From: Georges Khaznadar <georgesk@debian.org>
Date: Wed, 11 Oct 2023 11:43:22 +0200
Subject: dry-run

Implement two switches:

Implement two switches:
-n to make a "dry run" (syntax crontab -n foo)
-h to display a help message
---
 crontab.1 | 19 +++++++++++++++++--
 crontab.c | 40 +++++++++++++++++++++++++++++++---------
 2 files changed, 48 insertions(+), 11 deletions(-)

diff --git a/crontab.1 b/crontab.1
index 199bae8..2c87bc9 100644
--- a/crontab.1
+++ b/crontab.1
@@ -22,7 +22,9 @@
 .SH NAME
 crontab \- maintain crontab files for individual users (Vixie Cron)
 .SH SYNOPSIS
-crontab [ \-u user ] file
+crontab [ \-h]
+.br
+crontab [ \-u user ] [\-n] file
 .br
 crontab [ \-u user ] [ \-i ] { \-e | \-l | \-r }
 .SH DESCRIPTION
@@ -62,6 +64,12 @@ user is always allowed to setup a crontab.  For standard Debian systems, all
 users may use this command.
 .PP
 If the
+.I \-h
+option is given,
+.I crontab
+shows a help message and quits immediately.
+.PP
+If the
 .I \-u
 option is given, it specifies the name of the user whose crontab is to be
 used (when listing) or modified (when editing).  If this option is not given,
@@ -80,6 +88,13 @@ option for safety's sake.
 The first form of this command is used to install a new crontab from some
 named file or standard input if the pseudo-filename ``-'' is given.
 .PP
+If the
+.I \-n
+option is given, it means "dry run":
+.I crontab
+examines "your" crontab for its syntax, and outputs a success message if
+this syntax is correct, but nothing is written to any crontab.
+.PP
 The
 .I \-l
 option causes the current crontab to be displayed on standard output.  See
@@ -137,7 +152,7 @@ The files
 .I /etc/cron.allow
 and
 .I /etc/cron.deny
-if, they exist, must be either world-readable, or readable by group
+if they exist, must be either world-readable, or readable by group
 ``crontab''. If they are not, then cron will deny access to all users until the
 permissions are fixed.
 .PP
diff --git a/crontab.c b/crontab.c
index 8fe6f90..e403e34 100644
--- a/crontab.c
+++ b/crontab.c
@@ -46,7 +46,7 @@ static char rcsid[] = "$Id: crontab.c,v 2.13 1994/01/17 03:20:37 vixie Exp $";
 #endif
 
 
-enum opt_t	{ opt_unknown, opt_list, opt_delete, opt_edit, opt_replace };
+enum opt_t	{ opt_unknown, opt_list, opt_delete, opt_edit, opt_replace, opt_help };
 
 #if DEBUGGING
 static char	*Options[] = { "???", "list", "delete", "edit", "replace" };
@@ -60,6 +60,7 @@ static	char		Directory[MAX_FNAME];
 static	FILE		*NewCrontab = NULL;
 static	int		CheckErrorCount;
 static	int		PromptOnDelete;
+static  int             dry_run;
 static	enum opt_t	Option;
 static	struct passwd	*pw;
 static	void		list_cmd __P((void)),
@@ -79,13 +80,17 @@ static void
 usage(msg)
 	char *msg;
 {
-	fprintf(stderr, "%s: usage error: %s\n", ProgramName, msg);
-	fprintf(stderr, "usage:\t%s [-u user] file\n", ProgramName);
-	fprintf(stderr, "\t%s [ -u user ] [ -i ] { -e | -l | -r }\n", ProgramName);
-	fprintf(stderr, "\t\t(default operation is replace, per 1003.2)\n");
+	if (strlen(msg) > 0)
+	    fprintf(stderr, "%s: usage error: %s\n", ProgramName, msg);
+	fprintf(stderr, "usage:\t%s [-u user] [-n] file\n", ProgramName);
+	fprintf(stderr, "\t%s [ -u user ] [ -i ] { -e | -l | -r }\n\n", ProgramName);
+	fprintf(stderr, "\t-h\t(displays this help message)\n\n");
+	fprintf(stderr, "\tfile\t(default operation is replace, per 1003.2)\n");
+	fprintf(stderr, "\t-n\t(dry run: checks the syntax, then bails out)\n");
+	fprintf(stderr, "\t-u user\t(choose the user whose crontab is touched)\n\n");
 	fprintf(stderr, "\t-e\t(edit user's crontab)\n");
 	fprintf(stderr, "\t-l\t(list user's crontab)\n");
-	fprintf(stderr, "\t-r\t(delete user's crontab)\n");
+	fprintf(stderr, "\t-r\t(delete user's crontab)\n\n");
 	fprintf(stderr, "\t-i\t(prompt before deleting user's crontab)\n");
 	exit(ERROR_EXIT);
 }
@@ -133,6 +138,8 @@ main(argc, argv)
 	switch (Option) {
 	case opt_list:		list_cmd();
 				break;
+	case opt_help:          usage("");
+				break;
 	case opt_delete:	delete_cmd();
 				break;
 	case opt_edit:		edit_cmd();
@@ -152,9 +159,9 @@ main(argc, argv)
 }
 
 #if DEBUGGING
-const char *getoptarg = "u:lerix:";
+const char *getoptarg = "u:hnlerix:";
 #else
-const char *getoptarg = "u:leri";
+const char *getoptarg = "u:hnleri";
 #endif
 
 static void
@@ -188,6 +195,14 @@ parse_args(argc, argv)
 				usage("bad debug option");
 			break;
 #endif
+		case 'h':
+		  if (Option != opt_unknown)
+		    usage("only one operation permitted");
+		  Option = opt_help;
+		  break;
+		case 'n':
+		  dry_run = 1;
+		  break;
 		case 'u':
 			if (!(pw = getpwnam(optarg)))
 			{
@@ -916,7 +931,14 @@ replace_cmd() {
 		return (-1);
 	}
 
-
+	if (dry_run) {
+	    /* the option switch -n was used. Do not write the crontab file */
+	    /* instead display a pleasant message.                          */
+	    fprintf(stderr, "The syntax of the crontab file was successfully checked.\n");
+	    fclose(tmp); unlink(tn);
+	    return (0);
+	  }
+	
 #ifdef HAS_FCHMOD
 	if (fchmod(fileno(tmp), 0600) < OK)
 #else