Index: head/usr.bin/indent/indent.1
===================================================================
--- head/usr.bin/indent/indent.1
+++ head/usr.bin/indent/indent.1
@@ -30,7 +30,7 @@
 .\"	@(#)indent.1	8.1 (Berkeley) 7/1/93
 .\" $FreeBSD$
 .\"
-.Dd July 25, 2017
+.Dd August 7, 2017
 .Dt INDENT 1
 .Os
 .Sh NAME
@@ -119,7 +119,10 @@
 is named
 .Sq Pa /blah/blah/file ,
 the backup file is named
-.Sq Pa file.BAK .
+.Sq Pa file.BAK
+by default. The extension used for the backup file may be overridden using the
+.Ev SIMPLE_BACKUP_SUFFIX
+environment variable.
 .Pp
 If
 .Ar output-file
Index: head/usr.bin/indent/indent.c
===================================================================
--- head/usr.bin/indent/indent.c
+++ head/usr.bin/indent/indent.c
@@ -71,6 +71,8 @@
 					 * file */
 const char *out_name = "Standard Output";	/* will always point to name
 						 * of output file */
+const char *simple_backup_suffix = ".BAK";	/* Suffix to use for backup
+						 * files */
 char        bakfile[MAXPATHLEN] = "";
 
 int
@@ -99,8 +101,8 @@
 
     int         last_else = 0;	/* true iff last keyword was an else */
     const char *profile_name = NULL;
+    const char *envval = NULL;
 
-
     /*-----------------------------------------------*\
     |		      INITIALIZATION		      |
     \*-----------------------------------------------*/
@@ -160,6 +162,10 @@
     output = NULL;
     tabs_to_var = 0;
 
+    envval = getenv("SIMPLE_BACKUP_SUFFIX");
+    if (envval)
+        simple_backup_suffix = envval;
+
     /*--------------------------------------------------*\
     |   		COMMAND LINE SCAN		 |
     \*--------------------------------------------------*/
@@ -1234,7 +1240,7 @@
 	p--;
     if (*p == '/')
 	p++;
-    sprintf(bakfile, "%s.BAK", p);
+    sprintf(bakfile, "%s%s", p, simple_backup_suffix);
 
     /* copy in_name to backup file */
     bakchn = creat(bakfile, 0600);