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 Syntax ln [options] oldfile newfile ln [options] old-file-list directory Options -fforce creation (overwrite existing file) -ndon’t force -screate soft(symbolic) link

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

Hard Link across directories ln memo6.hard memos/memo6

Hard Link across accounts ln 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.soft ls – il rwxr--r-- 1 sarwar faculty 9352 May 28 23:09 Chapter 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 dangling symlinks 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