view toys/posix/ln.c @ 1531:3ff823086c99 draft

Teach ln -f to leave original target alone if link creation fails. Suggested by Ashwini Sharma, I wound up implementing it by creating the new link at a temporary name and renaming it over the old one instead of renaming the old file out of the way and putting it back if it failed. (Because "mkdir -p one/one/blah && ln -sf /bin/one one" would otherwise rename one/one out of the way and only notice it can't delete it way at the end when recovery's darn awkward, vs create new thing and if rename fails (including EISDIR) that's the main error path. And yes the temporary name is in the same directory as the destination so we never rename between mounts.) link over the old one instead of renaming the old file and renaming it back.
author Rob Landley <rob@landley.net>
date Wed, 22 Oct 2014 17:11:06 -0500
parents 0752b2d58909
children 58d9f1b61f0a
line wrap: on
line source

/* ln.c - Create filesystem links
 *
 * Copyright 2012 Andre Renaud <andre@bluewatersys.com>
 *
 * See http://opengroup.org/onlinepubs/9699919799/utilities/ln.html

USE_LN(NEWTOY(ln, "<1vnfs", TOYFLAG_BIN))

config LN
  bool "ln"
  default y
  help
    usage: ln [-sfnv] [FROM...] TO

    Create a link between FROM and TO.
    With only one argument, create link in current directory.

    -s	Create a symbolic link
    -f	Force the creation of the link, even if TO already exists
    -n	Symlink at destination treated as file
    -v	Verbose
*/

#define FOR_ln
#include "toys.h"

void ln_main(void)
{
  char *dest = toys.optargs[--toys.optc], *new;
  struct stat buf;
  int i;

  // With one argument, create link in current directory.
  if (!toys.optc) {
    toys.optc++;
    dest=".";
  }

  // Is destination a directory?
  if (((toys.optflags&FLAG_n) ? lstat : stat)(dest, &buf)
    || !S_ISDIR(buf.st_mode))
  {
    if (toys.optc>1) error_exit("'%s' not a directory");
    buf.st_mode = 0;
  }

  for (i=0; i<toys.optc; i++) {
    int rc;
    char *oldnew, *try = toys.optargs[i];

    if (S_ISDIR(buf.st_mode)) new = xmprintf("%s/%s", dest, basename(try));
    else new = dest;

    // Force needs to unlink the existing target (if any). Do that by creating
    // a temp version and renaming it over the old one, so we can retain the
    // old file in cases we can't replace it (such as hardlink between mounts).
    oldnew = new;
    if (toys.optflags & FLAG_f) {
      new = xmprintf("%s_XXXXXX", new);
      rc = mkstemp(new);
      if (rc >= 0) {
        close(rc);
        if (unlink(new)) perror_msg("unlink '%s'", new);
      }
    }

    rc = (toys.optflags & FLAG_s) ? symlink(try, new) : link(try, new);
    if (toys.optflags & FLAG_f) {
      if (!rc) {
        int temp;

        rc = rename(new, oldnew);
        temp = errno;
        if (rc && unlink(new)) perror_msg("unlink '%s'", new);
        errno = temp;
      }
      free(new);
      new = oldnew;
    }
    if (rc)
      perror_msg("cannot create %s link from '%s' to '%s'",
        (toys.optflags & FLAG_s) ? "symbolic" : "hard", try, new);
    else
      if (toys.optflags & FLAG_v) fprintf(stderr, "'%s' -> '%s'\n", new, try);

    if (new != dest) free(new);
  }
}