diff --git a/src/input.cpp b/src/input.cpp
index df4a2dfd7e..97eaa5cd58 100644
--- a/src/input.cpp
+++ b/src/input.cpp
@@ -265,7 +265,7 @@ char *Input::one(const char *single)
    command = first word
    narg = # of args
    arg[] = individual args
-   treat text between double quotes as one arg
+   treat text between single/double quotes as one arg
 ------------------------------------------------------------------------- */
 
 void Input::parse()
@@ -275,19 +275,17 @@ void Input::parse()
   strcpy(copy,line);
 
   // strip any # comment by resetting string terminator
-  // do not strip # inside double quotes
+  // do not strip # inside single/double quotes
 
-  int level = 0;
+  char quote = '\0';
   char *ptr = copy;
   while (*ptr) {
-    if (*ptr == '#' && level == 0) {
+    if (*ptr == '#' && !quote) {
       *ptr = '\0';
       break;
     }
-    if (*ptr == '"') {
-      if (level == 0) level = 1;
-      else level = 0;
-    }
+    if (*ptr == quote) quote = '\0';
+    else if (*ptr == '"' || *ptr == '\'') quote = *ptr;
     ptr++;
   }
 
@@ -302,9 +300,12 @@ void Input::parse()
   if (command == NULL) return;
 
   // point arg[] at each subsequent arg
-  // treat text between double quotes as one arg
+  // treat text between single/double quotes as one arg
   // insert string terminators in copy to delimit args
 
+  quote = '\0';
+  int iarg,argstart;
+
   narg = 0;
   while (1) {
     if (narg == maxarg) {
@@ -312,19 +313,23 @@ void Input::parse()
       arg = (char **) memory->srealloc(arg,maxarg*sizeof(char *),"input:arg");
     }
     arg[narg] = strtok(NULL," \t\n\r\f");
-    if (arg[narg] && arg[narg][0] == '\"') {
+    if (!arg[narg]) break;
+    if (!quote && (arg[narg][0] == '"' || arg[narg][0] == '\'')) {
+      quote = arg[narg][0];
+      argstart = narg;
       arg[narg] = &arg[narg][1];
-      if (arg[narg][strlen(arg[narg])-1] == '\"')
-	arg[narg][strlen(arg[narg])-1] = '\0';
-      else {
-	arg[narg][strlen(arg[narg])] = ' ';
-	ptr = strtok(arg[narg],"\"");
-	if (ptr == NULL) error->all("Unbalanced quotes in input line");
-      }
     }
-    if (arg[narg]) narg++;
-    else break;
+    if (quote && arg[narg][strlen(arg[narg])-1] == quote) {
+      for (iarg = argstart; iarg < narg; iarg++)
+	arg[iarg][strlen(arg[iarg])] = ' ';
+      arg[narg][strlen(arg[narg])-1] = '\0';
+      narg = argstart;
+      quote = '\0';
+    }
+    narg++;
   }
+
+  if (quote) error->all("Unbalanced quotes in input line");
 }
 
 /* ----------------------------------------------------------------------
@@ -335,18 +340,18 @@ void Input::parse()
 void Input::substitute(char *str, int flag)
 {
   // use work[] as scratch space to expand str
-  // do not replace $ inside double quotes as flagged by level
+  // do not replace $ inside single/double quotes
   // var = pts at variable name, ended by NULL
   //   if $ is followed by '{', trailing '}' becomes NULL
   //   else $x becomes x followed by NULL
   // beyond = pts at text following variable
 
   char *var,*value,*beyond;
-  int level = 0;
+  char quote = '\0';
   char *ptr = str;
 
   while (*ptr) {
-    if (*ptr == '$' && level == 0) {
+    if (*ptr == '$' && !quote) {
       if (*(ptr+1) == '{') {
 	var = ptr+2;
 	int i = 0;
@@ -379,10 +384,8 @@ void Input::substitute(char *str, int flag)
       }
       continue;
     }
-    if (*ptr == '"') {
-      if (level == 0) level = 1;
-      else level = 0;
-    }
+    if (*ptr == quote) quote = '\0';
+    else if (*ptr == '"' || *ptr == '\'') quote = *ptr;
     ptr++;
   }
 }
@@ -521,72 +524,107 @@ void Input::echo()
 
 void Input::ifthenelse()
 {
-  if (narg < 5) error->all("Illegal if command");
+  if (narg < 4) error->all("Illegal if command");
 
-  // flag = 0 for "then"
-  // flag = 1 for "else"
+  // substitute for variables in Boolean expression for "if"
+  // in case expression was enclosed in quotes
 
-  int flag = 0;
-  if (strcmp(arg[1],"==") == 0) {
-    if (atof(arg[0]) == atof(arg[2])) flag = 1;
-  } else if (strcmp(arg[1],"!=") == 0) {
-    if (atof(arg[0]) != atof(arg[2])) flag = 1;
-  } else if (strcmp(arg[1],"<") == 0) {
-    if (atof(arg[0]) < atof(arg[2])) flag = 1;
-  } else if (strcmp(arg[1],"<=") == 0) {
-    if (atof(arg[0]) <= atof(arg[2])) flag = 1;
-  } else if (strcmp(arg[1],">") == 0) {
-    if (atof(arg[0]) > atof(arg[2])) flag = 1;
-  } else if (strcmp(arg[1],">=") == 0) {
-    if (atof(arg[0]) >= atof(arg[2])) flag = 1;
-  } else error->all("Illegal if command");
+  substitute(arg[0],0);
 
-  // first = arg index of first "then" or "else" command
-  // last = arg index of last "then" or "else" command
-  
-  int iarg,first,last;
+  // evaluate Boolean expression for "if"
 
-  // identify range of commands within arg list for then or else
-  // for else, if no comands, just return
+  double btest = variable->evaluate_boolean(arg[0]);
 
-  if (strcmp(arg[3],"then") != 0) error->all("Illegal if command");
-  if (flag) {
-    iarg = first = 4;
-    while (iarg < narg && strcmp(arg[iarg],"else") != 0) iarg++;
+  // bound "then" commands
+
+  if (strcmp(arg[1],"then") != 0) error->all("Illegal if command");
+
+  int first = 2;
+  int iarg = first;
+  while (iarg < narg && 
+	 (strcmp(arg[iarg],"elif") != 0 && strcmp(arg[iarg],"else") != 0))
+    iarg++;
+  int last = iarg-1;
+
+  // execute "then" commands
+  // make copies of all arg string commands
+  // required because re-parsing a command via one() will wipe out args
+
+  if (btest != 0.0) {
+    int ncommands = last-first + 1;
+    if (ncommands <= 0) error->all("Illegal if command");
+
+    char **commands = new char*[ncommands];
+    ncommands = 0;
+    for (int i = first; i <= last; i++) {
+      int n = strlen(arg[i]) + 1;
+      if (n == 1) error->all("Illegal if command");
+      commands[ncommands] = new char[n];
+      strcpy(commands[ncommands],arg[i]);
+      ncommands++;
+    }
+    
+    for (int i = 0; i < ncommands; i++)
+      char *command = input->one(commands[i]);
+    
+    for (int i = 0; i < ncommands; i++) delete [] commands[i];
+    delete [] commands;
+
+    return;
+  }
+
+  // done if no "elif" or "else"
+
+  if (iarg == narg) return;
+
+  // check "elif" or "else" until find commands to execute
+  // substitute for variables and evaluate Boolean expression for "elif"
+  // bound and execute "elif" or "else" commands
+
+  while (1) {
+    if (iarg+2 > narg) error->all("Illegal if command");
+    if (strcmp(arg[iarg],"elif") == 0) {
+      substitute(arg[iarg+1],0);
+      btest = variable->evaluate_boolean(arg[iarg+1]);
+      first = iarg+2;
+    } else {
+      btest = 1.0;
+      first = iarg+1;
+    }
+
+    iarg = first;
+    while (iarg < narg && 
+	   (strcmp(arg[iarg],"elif") != 0 && strcmp(arg[iarg],"else") != 0))
+      iarg++;
     last = iarg-1;
-  } else {
-    iarg = 4;
-    while (iarg < narg && strcmp(arg[iarg],"else") != 0) iarg++;
-    if (iarg == narg) return;
-    first = iarg+1;
-    last = narg-1;
+
+    if (btest == 0.0) continue;
+
+    int ncommands = last-first + 1;
+    if (ncommands <= 0) error->all("Illegal if command");
+
+    char **commands = new char*[ncommands];
+    ncommands = 0;
+    for (int i = first; i <= last; i++) {
+      int n = strlen(arg[i]) + 1;
+      if (n == 1) error->all("Illegal if command");
+      commands[ncommands] = new char[n];
+      strcpy(commands[ncommands],arg[i]);
+      ncommands++;
+    }
+    
+    // execute the list of commands
+    
+    for (int i = 0; i < ncommands; i++)
+      char *command = input->one(commands[i]);
+    
+    // clean up
+
+    for (int i = 0; i < ncommands; i++) delete [] commands[i];
+    delete [] commands;
+
+    return;
   }
-
-  int ncommands = last-first + 1;
-  if (ncommands <= 0) error->all("Illegal if command");
-
-  // make copies of arg strings that are commands
-  // required because re-parsing commands via one() will wipe out args
-
-  char **commands = new char*[ncommands];
-  ncommands = 0;
-  for (int i = first; i <= last; i++) {
-    int n = strlen(arg[i]) + 1;
-    if (n == 1) error->all("Illegal if command");
-    commands[ncommands] = new char[n];
-    strcpy(commands[ncommands],arg[i]);
-    ncommands++;
-  }
-
-  // execute the list of commands
-
-  for (int i = 0; i < ncommands; i++)
-    char *command = input->one(commands[i]);
-
-  // clean up
-
-  for (int i = 0; i < ncommands; i++) delete [] commands[i];
-  delete [] commands;
 }
 
 /* ---------------------------------------------------------------------- */
@@ -685,17 +723,14 @@ void Input::next_command()
 
 void Input::print()
 {
-  if (narg == 0) error->all("Illegal print command");
+  if (narg != 1) error->all("Illegal print command");
 
-  // substitute for $ variables (no printing)
-  // print args one at a time, separated by spaces
+  // substitute for $ variables (no printing) and print arg
 
-  for (int i = 0; i < narg; i++) {
-    substitute(arg[i],0);
-    if (me == 0) {
-      if (screen) fprintf(screen,"%s ",arg[i]);
-      if (logfile) fprintf(logfile,"%s ",arg[i]);
-    }
+  substitute(arg[0],0);
+  if (me == 0) {
+    if (screen) fprintf(screen,"%s ",arg[0]);
+    if (logfile) fprintf(logfile,"%s ",arg[0]);
   }
 
   if (me == 0) {
diff --git a/src/variable.cpp b/src/variable.cpp
index 7bf2fde1cd..b1e4b51773 100644
--- a/src/variable.cpp
+++ b/src/variable.cpp
@@ -608,7 +608,7 @@ void Variable::copy(int narg, char **from, char **to)
      constant = PI
      thermo keyword = ke, vol, atoms, ...
      math operation = (),-x,x+y,x-y,x*y,x/y,x^y,
-                      x==y,x!=y,x<y,x<=y,x>y,x>=y,
+                      x==y,x!=y,x<y,x<=y,x>y,x>=y,x&&y,x||y,
                       sqrt(x),exp(x),ln(x),log(x),
 		      sin(x),cos(x),tan(x),asin(x),atan2(y,x),...
      group function = count(group), mass(group), xcm(group,x), ...
@@ -3069,3 +3069,175 @@ void Variable::print_tree(Tree *tree, int level)
   if (tree->right) print_tree(tree->right,level+1);
   return;
 }
+
+/* ----------------------------------------------------------------------
+   recursive evaluation of string str
+   called from "if" command in input script
+   str is a boolean expression containing one or more items:
+     number = 0.0, -5.45, 2.8e-4, ...
+     math operation = (),x==y,x!=y,x<y,x<=y,x>y,x>=y,x&&y,x||y
+------------------------------------------------------------------------- */
+
+double Variable::evaluate_boolean(char *str)
+{
+  int op,opprevious;
+  double value1,value2;
+  char onechar;
+  char *ptr;
+
+  double argstack[MAXLEVEL];
+  int opstack[MAXLEVEL];
+  int nargstack = 0;
+  int nopstack = 0;
+
+  int i = 0;
+  int expect = ARG;
+
+  while (1) {
+    onechar = str[i];
+    
+    // whitespace: just skip
+    
+    if (isspace(onechar)) i++;
+    
+    // ----------------
+    // parentheses: recursively evaluate contents of parens
+    // ----------------
+    
+    else if (onechar == '(') {
+      if (expect == OP) error->all("Invalid Boolean syntax in if command");
+      expect = OP;
+      
+      char *contents;
+      i = find_matching_paren(str,i,contents);
+      i++;
+      
+      // evaluate contents and push on stack
+      
+      argstack[nargstack++] = evaluate_boolean(contents);
+      
+      delete [] contents;
+      
+    // ----------------
+    // number: push value onto stack
+    // ----------------
+      
+    } else if (isdigit(onechar) || onechar == '.') {
+      if (expect == OP) error->all("Invalid Boolean syntax in if command");
+      expect = OP;
+      
+      // istop = end of number, including scientific notation
+      
+      int istart = i;
+      while (isdigit(str[i]) || str[i] == '.') i++;
+      if (str[i] == 'e' || str[i] == 'E') {
+	i++;
+	if (str[i] == '+' || str[i] == '-') i++;
+	while (isdigit(str[i])) i++;
+      }
+      int istop = i - 1;
+      
+      int n = istop - istart + 1;
+      char *number = new char[n+1];
+      strncpy(number,&str[istart],n);
+      number[n] = '\0';
+      
+      argstack[nargstack++] = atof(number);
+      
+      delete [] number;
+      
+    // ----------------
+    // Boolean operator, including end-of-string
+    // ----------------
+      
+    } else if (strchr("<>=!&|\0",onechar)) {
+      if (onechar == '=') {
+	if (str[i+1] != '=') 
+	  error->all("Invalid Boolean syntax in if command");
+	op = EQ;
+	i++;
+      } else if (onechar == '!') {
+	if (str[i+1] != '=') 
+	  error->all("Invalid Boolean syntax in if command");
+	op = NE;
+	i++;
+      } else if (onechar == '<') {
+	if (str[i+1] != '=') op = LT;
+	else {
+	  op = LE;
+	  i++;
+	}
+      } else if (onechar == '>') {
+	if (str[i+1] != '=') op = GT;
+	else {
+	  op = GE;
+	  i++;
+	}
+      } else if (onechar == '&') {
+	if (str[i+1] != '&') 
+	  error->all("Invalid Boolean syntax in if command");
+	op = AND;
+	i++;
+      } else if (onechar == '|') {
+	if (str[i+1] != '|') 
+	  error->all("Invalid Boolean syntax in if command");
+	op = OR;
+	i++;
+      } else op = DONE;
+      
+      i++;
+      
+      if (expect == ARG) error->all("Invalid Boolean syntax in if command");
+      expect = ARG;
+      
+      // evaluate stack as deep as possible while respecting precedence
+      // before pushing current op onto stack
+      
+      while (nopstack && precedence[opstack[nopstack-1]] >= precedence[op]) {
+	opprevious = opstack[--nopstack];
+	
+	value2 = argstack[--nargstack];
+	value1 = argstack[--nargstack];
+	
+	if (opprevious == EQ) {
+	  if (value1 == value2) argstack[nargstack++] = 1.0;
+	  else argstack[nargstack++] = 0.0;
+	} else if (opprevious == NE) {
+	  if (value1 != value2) argstack[nargstack++] = 1.0;
+	  else argstack[nargstack++] = 0.0;
+	} else if (opprevious == LT) {
+	  if (value1 < value2) argstack[nargstack++] = 1.0;
+	  else argstack[nargstack++] = 0.0;
+	} else if (opprevious == LE) {
+	  if (value1 <= value2) argstack[nargstack++] = 1.0;
+	  else argstack[nargstack++] = 0.0;
+	} else if (opprevious == GT) {
+	  if (value1 > value2) argstack[nargstack++] = 1.0;
+	  else argstack[nargstack++] = 0.0;
+	} else if (opprevious == GE) {
+	  if (value1 >= value2) argstack[nargstack++] = 1.0;
+	  else argstack[nargstack++] = 0.0;
+	} else if (opprevious == AND) {
+	  if (value1 != 0.0 && value2 != 0.0) argstack[nargstack++] = 1.0;
+	  else argstack[nargstack++] = 0.0;
+	} else if (opprevious == OR) {
+	  if (value1 != 0.0 || value2 != 0.0) argstack[nargstack++] = 1.0;
+	  else argstack[nargstack++] = 0.0;
+	}
+      }
+      
+      // if end-of-string, break out of entire formula evaluation loop
+      
+      if (op == DONE) break;
+      
+      // push current operation onto stack
+      
+      opstack[nopstack++] = op;
+      
+    } else error->all("Invalid Boolean syntax in if command");
+  }
+
+  if (nopstack) error->all("Invalid Boolean syntax in if command");
+  if (nargstack != 1) error->all("Invalid Boolean syntax in if command");
+  return argstack[0];
+}
diff --git a/src/variable.h b/src/variable.h
index 841abb3229..1109f57b4d 100644
--- a/src/variable.h
+++ b/src/variable.h
@@ -32,6 +32,7 @@ class Variable : protected Pointers {
   double compute_equal(int);
   void compute_atom(int, int, double *, int, int);
   int int_between_brackets(char *&);
+  double evaluate_boolean(char *);
 
  private:
   int me;