How to Lock the Script?

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# 8  
Old 11-25-2013
I sometimes have used mkfifo for an ad-hoc semaphore. Not because of any particular properties of fifo's -- only because mkfifo /tmp/myfifo fails the second time you run it, unlike touch or echo. It's even pretty portable. Cheap and ugly but effective.

If the shell itself had any sort of syntax for O_EXCL it wouldn't be necessary.


if ! mkfifo /tmp/myappname
        echo "Locked" >&2
        exit 1

trap "rm -f /tmp/myappname" EXIT

Last edited by Corona688; 11-25-2013 at 04:39 PM..
# 9  
Old 11-25-2013
Originally Posted by Corona688
If the shell itself had any sort of syntax for O_EXCL it wouldn't be necessary.
It may not qualify as syntax, but you can toggle O_EXCL with set's noclobber.

POSIX Shell Command Language:
Output redirection using the '>' format shall fail if the noclobber option is set (see the description of set -C) and the file named by the expansion of word exists and is a regular file.
OpenBSD's pdksh, exec.c:
	case IOWRITE:
		flags = O_WRONLY | O_CREAT | O_TRUNC;
		/* The stat() is here to allow redirections to
		 * things like /dev/null without error.
		if (Flag(FNOCLOBBER) && !(iop->flag & IOCLOB) &&
		    (stat(cp, &statb) < 0 || S_ISREG(statb.st_mode)))
			flags |= O_EXCL;

bash 4.3rc1 (freshly-tagged a couple hours ago) :: redir.c :: noclobber_open():
  /* If the file was not present (r != 0), make sure we open it
     exclusively so that if it is created before we open it, our open
     will fail.  Make sure that we do not truncate an existing file.
     Note that we don't turn on O_EXCL unless the stat failed -- if
     the file was not a regular file, we leave O_EXCL off. */
  flags &= ~O_TRUNC;
  if (r != 0)
      fd = open (filename, flags|O_EXCL, mode);
      return ((fd < 0 && errno == EEXIST) ? NOCLOBBER_REDIRECT : fd);

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