Is `mv dir dir2` atomic ? Post: 302170830

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Top Forums UNIX for Dummies Questions & Answers Is `mv dir dir2` atomic ? Post 302170830 by Perderabo on Tuesday 26th of February 2008 10:42:52 PM

02-26-2008

Administrator Emeritus

I'm really only familiar with the HP-UX version, but I'm guessing that Linux version must be similar. HP's mvdir is confined to operating in a filesystem...never between filesystems. To understand it, consider the first unix implemention of it... If you have /parent/A which has a lot of files in it and you move it to /parent/B, the original version would invoke the link system call and link A to B. At this point, both A and B exist and they are just two names to the same directory. Then, as soon as possible, it would unlink A. This would leave just B. The operation took two very quick system calls. This was NOT atomic. Between the system calls, the filesystem was in an illegal state. A and B should not be links to the same directory even for a millisecond. Today, instead of two system calls, we use just one...and rename(). This is absolutely atomic. For all intents and purposes A just morphs into B. A and B must be in the same filesystem, but not necessarily have the same exact parent as I have indicated here.

Perderabo

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LEARN ABOUT OSF1

unlink

link(1) 						      General Commands Manual							   link(1)

NAME

       link, unlink - Creates an additional directory entry for an existing file

SYNOPSIS

       link file1 file2

       unlink file

STANDARDS

       Interfaces documented on this reference page conform to industry standards as follows:

       link:  XCU5.0

       unlink:	XCU5.0

       Refer to the standards(5) reference page for more information about industry standards and associated tags.

DESCRIPTION

       The  link  command  performs  the  link()  system call to create an additional directory entry for an existing file, file1.  In effect, the
       underlying file then has two names, file1 and file2 (either of these arguments can be a pathname).  The old and	new  entries  share  equal
       access rights to the underlying file.

       The  unlink  command  performs  the unlink() system call to remove a link to file created by link. The unlink command removes the directory
       entry specified by the file parameter and, if the entry is a hard link, decrements the link count of the file referenced by the link.

       You should be familiar with the link() and unlink() system calls before you use these commands.

       The link and unlink commands do not issue error messages when the associated system call is unsuccessful.

NOTES

       The link and unlink commands cannot be used to link and unlink directories.

EXIT STATUS

       The link and unlink commands both exit with the following: The link() or unlink() system call succeeded.  Too few  or  too  many  arguments
       specified.  The link() or unlink() system call failed.

SEE ALSO

       Commands:  fsck(8), ln(1)

       Functions:  link(2), unlink(2)

																	   link(1)