diff --git a/usr.bin/Makefile b/usr.bin/Makefile
--- a/usr.bin/Makefile
+++ b/usr.bin/Makefile
@@ -61,6 +61,7 @@
 	hexdump \
 	id \
 	ident \
+	preserve \
 	ipcrm \
 	ipcs \
 	join \
diff --git a/usr.bin/preserve/Makefile b/usr.bin/preserve/Makefile
new file mode 100644
--- /dev/null
+++ b/usr.bin/preserve/Makefile
@@ -0,0 +1,3 @@
+PROG = preserve
+
+.include <bsd.prog.mk>
diff --git a/usr.bin/preserve/preserve.1 b/usr.bin/preserve/preserve.1
new file mode 100644
--- /dev/null
+++ b/usr.bin/preserve/preserve.1
@@ -0,0 +1,112 @@
+.\"-
+.\" Copyright (c) 2023 Dag-Erling Smørgrav
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+.\" SUCH DAMAGE.
+.\"
+.Dd March 29, 2023
+.Dt PRESERVE 1
+.Os
+.Sh NAME
+.Nm preserve
+.Nd run a command and replace its output only if it has changed
+.Sh SYNOPSIS
+.Nm
+.Op Fl v
+.Op Fl o Ar filename
+.Ar command
+.Op Ar args
+.Sh DESCRIPTION
+The
+.Nm
+utility runs the specified command, which is expected to either write to
+.Va stdout
+or produce a single output file.
+.Pp
+If the
+.Fl o
+option is provided before the command, the command is assumed to write
+its output to
+.Va stdout ,
+and
+.Nm
+redirects it to a temporary file.
+.Pp
+If the
+.Fl o
+option is not provided,
+.Nm
+scans
+.Ar args
+to determine the output file name.
+If
+.Ar args
+includes a word that begins with
+.Dq Fl o ,
+the remainder of that word is taken to be the output file name.
+Otherwise, if it includes the word
+.Dq Fl o
+in a non-terminal position, the next word is taken to be the output
+file name.
+Otherwise, if the last word of
+.Ar args
+does not begin with a hyphen, it is taken to be the output file name.
+In all cases, the actual filename is replaced in
+.Ar args
+with a temporary filename.
+.Pp
+If
+.Nm
+is unable to determine the output file name, the original command is
+executed without further interference.
+.Pp
+Otherwse, if the command succeeds and the final output file either
+does not exist, or exists but its contents differ from those of the
+temporary file, the temporary file is renamed to the final output
+file.
+In all other cases, the temporary file is deleted.
+.Pp
+The following options are available:
+.Bl -tag -width Fl
+.It Fl o Ar filename
+Assume that
+.Ar command
+writes its output to
+.Va stdout
+rather than to a file, and use
+.Ar filename
+as the final output file name.
+.It Fl v
+Write additional information to
+.Va stderr
+explaining whether the output file was retained or replaced and why.
+.El
+.Sh EXIT STATUS
+The exit status of the
+.Nm
+utility is that of the command it ran, unless it failed to run the
+command or an error occurred while handling the output, in which case
+its exit status is 1.
+.Sh AUTHORS
+The
+.Nm
+command and this manual page were written by
+.An Dag-Erling Sm\(/orgrav Aq Mt des@FreeBSD.org .
diff --git a/usr.bin/preserve/preserve.c b/usr.bin/preserve/preserve.c
new file mode 100644
--- /dev/null
+++ b/usr.bin/preserve/preserve.c
@@ -0,0 +1,221 @@
+/*-
+ * Copyright (c) 2023 Dag-Erling Smørgrav
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include <sys/stat.h>
+#include <sys/wait.h>
+
+#include <err.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <signal.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+static bool vflag;
+
+static bool
+files_are_identical(const char *afn, const char *bfn)
+{
+	static char abuf[4096], bbuf[4096];
+	struct stat asb, bsb;
+	ssize_t alen, blen;
+	off_t off;
+	int afd = -1, bfd = -1, i;
+	bool identical = false;
+
+	if ((afd = open(afn, O_RDONLY)) < 0) {
+		if (errno != ENOENT)
+			warn("%s", afn);
+		goto done;
+	}
+	if ((bfd = open(bfn, O_RDONLY)) < 0) {
+		if (errno != ENOENT)
+			warn("%s", bfn);
+		goto done;
+	}
+	if (fstat(afd, &asb) != 0 || fstat(bfd, &bsb) != 0) {
+		/* can't happen */
+		warn("fstat()");
+		goto done;
+	}
+	if (asb.st_size != bsb.st_size) {
+		if (vflag) {
+			warnx("%s and %s differ in length: %zu %c %zu",
+			    afn, bfn, (size_t)asb.st_size,
+			    asb.st_size < bsb.st_size ? '<' : '>',
+			    (size_t)bsb.st_size);
+		}
+		goto done;
+	}
+	off = 0;
+	do {
+		if ((alen = read(afd, abuf, sizeof(abuf))) < 0) {
+			warn("%s", afn);
+			goto done;
+		}
+		if ((blen = read(bfd, bbuf, sizeof(bbuf))) < 0) {
+			warn("%s", bfn);
+			goto done;
+		}
+		if (alen != blen) {
+			/* either or both have changed since fstat() */
+			if (vflag)
+				warnx("%s and %s differ in length\n", afn, bfn);
+			goto done;
+		}
+		if (memcmp(abuf, bbuf, alen) != 0) {
+			if (vflag) {
+				for (i = 0; i < alen; i++)
+					if (abuf[i] != bbuf[i])
+						break;
+				warnx("%s and %s differ at offset %zu\n",
+				    afn, bfn, (size_t)(off + i));
+			}
+			goto done;
+		}
+		off += alen;
+	} while (alen > 0);
+	if (vflag)
+		warnx("%s and %s are identical", afn, bfn);
+	identical = true;
+done:
+	if (afd >= 0)
+		close(afd);
+	if (bfd >= 0)
+		close(bfd);
+	return (identical);
+}
+
+static void
+usage(void)
+{
+	fprintf(stderr, "usage: preserve [-v] [-o file] command [args]\n");
+	exit(1);
+}
+
+int
+main(int argc, char *argv[])
+{
+	char *fn = NULL, *tmp = NULL;
+	pid_t pid;
+	int opt, serrno, wstatus;
+
+	while ((opt = getopt(argc, argv, "o:v")) != -1)
+		switch (opt) {
+		case 'o':
+			fn = optarg;
+			break;
+		case 'v':
+			vflag = true;
+			break;
+		default:
+			usage();
+		}
+
+	argc -= optind;
+	argv += optind;
+	if (argc == 0)
+		usage();
+
+	if (fn != NULL) {
+		if (asprintf(&tmp, "%s.tmp", fn) < 0)
+			err(1, "asprintf()");
+		if (freopen(tmp, "w", stdout) == NULL)
+			err(1, "%s", tmp);
+	} else if (argc > 1) {
+		/* look for output file name in arguments */
+		for (int i = 1; i < argc; i++) {
+			if (argv[i][0] == '-' && argv[i][1] == 'o') {
+				if (argv[i][2] != '\0') {
+					fn = &argv[i][2];
+					if (asprintf(&tmp, "-o%s.tmp", fn) < 0)
+						err(1, "asprintf()");
+					argv[i] = tmp;
+					tmp += 2;
+				} else if (i + 1 < argc) {
+					fn = argv[i + 1];
+					if (asprintf(&tmp, "%s.tmp", fn) < 0)
+						err(1, "asprintf()");
+					argv[i + 1] = tmp;
+				}
+				break;
+			}
+		}
+		if (fn == NULL && argv[argc - 1][0] != '-') {
+			fn = argv[argc - 1];
+			if (asprintf(&tmp, "%s.tmp", fn) < 0)
+				err(1, "asprintf()");
+			argv[argc - 1] = tmp;
+		}
+	}
+	if (fn == NULL) {
+		/* still no output file, just run the original command */
+		if (vflag)
+			warnx("unable to determine output file name");
+		execvp(argv[0], argv);
+		err(1, "%s", argv[0]);
+	}
+
+	if ((pid = fork()) < 0)
+		err(1, "fork()");
+	if (pid == 0) {
+		/* child */
+		execvp(argv[0], argv);
+		err(1, "%s", argv[0]);
+	}
+
+	/* parent */
+	setprogname(argv[0]);
+	(void)freopen("/dev/null", "a", stdout);
+
+	if (waitpid(pid, &wstatus, 0) < 0)
+		err(1, "waitpid()");
+	if (WIFSIGNALED(wstatus)) {
+		(void)unlink(tmp);
+		raise(WTERMSIG(wstatus));
+	}
+	if (!WIFEXITED(wstatus) || WEXITSTATUS(wstatus) != 0) {
+		(void)unlink(tmp);
+		exit(wstatus & 0xff);
+	}
+	if (files_are_identical(fn, tmp)) {
+		if (vflag)
+			warnx("removing %s", tmp);
+		(void)unlink(tmp);
+	} else {
+		if (vflag)
+			warnx("replacing %s with %s", fn, tmp);
+		if (rename(tmp, fn) != 0) {
+			serrno = errno;
+			(void)unlink(tmp);
+			errno = serrno;
+			err(1, "%s", fn);
+		}
+	}
+	exit(0);
+}