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What does size of a directory mean in output of ls -l command?

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3 Answers 3

up vote 21 down vote accepted

This is the size of space on the disk that is used to store the meta information for the directory (i.e. the table of files that belong to this directory). If it is i.e. 1024 this means that 1024 bytes on the disk are used (it always allocate full blocks) for this purpose.

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and when a directory contains thousands of files, the size of the directory itself can easily be > 10KB –  glenn jackman Feb 23 '11 at 20:28
    
Sure. Let's say each filename has on average 10 characters. Then the storage for 1000 file names alone would take almost 10KB as space. However, it depends also on the file system. Some file systems have limits on the number of files in a directory. –  txwikinger Mar 9 '11 at 21:10
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@txwikinger I think directories contain lists of inodes, not actual file names. –  Ярослав Рахматуллин Mar 18 '13 at 6:36
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@ЯрославРахматуллин A directory entry consists of an inode number and a filename. –  200_success Oct 1 '13 at 7:45

A directory is a just a directory, like a phone directory. It's just a file with a list of numbers and a name next to them. Each number references a file in the file-system (an inode number) and the name is the file name.

You need disk space to save that data, how it is allocated and grown and shrunk is filesystem dependant.

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you could include the actual details of what meta-data is kept and the block allocation considerations regarding the grow of size. it would make a more exact answer complete with the already given nice allegory. :) –  naxa Jan 9 at 13:19
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@naxa, that's filesystem dependant, and even within a filesystem (like ext4), that depends on which options you enable, so I'd rather leave it as "it's filesystem dependant", rather than try and be exhaustive which would not be very relevant for this question. –  Stéphane Chazelas Jan 9 at 13:54

A directory reserves 4096 bytes (at minimum) for meta-data about itself and its contents.

Also, 4096 bytes is the default allocation unit (block) for ext2/ext3/ext4 filesystem and therefor a directory cannot be any smaller.

On different filesystems you might find directories with different default sizes, that is due to the default block size of the filesystem.

Directory sizes can also grow dynamically as they get filled, but once filled the space reserved for meta-data cannot be re-allocated without removal of the directory.

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What meta-data are you referring to? –  Stéphane Chazelas Oct 1 '12 at 21:27
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details depend on the filesystem but generally includes meta-data like filename, inode number, file type (file, dir, fifo, socket, device node, symlink, etc), owner, group, permissions, attributes, ACLs, symlink targets. –  cas Oct 1 '12 at 21:58
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@CraigSanders, the directory contains object name and corresponding inode number only. The inode contains the type of object, owner/group, permissions, reference to wherever ACLs and extended attributes are kept, ... Anything else will cause severe breakage of POSIX filesystem semantics. –  vonbrand Mar 15 '13 at 15:05

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