tailf: add option -n to specifying output lines

It will be useful if we can print out the last n lines instead of the last
10, just like tail.

There are examples:

tailf -n 5 file1
tailf --lines 10 file2
tailf -20 file3

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Karel Zak <kzak@redhat.com>

2 files changed, 36 insertions, 10 deletions

diff --git a/text-utils/tailf.1 b/text-utils/tailf.1
index ccce6628..8bc53fca 100644
--- a/text-utils/tailf.1
+++ b/text-utils/tailf.1
@@ -26,7 +26,8 @@
 .SH NAME
 tailf \- follow the growth of a log file
 .SH SYNOPSIS
-.BI tailf " file"
+.B tailf
+[\fIOPTION\fR] \fIfile\fR
 .SH DESCRIPTION
 .B tailf
 will print out the last 10 lines of a file and then wait for the file to
@@ -40,10 +41,13 @@ does not occur periodically when no log activity is happening.
 is extremely useful for monitoring log files on a laptop when logging is
 infrequent and the user desires that the hard disk spin down to conserve
 battery life.
-.SH BUGS
-An option could be provided to print out the last
-.I n
-lines instead of the last 10.
+.PP
+Mandatory arguments to long options are mandatory for short options too.
+.TP
+\fB\-n\fR, \fB\-\-lines\fR=\fIN\fR, \fB\-N\fR
+output the last
+.I N
+lines, instead of the last 10.
 .SH AUTHOR
 This program was written by Rik Faith (faith@acm.org) and may be freely
 distributed under the terms of the X11/MIT License.  There is ABSOLUTELY
diff --git a/text-utils/tailf.c b/text-utils/tailf.c
index 5b1d1a4e..79a194da 100644
--- a/text-utils/tailf.c
+++ b/text-utils/tailf.c
@@ -33,8 +33,12 @@
 #include <errno.h>
 #include <string.h>
 #include <sys/stat.h>
+#include <ctype.h>
+#include "errs.h"
 #include "nls.h"
 
+#define DEFAULT_LINES  10
+
 static size_t filesize(const char *filename)
 {
     struct stat sb;
@@ -86,19 +90,37 @@ int main(int argc, char **argv)
     FILE       *str;
     const char *filename;
     int        count, wcount;
+    int        lines = DEFAULT_LINES;
 
     setlocale(LC_ALL, "");
     bindtextdomain(PACKAGE, LOCALEDIR);
     textdomain(PACKAGE);
 
-    if (argc != 2) {
-	fprintf(stderr, _("Usage: tailf logfile\n"));
-	exit(1);
+    argc--;
+    argv++;
+
+    for (; argc > 0 && argv[0][0] == '-'; argc--, argv++) {
+	if (!strcmp(*argv, "-n") || !strcmp(*argv, "--lines")) {
+	    argc--;
+	    argv++;
+	    if (argc > 0 && (lines = atoi(argv[0])) <= 0)
+		errx(EXIT_FAILURE, _("Invalid number of lines"));
+	}
+	else if (isdigit(argv[0][1])) {
+	    if ((lines = atoi(*argv + 1)) <= 0)
+		errx(EXIT_FAILURE, _("Invalid number of lines"));
+	}
+	else
+		errx(EXIT_FAILURE, _("Invalid option"));
     }
 
-    filename = argv[1];
+    if (argc != 1) {
+	fprintf(stderr, _("Usage: tailf [-n N | -N] logfile\n"));
+	exit(EXIT_FAILURE);
+    }
 
-    tailf(filename, 10);
+    filename = argv[0];
+    tailf(filename, lines);
 
     for (osize = filesize(filename);;) {
 	nsize = filesize(filename);