  if (narg < 4) error->all(FLERR,"Illegal fix indent command");
+  if (narg < 4) utils::missing_cmd_args(FLERR, "fix indent", error);
 
   scalar_flag = 1;
   vector_flag = 1;
@@ -405,7 +405,7 @@ double FixIndent::compute_vector(int n)
 
 void FixIndent::options(int narg, char **arg)
 {
-  if (narg < 0) error->all(FLERR,"Illegal fix indent command");
+  if (narg < 0) utils::missing_cmd_args(FLERR, "fix indent", error);
 
   istyle = NONE;
   xstr = ystr = zstr = rstr = pstr = nullptr;
@@ -416,7 +416,7 @@ void FixIndent::options(int narg, char **arg)
   int iarg = 0;
   while (iarg < narg) {
     if (strcmp(arg[iarg],"sphere") == 0) {
-      if (iarg+5 > narg) error->all(FLERR,"Illegal fix indent command");
+      if (iarg+5 > narg) utils::missing_cmd_args(FLERR, "fix indent sphere", error);
 
       if (utils::strmatch(arg[iarg+1],"^v_")) {
         xstr = utils::strdup(arg[iarg+1]+2);
@@ -435,7 +435,7 @@ void FixIndent::options(int narg, char **arg)
       iarg += 5;
 
     } else if (strcmp(arg[iarg],"cylinder") == 0) {
-      if (iarg+5 > narg) error->all(FLERR,"Illegal fix indent command");
+      if (iarg+5 > narg) utils::missing_cmd_args(FLERR, "fix indent cylinder", error);
 
       if (strcmp(arg[iarg+1],"x") == 0) {
         cdim = 0;
@@ -461,7 +461,7 @@ void FixIndent::options(int narg, char **arg)
         if (utils::strmatch(arg[iarg+3],"^v_")) {
           ystr = utils::strdup(arg[iarg+3]+2);
         } else yvalue = utils::numeric(FLERR,arg[iarg+3],false,lmp);
-      } else error->all(FLERR,"Illegal fix indent command");
+      } else error->all(FLERR,"Unknown fix indent cylinder argument: {}", arg[iarg+1]);
 
       if (utils::strmatch(arg[iarg+4],"^v_")) {
         rstr = utils::strdup(arg[iarg+4]+2);
@@ -471,11 +471,11 @@ void FixIndent::options(int narg, char **arg)
       iarg += 5;
 
     } else if (strcmp(arg[iarg],"plane") == 0) {
-      if (iarg+4 > narg) error->all(FLERR,"Illegal fix indent command");
+      if (iarg+4 > narg) utils::missing_cmd_args(FLERR, "fix indent plane", error);
       if (strcmp(arg[iarg+1],"x") == 0) cdim = 0;
       else if (strcmp(arg[iarg+1],"y") == 0) cdim = 1;
       else if (strcmp(arg[iarg+1],"z") == 0) cdim = 2;
-      else error->all(FLERR,"Illegal fix indent command");
+      else error->all(FLERR,"Unknown fix indent plane argument: {}", arg[iarg+1]);
 
       if (utils::strmatch(arg[iarg+2],"^v_")) {
         pstr = utils::strdup(arg[iarg+2]+2);
@@ -483,23 +483,23 @@ void FixIndent::options(int narg, char **arg)
 
       if (strcmp(arg[iarg+3],"lo") == 0) planeside = -1;
       else if (strcmp(arg[iarg+3],"hi") == 0) planeside = 1;
-      else error->all(FLERR,"Illegal fix indent command");
+      else error->all(FLERR,"Unknown fix indent plane argument: {}", arg[iarg+3]);
       istyle = PLANE;
       iarg += 4;
 
     } else if (strcmp(arg[iarg],"units") == 0) {
-      if (iarg+2 > narg) error->all(FLERR,"Illegal fix indent command");
+      if (iarg+2 > narg) utils::missing_cmd_args(FLERR, "fix indent units", error);
       if (strcmp(arg[iarg+1],"box") == 0) scaleflag = 0;
       else if (strcmp(arg[iarg+1],"lattice") == 0) scaleflag = 1;
-      else error->all(FLERR,"Illegal fix indent command");
+      else error->all(FLERR,"Unknown fix indent units argument: {}", arg[iarg+1]);
       iarg += 2;
 
     } else if (strcmp(arg[iarg],"side") == 0) {
-      if (iarg+2 > narg) error->all(FLERR,"Illegal fix indent command");
+      if (iarg+2 > narg) utils::missing_cmd_args(FLERR, "fix indent side", error);
       if (strcmp(arg[iarg+1],"in") == 0) side = INSIDE;
       else if (strcmp(arg[iarg+1],"out") == 0) side = OUTSIDE;
-      else error->all(FLERR,"Illegal fix indent command");
+      else error->all(FLERR,"Unknown fix indent side argument: {}", arg[iarg+1]);
       iarg += 2;
-    } else error->all(FLERR,"Illegal fix indent command");
+    } else error->all(FLERR,"Unknown fix indent argument: {}", arg[iarg]);
   }
 }
diff --git a/src/fix_nh.cpp b/src/fix_nh.cpp
index cb0408a50c..89fbf2ef45 100644
--- a/src/fix_nh.cpp
+++ b/src/fix_nh.cpp
@@ -60,7 +60,7 @@ FixNH::FixNH(LAMMPS *lmp, int narg, char **arg) :
   eta_mass(nullptr), etap(nullptr), etap_dot(nullptr), etap_dotdot(nullptr),
   etap_mass(nullptr)
 {
-  if (narg < 4) error->all(FLERR,"Illegal fix nvt/npt/nph command");
+  if (narg < 4) utils::missing_cmd_args(FLERR, "fix nvt/npt/nph", error);
 
   restart_global = 1;
   dynamic_group_allow = 1;
@@ -130,7 +130,7 @@ FixNH::FixNH(LAMMPS *lmp, int narg, char **arg) :
 
   while (iarg < narg) {
     if (strcmp(arg[iarg],"temp") == 0) {
-      if (iarg+4 > narg) error->all(FLERR,"Illegal fix nvt/npt/nph command");
+      if (iarg+4 > narg) utils::missing_cmd_args(FLERR, "fix nvt/npt/nph temp", error);
       tstat_flag = 1;
       t_start = utils::numeric(FLERR,arg[iarg+1],false,lmp);
       t_target = t_start;
@@ -142,7 +142,7 @@ FixNH::FixNH(LAMMPS *lmp, int narg, char **arg) :
       iarg += 4;
 
     } else if (strcmp(arg[iarg],"iso") == 0) {
-      if (iarg+4 > narg) error->all(FLERR,"Illegal fix nvt/npt/nph command");
+      if (iarg+4 > narg) utils::missing_cmd_args(FLERR, "fix nvt/npt/nph iso", error);
       pcouple = XYZ;
       p_start[0] = p_start[1] = p_start[2] = utils::numeric(FLERR,arg[iarg+1],false,lmp);
       p_stop[0] = p_stop[1] = p_stop[2] = utils::numeric(FLERR,arg[iarg+2],false,lmp);
@@ -155,7 +155,7 @@ FixNH::FixNH(LAMMPS *lmp, int narg, char **arg) :
       }
       iarg += 4;
     } else if (strcmp(arg[iarg],"aniso") == 0) {
-      if (iarg+4 > narg) error->all(FLERR,"Illegal fix nvt/npt/nph command");
+      if (iarg+4 > narg) utils::missing_cmd_args(FLERR, "fix nvt/npt/nph aniso", error);
       pcouple = NONE;
       p_start[0] = p_start[1] = p_start[2] = utils::numeric(FLERR,arg[iarg+1],false,lmp);
       p_stop[0] = p_stop[1] = p_stop[2] = utils::numeric(FLERR,arg[iarg+2],false,lmp);
@@ -168,7 +168,7 @@ FixNH::FixNH(LAMMPS *lmp, int narg, char **arg) :
       }
       iarg += 4;
     } else if (strcmp(arg[iarg],"tri") == 0) {
-      if (iarg+4 > narg) error->all(FLERR,"Illegal fix nvt/npt/nph command");
+      if (iarg+4 > narg) utils::missing_cmd_args(FLERR, "fix nvt/npt/nph tri", error);
       pcouple = NONE;
       scalexy = scalexz = scaleyz = 0;
       p_start[0] = p_start[1] = p_start[2] = utils::numeric(FLERR,arg[iarg+1],false,lmp);
@@ -191,7 +191,7 @@ FixNH::FixNH(LAMMPS *lmp, int narg, char **arg) :
       }
       iarg += 4;
     } else if (strcmp(arg[iarg],"x") == 0) {
-      if (iarg+4 > narg) error->all(FLERR,"Illegal fix nvt/npt/nph command");
+      if (iarg+4 > narg) utils::missing_cmd_args(FLERR, "fix nvt/npt/nph x", error);
       p_start[0] = utils::numeric(FLERR,arg[iarg+1],false,lmp);
       p_stop[0] = utils::numeric(FLERR,arg[iarg+2],false,lmp);
       p_period[0] = utils::numeric(FLERR,arg[iarg+3],false,lmp);
@@ -199,7 +199,7 @@ FixNH::FixNH(LAMMPS *lmp, int narg, char **arg) :
       deviatoric_flag = 1;
       iarg += 4;
     } else if (strcmp(arg[iarg],"y") == 0) {
-      if (iarg+4 > narg) error->all(FLERR,"Illegal fix nvt/npt/nph command");
+      if (iarg+4 > narg) utils::missing_cmd_args(FLERR, "fix nvt/npt/nph y", error);
       p_start[1] = utils::numeric(FLERR,arg[iarg+1],false,lmp);
       p_stop[1] = utils::numeric(FLERR,arg[iarg+2],false,lmp);
       p_period[1] = utils::numeric(FLERR,arg[iarg+3],false,lmp);
@@ -207,7 +207,7 @@ FixNH::FixNH(LAMMPS *lmp, int narg, char **arg) :
       deviatoric_flag = 1;
       iarg += 4;
     } else if (strcmp(arg[iarg],"z") == 0) {
-      if (iarg+4 > narg) error->all(FLERR,"Illegal fix nvt/npt/nph command");
+      if (iarg+4 > narg) utils::missing_cmd_args(FLERR, "fix nvt/npt/nph z", error);
       p_start[2] = utils::numeric(FLERR,arg[iarg+1],false,lmp);
       p_stop[2] = utils::numeric(FLERR,arg[iarg+2],false,lmp);
       p_period[2] = utils::numeric(FLERR,arg[iarg+3],false,lmp);
@@ -218,7 +218,7 @@ FixNH::FixNH(LAMMPS *lmp, int narg, char **arg) :
         error->all(FLERR,"Invalid fix nvt/npt/nph command for a 2d simulation");
 
     } else if (strcmp(arg[iarg],"yz") == 0) {
-      if (iarg+4 > narg) error->all(FLERR,"Illegal fix nvt/npt/nph command");
+      if (iarg+4 > narg) utils::missing_cmd_args(FLERR, "fix nvt/npt/nph yz", error);
       p_start[3] = utils::numeric(FLERR,arg[iarg+1],false,lmp);
       p_stop[3] = utils::numeric(FLERR,arg[iarg+2],false,lmp);
       p_period[3] = utils::numeric(FLERR,arg[iarg+3],false,lmp);
@@ -229,7 +229,7 @@ FixNH::FixNH(LAMMPS *lmp, int narg, char **arg) :
       if (dimension == 2)
         error->all(FLERR,"Invalid fix nvt/npt/nph command for a 2d simulation");
     } else if (strcmp(arg[iarg],"xz") == 0) {
-      if (iarg+4 > narg) error->all(FLERR,"Illegal fix nvt/npt/nph command");
+      if (iarg+4 > narg) utils::missing_cmd_args(FLERR, "fix nvt/npt/nph xz", error);
       p_start[4] = utils::numeric(FLERR,arg[iarg+1],false,lmp);
       p_stop[4] = utils::numeric(FLERR,arg[iarg+2],false,lmp);
       p_period[4] = utils::numeric(FLERR,arg[iarg+3],false,lmp);
@@ -240,7 +240,7 @@ FixNH::FixNH(LAMMPS *lmp, int narg, char **arg) :
       if (dimension == 2)
         error->all(FLERR,"Invalid fix nvt/npt/nph command for a 2d simulation");
     } else if (strcmp(arg[iarg],"xy") == 0) {
-      if (iarg+4 > narg) error->all(FLERR,"Illegal fix nvt/npt/nph command");
+      if (iarg+4 > narg) utils::missing_cmd_args(FLERR, "fix nvt/npt/nph xy", error);
       p_start[5] = utils::numeric(FLERR,arg[iarg+1],false,lmp);
       p_stop[5] = utils::numeric(FLERR,arg[iarg+2],false,lmp);
       p_period[5] = utils::numeric(FLERR,arg[iarg+3],false,lmp);
@@ -250,7 +250,7 @@ FixNH::FixNH(LAMMPS *lmp, int narg, char **arg) :
       iarg += 4;
 
     } else if (strcmp(arg[iarg],"couple") == 0) {
-      if (iarg+2 > narg) error->all(FLERR,"Illegal fix nvt/npt/nph command");
+      if (iarg+2 > narg) utils::missing_cmd_args(FLERR, "fix nvt/npt/nph couple", error);
       if (strcmp(arg[iarg+1],"xyz") == 0) pcouple = XYZ;
       else if (strcmp(arg[iarg+1],"xy") == 0) pcouple = XY;
       else if (strcmp(arg[iarg+1],"yz") == 0) pcouple = YZ;
@@ -260,18 +260,18 @@ FixNH::FixNH(LAMMPS *lmp, int narg, char **arg) :
       iarg += 2;
 
     } else if (strcmp(arg[iarg],"drag") == 0) {
-      if (iarg+2 > narg) error->all(FLERR,"Illegal fix nvt/npt/nph command");
+      if (iarg+2 > narg) utils::missing_cmd_args(FLERR, "fix nvt/npt/nph drag", error);
       drag = utils::numeric(FLERR,arg[iarg+1],false,lmp);
-      if (drag < 0.0) error->all(FLERR,"Illegal fix nvt/npt/nph command");
+      if (drag < 0.0) error->all(FLERR, "Invalid fix nvt/npt/nph drag argument: {}", drag);
       iarg += 2;
     } else if (strcmp(arg[iarg],"ptemp") == 0) {
-      if (iarg+2 > narg) error->all(FLERR,"Illegal fix nvt/npt/nph command");
+      if (iarg+2 > narg) utils::missing_cmd_args(FLERR, "fix nvt/npt/nph ptemp", error);
       p_temp = utils::numeric(FLERR,arg[iarg+1],false,lmp);
       p_temp_flag = 1;
-      if (p_temp <= 0.0) error->all(FLERR,"Illegal fix nvt/npt/nph command");
+      if (p_temp <= 0.0) error->all(FLERR, "Invalid fix nvt/npt/nph ptemp argument: {}", p_temp);
       iarg += 2;
     } else if (strcmp(arg[iarg],"dilate") == 0) {
-      if (iarg+2 > narg) error->all(FLERR,"Illegal fix nvt/npt/nph command");
+      if (iarg+2 > narg) utils::missing_cmd_args(FLERR, "fix nvt/npt/nph dilate", error);
       if (strcmp(arg[iarg+1],"all") == 0) allremap = 1;
       else {
         allremap = 0;
@@ -284,62 +284,62 @@ FixNH::FixNH(LAMMPS *lmp, int narg, char **arg) :
       iarg += 2;
 
     } else if (strcmp(arg[iarg],"tchain") == 0) {
-      if (iarg+2 > narg) error->all(FLERR,"Illegal fix nvt/npt/nph command");
+      if (iarg+2 > narg) utils::missing_cmd_args(FLERR, "fix nvt/npt/nph tchain", error);
       mtchain = utils::inumeric(FLERR,arg[iarg+1],false,lmp);
       // used by FixNVTSllod to preserve non-default value
       mtchain_default_flag = 0;
-      if (mtchain < 1) error->all(FLERR,"Illegal fix nvt/npt/nph command");
+      if (mtchain < 1) error->all(FLERR, "Invalid fix nvt/npt/nph tchain argument: {}", mtchain);
       iarg += 2;
     } else if (strcmp(arg[iarg],"pchain") == 0) {
-      if (iarg+2 > narg) error->all(FLERR,"Illegal fix nvt/npt/nph command");
+      if (iarg+2 > narg) utils::missing_cmd_args(FLERR, "fix nvt/npt/nph pchain", error);
       mpchain = utils::inumeric(FLERR,arg[iarg+1],false,lmp);
-      if (mpchain < 0) error->all(FLERR,"Illegal fix nvt/npt/nph command");
+      if (mpchain < 0) error->all(FLERR, "Invalid fix nvt/npt/nph pchain argument: {}", mpchain);
       iarg += 2;
     } else if (strcmp(arg[iarg],"mtk") == 0) {
-      if (iarg+2 > narg) error->all(FLERR,"Illegal fix nvt/npt/nph command");
+      if (iarg+2 > narg) utils::missing_cmd_args(FLERR, "fix nvt/npt/nph mtk", error);
       mtk_flag = utils::logical(FLERR,arg[iarg+1],false,lmp);
       iarg += 2;
     } else if (strcmp(arg[iarg],"tloop") == 0) {
-      if (iarg+2 > narg) error->all(FLERR,"Illegal fix nvt/npt/nph command");
+      if (iarg+2 > narg) utils::missing_cmd_args(FLERR, "fix nvt/npt/nph tloop", error);
       nc_tchain = utils::inumeric(FLERR,arg[iarg+1],false,lmp);
-      if (nc_tchain < 0) error->all(FLERR,"Illegal fix nvt/npt/nph command");
+      if (nc_tchain < 0) error->all(FLERR, "Invalid fix nvt/npt/nph tloop argument: {}", nc_tchain);
       iarg += 2;
     } else if (strcmp(arg[iarg],"ploop") == 0) {
-      if (iarg+2 > narg) error->all(FLERR,"Illegal fix nvt/npt/nph command");
+      if (iarg+2 > narg) utils::missing_cmd_args(FLERR, "fix nvt/npt/nph ploop", error);
       nc_pchain = utils::inumeric(FLERR,arg[iarg+1],false,lmp);
-      if (nc_pchain < 0) error->all(FLERR,"Illegal fix nvt/npt/nph command");
+      if (nc_pchain < 0) error->all(FLERR, "Invalid fix nvt/npt/nph ploop argument: {}", nc_pchain);
       iarg += 2;
     } else if (strcmp(arg[iarg],"nreset") == 0) {
-      if (iarg+2 > narg) error->all(FLERR,"Illegal fix nvt/npt/nph command");
+      if (iarg+2 > narg) utils::missing_cmd_args(FLERR, "fix nvt/npt/nph nreset", error);
       nreset_h0 = utils::inumeric(FLERR,arg[iarg+1],false,lmp);
-      if (nreset_h0 < 0) error->all(FLERR,"Illegal fix nvt/npt/nph command");
+      if (nreset_h0 < 0) error->all(FLERR, "Invalid fix nvt/npt/nph nreset argument: {}", nreset_h0);
       iarg += 2;
     } else if (strcmp(arg[iarg],"scalexy") == 0) {
-      if (iarg+2 > narg) error->all(FLERR,"Illegal fix nvt/npt/nph command");
+      if (iarg+2 > narg) utils::missing_cmd_args(FLERR, "fix nvt/npt/nph scalexy", error);
       scalexy = utils::logical(FLERR,arg[iarg+1],false,lmp);
       iarg += 2;
     } else if (strcmp(arg[iarg],"scalexz") == 0) {
-      if (iarg+2 > narg) error->all(FLERR,"Illegal fix nvt/npt/nph command");
+      if (iarg+2 > narg) utils::missing_cmd_args(FLERR, "fix nvt/npt/nph scalexz", error);
       scalexz = utils::logical(FLERR,arg[iarg+1],false,lmp);
       iarg += 2;
     } else if (strcmp(arg[iarg],"scaleyz") == 0) {
-      if (iarg+2 > narg) error->all(FLERR,"Illegal fix nvt/npt/nph command");
+      if (iarg+2 > narg) utils::missing_cmd_args(FLERR, "fix nvt/npt/nph scaleyz", error);
       scaleyz = utils::logical(FLERR,arg[iarg+1],false,lmp);
       iarg += 2;
     } else if (strcmp(arg[iarg],"flip") == 0) {
-      if (iarg+2 > narg) error->all(FLERR,"Illegal fix nvt/npt/nph command");
+      if (iarg+2 > narg) utils::missing_cmd_args(FLERR, "fix nvt/npt/nph flip", error);
       flipflag = utils::logical(FLERR,arg[iarg+1],false,lmp);
       iarg += 2;
     } else if (strcmp(arg[iarg],"update") == 0) {
-      if (iarg+2 > narg) error->all(FLERR,"Illegal fix nvt/npt/nph command");
+      if (iarg+2 > narg) utils::missing_cmd_args(FLERR, "fix nvt/npt/nph update", error);
       if (strcmp(arg[iarg+1],"dipole") == 0) dipole_flag = 1;
       else if (strcmp(arg[iarg+1],"dipole/dlm") == 0) {
         dipole_flag = 1;
         dlm_flag = 1;
-      } else error->all(FLERR,"Illegal fix nvt/npt/nph command");
+      } else error->all(FLERR, "Invalid fix nvt/npt/nph update argument: {}", arg[iarg+1]);
       iarg += 2;
     } else if (strcmp(arg[iarg],"fixedpoint") == 0) {
-      if (iarg+4 > narg) error->all(FLERR,"Illegal fix nvt/npt/nph command");
+      if (iarg+4 > narg) utils::missing_cmd_args(FLERR, "fix nvt/npt/nph fixedpoint", error);
       fixedpoint[0] = utils::numeric(FLERR,arg[iarg+1],false,lmp);
       fixedpoint[1] = utils::numeric(FLERR,arg[iarg+2],false,lmp);
       fixedpoint[2] = utils::numeric(FLERR,arg[iarg+3],false,lmp);
@@ -359,7 +359,7 @@ FixNH::FixNH(LAMMPS *lmp, int narg, char **arg) :
     } else if (strcmp(arg[iarg],"ext") == 0) {
       iarg += 2;
 
-    } else error->all(FLERR,"Illegal fix nvt/npt/nph command");
+    } else error->all(FLERR,"Unknown fix nvt/npt/nph keyword: {}", arg[iarg]);
   }
 
   // error checks