Chapter 11

File Sharing

Sharing Techniques

• Duplicate files• Common login• Setting appropriate access permissions on

shared files• Common group for team members• Sharing via links

Sharing via Links

• A link is a connection between the file to be shared and the directory entries of the users who want to have access to this file

• 2 types of links exist:– hard links– soft (symbolic) links

Figure 11.1 (a) Logical structure of current directory;

(b) contents of current directory

Figure 11.1 (c) relationship among a directory entry, inode, and file contents

Hard Links

• A hard link is a pointer to the inode of a file• Established using the ln command• The link count of the file is incremented• Both the original file and the new entry point

to the same inode• When deleted, the link count is decremented,

and the file is only deleted if the resulting link count is zero

Figure 11.2 Establishing a hard link

ln Chapter3 Chapter3.hard

ln command

Syntaxln [options] oldfile newfileln [options] old-file-list directory

Options-f force creation (overwrite existing

file)-n don’t force-s create soft(symbolic) link

Figure 11.2 (c) hard link implementation by establishing a pointer to inode of the file

Hard Link across directoriesln memo6.hard memos/memo6

Hard Link across accountsln linuxbook/examples/demo1 ../bob/dir1

sarwar must have x-permission for bob and wx-permission for dir1

Limitations of Hard Links

• Links cannot be established across file systems• If one of the files is moved to a different file

system, it is copied instead, and the link counts of both files adjusted accordingly

• Only superusers can create hard links to directories

Soft (Symbolic) Links

• Established using the ln -s command• The link count of the file is not incremented• The created file is of the special type “link”

denoted by “l” in directory listings• The linked file is an actual file that contains the

path to the original file• Symbolic links can be created across file systems• Symbolic links to directories can be created by

any user

Figure 11.5 Establishing a soft link

ln -s Chapter3 Chapter3.soft

Soft links in directory listing

ln -s Chapter3 Chapter3.softls – il52473 -rwxr--r-- 1 sarwar faculty 9352 May 28 23:09 Chapter352479 lrwxr--r-- 2 sarwar faculty 8 Oct 13 14:24 Chapter3.soft --> Chapter3

Figure 11.5 (c) soft link implementation by establishing a “pointer” to (pathname of) the existing file in the link file

Soft Link across accounts

Types of Symbolic Links

• absolute• relative• other_fs• messy• lengthy• danglingsymlinks command used for examining and

repairing links

Drawbacks of Soft Links

• If the original file is moved to a different location, it can no longer be accessed via the symbolic link (dangling link)

• Extra space on disk and extra inode to store the link file

• Extra time required for access to the original file: the link file has to be read first, then path followed to target file