diff --git a/py/desispec/io/util.py b/py/desispec/io/util.py
index c9857fadc..320ea8bac 100644
--- a/py/desispec/io/util.py
+++ b/py/desispec/io/util.py
@@ -1134,6 +1134,8 @@ def backup_filename(filename):
 
     Returns:
         New filename.N, or filename if original file didn't already exist
+
+    if filename=='/dev/null' or filename doesn't exist, just return filename
     """
     if filename == '/dev/null' or not os.path.exists(filename):
         return filename
diff --git a/py/desispec/parallel.py b/py/desispec/parallel.py
index 7a5083391..ad5e97692 100644
--- a/py/desispec/parallel.py
+++ b/py/desispec/parallel.py
@@ -10,6 +10,7 @@
 import os
 import sys
 import time
+import glob
 import io
 from contextlib import contextmanager
 import logging
@@ -380,6 +381,13 @@ def stdouterr_redirected(to=None, comm=None, overwrite=False):
     # The DESI loggers.
     desi_loggers = desiutil.log._desiutil_log_root
 
+    def _rank_filename(base, rank):
+        """
+        Return standard name for output file of individual rank.
+        Rank can be a wildcard to generate a glob string.
+        """
+        return f"{base}-rank{rank}"
+
     def _redirect(out_to, err_to):
 
         # Flush the C-level buffers
@@ -424,6 +432,29 @@ def _redirect(out_to, err_to):
             ch.setFormatter(formatter)
             logger.addHandler(ch)
 
+    def _combine_individual_outputs(to, nproc=None):
+        """
+        Combine individual {to}_{n} files into a single {to} file
+
+        If nproc is specified, assume there are exactly that many per-rank
+        files; otherwise glob to find out what is there.
+        """
+        if nproc is None:
+            individual_files = sorted(glob.glob(_rank_filename(to, '*')))
+        else:
+            individual_files = [_rank_filename(to, p) for p in range(nproc)]
+
+        with open(to, "w") as outfile:
+            for p, fname in enumerate(individual_files):
+                outfile.write("================ Start of Process {} ================\n".format(p))
+                with open(fname) as infile:
+                    outfile.write(infile.read())
+                outfile.write("================= End of Process {} =================\n\n".format(p))
+
+        # only remove input files after successfully finishing merging
+        for fname in individual_files:
+            os.remove(fname)
+
     # redirect both stdout and stderr to the same file
 
     if to is None:
@@ -432,22 +463,30 @@ def _redirect(out_to, err_to):
     if rank == 0:
         log = get_logger()
         log.info("Begin log redirection to {} at {}".format(to, time.asctime()))
-        if not overwrite:
-            backup_filename(to)
-
-    #- all ranks wait for logfile backup
-    if comm is not None:
-        comm.barrier()
 
     # Save the original file descriptors so we can restore them later
     saved_fd_out = os.dup(fd_out)
     saved_fd_err = os.dup(fd_err)
 
-    try:
-        pto = to
-        if to != "/dev/null":
-            pto = "{}_{}".format(to, rank)
+    # Determine individual per-rank output filenames and check if there are
+    # leftover per-rank files from a previous crash  that need to be merged
+    # before proceeding. Leftovers might have come from a run with a different
+    # nproc, so don't enforce that here.
+    pto = to
+    if to != "/dev/null":
+        pto = _rank_filename(to, rank)
+        if rank == 0 and os.path.exists(pto):
+            _combine_individual_outputs(to)
+
+    # backup previous output file if needed
+    if rank == 0 and not overwrite:
+        backup_filename(to)
+
+    # all ranks wait for logfile backup
+    if comm is not None:
+        comm.barrier()
 
+    try:
         # open python file, which creates low-level POSIX file
         # descriptor.
         file = open(pto, "w")
@@ -475,14 +514,7 @@ def _redirect(out_to, err_to):
 
         # concatenate per-process files
         if rank == 0 and to != "/dev/null":
-            with open(to, "w") as outfile:
-                for p in range(nproc):
-                    outfile.write("================ Start of Process {} ================\n".format(p))
-                    fname = "{}_{}".format(to, p)
-                    with open(fname) as infile:
-                        outfile.write(infile.read())
-                    outfile.write("================= End of Process {} =================\n\n".format(p))
-                    os.remove(fname)
+            _combine_individual_outputs(to, nproc)
 
         if nproc > 1:
             comm.barrier()