File Attributes

Commands

Recall

• Linux has a hierarchical file system with access controls for the user, her group, and all others
• A file is a sequence of characters (or bytes)
• The size of the file is not stored in the file
• The name of the file is not stored in the file
• The is no end-of-file (EOF) marker in the file
• There are ordinary, directory & device files

Permissions for chmod

• ls -l shows: Type&Perms, Lnks, Owner, Group, Bytes,LastModTime, FileName
• u: user, g: group, o: other, a: all
• r: read, w: write, x: execute
• s: setuid permissions. When you run a setuid program, take on the permissions of the owner of the file.(u+s).
• Setgid: set group id (g+s), get permissions of the group.
• Directory permission
  x: user can cd and ls a file if the name is known
  r: can read the contents of the directory (ls all files)
  w: can create new files in the directory and rm

  -rw-r----	This ordinary file can be viewed by the owner and anyone in the group. It can be changed by the owner only.
  -rw-r--r-	This ordinary file can be viewed by anyone and can be changed by the owner only.
  -rwxr-xr-x	Ditto. But this file can also be executed by anyone
  drwxrwxrwx	This directory allows anyone to cd into it, and to create or delete files. Dangerous.
  drwxr-xr-x	This directory allows full access to the owner. All others can cd into it and list the files it contains.
  lrwxrwxrwx	This file is a symbolic link to the directory /tmp

• chmod ug+rx file
• chmod o-w file
• chmod 700 file
• read is 4, write is 2, execute is 1
• chown
  The owner is the login ID (actually a number) of the person who created the file
  This can be modified by root, for example if the account is being deleted
• chgrp
  The sysAdmin can set up various groups of users.
  All members of the group get the permissions specified in the group field
  To see if you are member of a group, us the .groups. command

Links

• Pointer to a file, not a copy (a nickname)
• Used to give others access. You give them read and execute access as necessary, they crete the link
• 2 different types of links
• Hard Links - not across file systems, super-user.
• File exists as long as 1+ hard link exists
Hard links provide another name for the same file
multiple directory entries, but a single file
Increase the link count and use same inode #
Must be on same disk and may not be a directory
ln file1 file2
ln file1 dirName
Symbolic Links - indirect pointer, can link directories (~shortcuts)
Separate file that contains path of original
Possible to get a .dangling. link
ln -s /tmp myTmp
rm file and symbolic link not usable, but still there
When cd ing to symbolic link, ksh reports (via pwd) that your directory is the link; csh and bourne report the name of the original directory
Built-in variable cwd contains the link directory when you cd to it

Umask and Find

• find - search for files
  Recursively search the file system from a specified starting point looking for any file that matches the criteria and take the specified action
• find . -name hello
• name can be a regular expression (e.g. *.txt)

All work herein is subject to copyright. Original content to Dr. Deborah Whitfield, text content (Your unix/Linux) to Prentice Hall publishing.