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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mvdir

mvdir(1)						      General Commands Manual							  mvdir(1)

NAME

       mvdir - Moves (renames) a directory

SYNOPSIS

       mvdir directory1 directory2

DESCRIPTION

       The  mvdir  command renames directories within a file system. To use mvdir, you must have write permission to directory1 and directory2 and
       to the parent directories of directory1 and directory2. The directory1 argument must name an existing directory.  If  directory2  does  not
       exist,  directory1  is moved to directory2.  If directory2 exists, directory1 becomes a subdirectory of directory2.  directory2 cannot be a
       subdirectory of directory1.

       Directories cannot be moved across file systems.

       You can also rename a directory with the mv command.

NOTES

       This command was made obsolete by the mv command.

EXAMPLES

       To rename or move a directory to another location, enter: mvdir appendixes manual

       If manual does not exist, then this renames the directory appendixes to manual.

       If a directory named manual already exists, this moves appendixes and its  contents  to	manual/appendixes.   In  other	words,	appendixes
       becomes a subdirectory of manual.

SEE ALSO

       Commands:  mkdir(1), mv(1)

       Functions:  rename(2)

																	  mvdir(1)