pidof: allow to suppress output

Often pidof is used in shell scripts in this form:

    if pidof daemon >/dev/null; then
        ...
    fi

The redirection to /dev/null is needed because otherwise the script
would output the found PIDs.
Let's add a -q option which, similary to grep, just sets the exit code.
Also exit on first match, as there is no reason to proceed further when
at least a process is matched.

Tested with:

    $ ./pidof bash
    17701 14019 5276 2967
    $ echo $?
    0
    $ ./pidof bashx
    $ echo $?
    1
    $ ./pidof -q bash
    $ echo $?
    0
    $ ./pidof -q bashx
    $ echo $?
    1

pidof.1

@@ -15,17 +15,17 @@
 .\" along with this program; if not, write to the Free Software
 .\" Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 .\"
-.TH PIDOF 1 "2020-06-04" "" "User Commands"
+.TH PIDOF 1 "2020-12-22" "" "User Commands"
 .SH NAME
 pidof -- find the process ID of a running program
 .SH SYNOPSIS
 .B pidof
 .RB [ \-s ]
 .RB [ \-c ]
+.RB [ \-q ]
+.RB [ \-w ]
 .RB [ \-x ]
 .RB [ \-o
-.IR omitpid[,omitpid...] ]
-.RB [ \-o
 .IR omitpid[,omitpid...]... ]
 .RB [ \-S
 .IR separator ]
@@ -42,12 +42,14 @@ Single shot - this instructs the program to only return one \fIpid\fP.
 Only return process ids that are running with the same root directory.
 This option is ignored for non-root users, as they will be unable to check
 the current root directory of processes they do not own.
-.IP \-x
+.IP \-q
-Scripts too - this causes the program to also return process id's of
+Quiet mode, suppress any output and only sets the exit status accordingly.
-shells running the named scripts.
 .IP \-w
 Show also processes that do not have visible command line (e.g. kernel
 worker threads).
+.IP \-x
+Scripts too - this causes the program to also return process id's of
+shells running the named scripts.
 .IP "-o \fIomitpid\fP"
 Tells \fIpidof\fP to omit processes with that process id. The special
 pid \fB%PPID\fP can be used to name the parent process of the \fIpidof\fP

pidof.c

@@ -56,7 +56,7 @@ static int opt_single_shot    = 0;  /* -s */
 static int opt_scripts_too    = 0;  /* -x */
 static int opt_rootdir_check  = 0;  /* -c */
 static int opt_with_workers   = 0;  /* -w */
-
+static int opt_quiet          = 0;  /* -q */
 
 static char *pidof_root = NULL;
 
@@ -70,8 +70,9 @@ static int __attribute__ ((__noreturn__)) usage(int opt)
 	fputs(USAGE_OPTIONS, fp);
 	fputs(_(" -s, --single-shot         return one PID only\n"), fp);
 	fputs(_(" -c, --check-root          omit processes with different root\n"), fp);
-	fputs(_(" -x                        also find shells running the named scripts\n"), fp);
+	fputs(_(" -q,                       quiet mode, only set the exit code\n"), fp);
 	fputs(_(" -w, --with-workers        show kernel workers too\n"), fp);
+	fputs(_(" -x                        also find shells running the named scripts\n"), fp);
 	fputs(_(" -o, --omit-pid <PID,...>  omit processes with PID\n"), fp);
 	fputs(_(" -S, --separator SEP       use SEP as separator put between PIDs"), fp);
 	fputs(USAGE_SEPARATOR, fp);
@@ -294,13 +295,14 @@ int main (int argc, char **argv)
 	int first_pid = 1;
 
 	const char *separator = " ";
-	const char *opts = "scnxwmo:S:?Vh";
+	const char *opts = "scnqxwmo:S:?Vh";
 
 	static const struct option longopts[] = {
 		{"check-root", no_argument, NULL, 'c'},
 		{"single-shot", no_argument, NULL, 's'},
 		{"omit-pid", required_argument, NULL, 'o'},
 		{"separator", required_argument, NULL, 'S'},
+		{"quiet", no_argument, NULL, 'q'},
 		{"with-workers", no_argument, NULL, 'w'},
 		{"help", no_argument, NULL, 'h'},
 		{"version", no_argument, NULL, 'V'},
@@ -318,6 +320,9 @@ int main (int argc, char **argv)
 	/* process command-line options */
 	while ((opt = getopt_long (argc, argv, opts, longopts, NULL)) != -1) {
 		switch (opt) {
+		case 'q':
+			opt_quiet = 1;
+			/* fallthrough */
 		case 's':
 			opt_single_shot = 1;
 			break;
@@ -372,12 +377,14 @@ int main (int argc, char **argv)
 
 			found = 1;
 			for (i = proc_count - 1; i >= 0; i--) {	/* and display their PIDs */
+				if (!opt_quiet) {
 					if (first_pid) {
 						first_pid = 0;
 						printf ("%ld", (long) procs[i].pid);
 					} else {
 						printf ("%s%ld", separator, (long) procs[i].pid);
 					}
+				}
 				if (opt_single_shot) break;
 			}
 
@@ -386,7 +393,7 @@ int main (int argc, char **argv)
 	}
 
 	/* final line feed */
-	if (found) printf("\n");
+	if (!opt_quiet && found) printf("\n");
 
 	/* some cleaning */
 	safe_free(procs);