Add readlink(1).

utils/.gitignore

@@ -35,6 +35,7 @@ passwd
 ps
 pstree
 pwd
+readlink
 realpath
 rm
 rmdir

utils/Makefile

@@ -47,6 +47,7 @@ passwd \
 ps \
 pstree \
 pwd \
+readlink \
 realpath \
 rm \
 rmdir \
@@ -72,6 +73,9 @@ BINARIES=\
 $(BINARIES_EXCEPT_INSTALL) \
 xinstall
 
+MANPAGES=\
+readlink.1
+
 all: $(BINARIES)
 
 .PHONY: all install clean
@@ -80,6 +84,8 @@ install: all
 	mkdir -p $(DESTDIR)$(BINDIR)
 	install $(BINARIES_EXCEPT_INSTALL) $(DESTDIR)$(BINDIR)
 	install xinstall $(DESTDIR)$(BINDIR)/install
+	mkdir -p $(DESTDIR)$(MANDIR)/man1
+	cp $(MANPAGES) $(DESTDIR)$(MANDIR)/man1
 
 %: %.c
 	$(CC) -std=gnu11 $(CFLAGS) $(CPPFLAGS) $< -o $@

utils/readlink.1

@@ -0,0 +1,36 @@
+.Dd September 29, 2016
+.Dt READLINK 1
+.Os
+.Sh NAME
+.Nm readlink
+.Nd print symbolic link target
+.Sh SYNOPSIS
+.Nm
+.Op Fl fn
+.Ar path
+.Sh DESCRIPTION
+.Nm
+prints the target of the symbolic link specified by
+.Ar path .
+.Pp
+The options are as follows:
+.Bl -tag -width "12345678"
+.It Fl f , Fl \-canonicalize
+Print the absolute path of
+.Ar path
+(which need not be a symbolic link) with all symbolic links expanded,
+and no
+.Pa \&.
+and
+.Pa ..
+path components.
+.It Fl n , Fl \-no-newline
+Do not append a newline.
+.El
+.Sh EXIT STATUS
+.Nm
+will exit with 0 on success and non-zero otherwise.
+.Sh SEE ALSO
+.Xr readlink 1 ,
+.Xr readlink 3 ,
+.Xr realpath 3

utils/readlink.c

@@ -0,0 +1,127 @@
+/*
+ * Copyright (c) 2016 Nicholas De Nova.
+ *
+ * Permission to use, copy, modify, and/or distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ *
+ * readlink.c
+ * Print symbolic link target.
+ */
+
+#include <err.h>
+#include <errno.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+static void compact_arguments(int* argc, char*** argv)
+{
+	for ( int i = 0; i < *argc; i++ )
+	{
+		while ( i < *argc && !(*argv)[i] )
+		{
+			for ( int n = i; n < *argc; n++ )
+				(*argv)[n] = (*argv)[n+1];
+			(*argc)--;
+		}
+	}
+}
+
+int main(int argc, char* argv[])
+{
+	bool append_newline = true;
+	bool canonicalize = false;
+
+	for ( int i = 1; i < argc; i++ )
+	{
+		const char* arg = argv[i];
+		if ( arg[0] != '-' || !arg[1] )
+			continue;
+		argv[i] = NULL;
+		if ( !strcmp(arg, "--") )
+			break;
+		if ( arg[1] != '-' )
+		{
+			char c;
+			while ( (c = *++arg) )
+				switch ( c )
+				{
+				case 'f': canonicalize = true; break;
+				case 'n': append_newline = false; break;
+				default:
+					errx(1, "unknown option -- '%c'", c);
+				}
+		}
+		else if ( !strcmp(arg, "--canonicalize") )
+			canonicalize = true;
+		else if ( !strcmp(arg, "--no-newline") )
+			append_newline = false;
+		else
+			errx(1, "unknown option: %s", arg);
+	}
+
+	compact_arguments(&argc, &argv);
+
+	if ( argc < 2 )
+		errx(1, "missing path operand");
+	else if ( argc > 2 )
+		errx(1, "unexpected extra operand");
+
+	char* buffer;
+	if ( canonicalize )
+	{
+		buffer = realpath(argv[1], NULL);
+		if ( !buffer )
+			err(1, "%s", argv[1]);
+	}
+	else
+	{
+		size_t buffer_size = 4096;
+		while ( true )
+		{
+			buffer = malloc(buffer_size);
+			if ( !buffer )
+				err(1, "malloc");
+
+			ssize_t path_size = readlink(argv[1], buffer, buffer_size);
+			if ( path_size < 0 )
+				err(1, "%s", argv[1]);
+
+			if ( (size_t) path_size == buffer_size )
+			{
+				free(buffer);
+				if ( SIZE_MAX / 2 < buffer_size )
+				{
+					errno = ENOMEM;
+					err(1, "malloc");
+					return 1;
+				}
+				buffer_size *= 2;
+				continue;
+			}
+
+			buffer[path_size] = '\0';
+			break;
+		}
+	}
+
+	fputs(buffer, stdout);
+	if ( append_newline )
+		putchar('\n');
+
+	if ( ferror(stdout) || fflush(stdout) == EOF )
+		err(1, "stdout");
+
+	return 0;
+}