sh: implement PS1 \D to print current time

\D{format} yields the result of calling strftime(3) with the provided
format and the current time.

When PS4 can use this, it will enable us to easily generate timestamps
when tracing script execution.

Differential Revision:	https://reviews.freebsd.org/D35840

bin/sh/parser.c

@@ -43,6 +43,7 @@ static char sccsid[] = "@(#)parser.c	8.7 (Berkeley) 5/16/95";
 #include <stdlib.h>
 #include <unistd.h>
 #include <stdio.h>
+#include <time.h>
 
 #include "shell.h"
 #include "parser.h"
@@ -2100,6 +2101,49 @@ getprompt(void *unused __unused)
 			}
 			break;
 
+		/*
+		 * Print the current time as per provided strftime format.
+		 */
+		case 'D': {
+			char tfmt[128] = "%X"; /* \D{} means %X. */
+			struct tm *now;
+
+			if (fmt[1] != '{') {
+				/*
+				 * "\D" but not "\D{", so treat the '\'
+				 * literally and rewind fmt to treat 'D'
+				 * literally next iteration.
+				 */
+				ps[i] = '\\';
+				fmt--;
+				break;
+			}
+			fmt += 2; /* Consume "D{". */
+			if (fmt[0] != '}') {
+				char *end;
+
+				end = memccpy(tfmt, fmt, '}', sizeof(tfmt));
+				if (end == NULL) {
+					/*
+					 * Format too long or no '}', so
+					 * ignore "\D{" altogether.
+					 * The loop will do i++, but nothing
+					 * was written to ps, so do i-- here.
+					 * Rewind fmt for similar reason.
+					 */
+					i--;
+					fmt--;
+					break;
+				}
+				*--end = '\0'; /* Ignore the copy of '}'. */
+				fmt += end - tfmt;
+			}
+			now = localtime(&(time_t){time(NULL)});
+			i += strftime(&ps[i], PROMPTLEN - i - 1, tfmt, now);
+			i--; /* The loop will do i++. */
+			break;
+		}
+
 		/*
 		 * Hostname.
 		 *

bin/sh/sh.1

@@ -1427,6 +1427,16 @@ unless you are the superuser, in which case it defaults to
 may include any of the following formatting sequences,
 which are replaced by the given information:
 .Bl -tag -width indent
+.It Li \eD{format}
+The current time in
+.Xr strftime 3
+.Ar format .
+The braces are required.
+Empty
+.Ar format
+is equivalent to
+\&%X,
+national representation of the time.
 .It Li \eH
 This system's fully-qualified hostname (FQDN).
 .It Li \eh