The Filesystem

• mkdir
• rmdir
• rm -r
• mknod
• grep
• mount
• umount
• fuser
• lsof
• ls -l
• ls -ld
• chmod
• chown
• chgrp

/etc/fstab

• In UNIX it is presented as a single hierarchy that starts at the directory /
• / is called the root of the filesystem
• A filesytem can also be a small portion of that tree such as a directory and the files and directories below it

• /homes in this example can be considered a filesystem
• It is mounted as part of the overall UNIX filesystem tree which begins at /

Pathnames

Two Types of Pathnames:

•      Absolute (Full) - starts from the root directory (/)
•      Relative - starts from the current directory.

Terminology: file, filename, pathname, and path

• pathname usually infers the full or absolute path to a file.
• A pathname cannot be more than 255 character long:

Way around this limitation

• you can use the cd command to traverse to an intermediate directory
• use the relative pathname to access the file with the long path

• limited in length (255 characters)
• can't contain nulls (\0)
• spaces must be quoted

$ cat "my file.txt"

PATH variable

• The path of a regular user is different than that of the root user
• What are the differences?

$ echo $PATH
/usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin:/home/pattyo/bin

# echo $PATH
/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin:/usr/bin/X11:/usr/X11R6/bin:/root/bin

Mounting and Unmounting Filesystems

• A filesystem can be a disk partition
• or a cdrom or floppy device
• or a filesystem on a remote server

• Filesystems are attached to the file tree using the mount command
• mount maps a directory within the existing file tree

mount [options] device directory
mount [options] server:directory directory

mount /mnt/cdrom

1. root on the instructor machine must have /mnt/cdrom exported
2. client must have the directory /mnt/pub created

• this is the mount point

# mkdir /mnt/pub

3. now mount the instructors cdrom filesystem

   # mount 10.2.128.30:/var/ftp/pub  /mnt/pub

   # cd /mnt/pub

   # umount /mnt/pub
   umount: /mnt/pub: device is busy

5. Why are you seeing this error message?

6. Use the fuser or lsof command to see which processes are access the filesystem you are trying to umount.

   # fuser -mv /mnt/pub  

• Note the ACCESS column in the fuser command. 
• Your book has a table of activities that the process may be doing to prevent you from umounting the filesystem. (page 66)

7. Use the ps command to get information about the offending processes.

   # ps -fp "pid1 pid2 pid3"

Filesystem Organization

Types of Files

• Regular files
• Directories
• Character device files
• Block device files
• Local domain sockets
• Named popes
• Symbolic links

• text file
• data file
• executable program
• shared libraries

• Contains named references to other files
• Creating directories
•  mkdir <directory>
• Removing directories
• rmdir <directory>
• What is the main caveat associated with the rmdir command?
• rm -r <directory>
• recursively remove the directory and all files and directories beneath it
• Be Careful with this command!

• Provides a way for an application to interface with device drivers via the filesystem
• Character and block devices look like regular files
• Device files are not the same as device drivers!

• Block and Character Devices
• b - random access devices
• c - character or stream devices

• Major and minor numbers
• major: which driver, indexed into kernel
• minor: address of the device, the device driver parameter

In Unix everything is a file. Take a look inside the /dev directory

     $ ls -l /dev/hda?
    brw-rw----    1 root   disk 3,   1 Apr 11 07:25 /dev/hda1
     brw-rw----    1 root   disk 3,   2 Apr 11 07:25 /dev/hda2
     brw-rw----    1 root   disk 3,   3 Apr 11 07:25 /dev/hda3
     ...
     $ ls -l /dev/tty[0-9]
     crw--w----   1 pattyo  sysadmin 4,  0 Aug 22 23:34 /dev/tty0
     crw--w----    1 pattyo  tty      4,  1 Sep 11 10:49 /dev/tty1
     crw-------    1 root    root     4,  2 Aug 22 23:35 /dev/tty2
     ...


• Device files are created with the mknod command
• Notice that they can go anywhere, not just /dev
• What is the implication of having them in /tmp?
 
# ls -l /dev/hda
brw-rw----    1 root  disk  3, 0 Aug 30 /dev/hda

# mknod /tmp/pattyo b 3 0

# fdisk -l /tmp/pattyo

# fdisk -l /dev/hda

# rm /tmp/pattyo
 
• Remove device files with rm

Local domain sockets

• used by processes on local machine to communicate with each other
• also known as "UNIX domain sockets"
• Used by printing system, X Window System, and syslog

$ ls -l /dev | grep '^s'
srwx------    1 pattyo   root   0 Aug 22 23:35 gpmctl
srw-rw-rw-    1 root     root   0 Aug 22 23:34 log

• Another mechanism for interprocess communication
• created with mknod and removed with rm
• typically found in /dev
• rarely used

$ ls -l /dev | grep '^p'
prw-------    1 root     root   0 Aug 22 23:30 initctl

• Allows a file to have more than one name.

Syntax:  

$ ln source_file target_file

• One inode referenced by multiple names.
• Share same data blocks!
• The owner of the file remains the same.
• The file permissions remain the same.
• Hard links may not span partitions and may not refer to directories.

• Indirect pointer to a file.
• Different inode.
• May span physical partitions since they point to a pathname and not a disklocation.

Soft link Directory inode entry
 
 

inode	Name
1505	myfile
1766	myfile-lnk

Hard and symbolic links.
Link Figure  from: O'Reilly, Essential System Administration, Second Edition, page 41

First picture, top left

• index and hlink share the inode N1 and its associated data blocks.
• The symbolic link slink has a different inode, N2, and therefore different data blocks.
• The contents of inode N2's data blocks refer to the pathname to index.
• Accessing slink thus eventually reaches the data blocks for inode N1.

• Wben index is deleted, hlink is associated with inode N1 by its own directory entry.
• Accessing slink will generate an error since the pathnmae it references doesn't exist.

• index is created and gets a new inode, N3
• This new file has no relationship to hlink.
• The new file acts as the target for slink.

• Take a look at the files in /etc/rc.d/rc3.d.
• Do a long listing.
• In which directory are the links?
• In which directory are the scripts?

# cd /etc/rc.d/rc3.d
# ls -l

# ln -s /etc/rc.d/init.d /etc/init.d

# cd /etc
# ls -l | grep init

File Attributes

• Permission bits with octal values 4000 and 2000

• Allow programs to access files and processes they would normally not have access to


# ls -l /usr/bin/sudo

  ---s--x--x   1 root root 80764 Jul 23 /usr/bin/sudo

•   Use chmod to create permissions like the one above for sudo

# touch /tmp/myprog
# ls -l /tmp/myprog
# chmod 4111 /tmp/myprog
# ls -l !$

---s--x--x   1 pattyo sysadmin  0 Mar  4 14:00 /tmp/myprog
      ^^^

• Used with directories
• makes newly created files in the directory take the same group permissions as the parent directory
• easier to share files within a group of users

$ mkdir /tmp/myproj
$ ls -ld /tmp/myproj

$ chmod g+s /tmp/myproj

OR

$ chmod 2755 /tmp/myproj

$ ls -ld /tmp/myproj
drwxr-sr-x  2 pattyo sysadmin 4096 Sep 25 13:13 test
     ^^^

Files

• has an octal value of 1000
• a long time ago it was used to keep programs resident in memory, back when memory was small and expensive
• today it is ignored by executables

Directories

• prevent users from deleting or renaming files unless they are the owner
• having write permissions on the directory is not enough, you must own the directory or file
• used on /tmp so users can't remove each others files and directories

$ ls -ld /tmp
drwxrwxrwt  9 root root  4096 Apr 14 14:17 /tmp
      ^^^
$ mkdir /tmp/mytmp
$ chmod 1755 /tmp/mytmp
$ ls -ld /tmp/mytmp
drwxr-xr-t    2 pattyo sysadmin  1024 Mar  4 14:02 /tmp/mytmp

Changing Ownership and Group

• changes a file or directories user ownership

# chown someuser /home/someuser

• changes a file or directories group ownership

# chgrp admin /prj/engineering

 Syntax:
 chown user.group file-or-dir

# chown student.admin testdir

• It would be tedious to change into every sub directory and change permissions
• to recursively change the permissions on every file and subdirectory

# chown -R someuer /home/someuser
# chgrp -R staff someuser

Additional Attributes

• Can be problematic so rarely used
• Can make the a file undeletable and only appendable
  • can confuse software
  • only provide protection against users who don't know how to change permissions back

$ cp /etc/passwd .
$ sudo chattr +ia passwd
$ ls -l passwd
-rw-r--r--    1 pattyo   sysadmin 1947 Mar  4 14:15 passwd
$ lsattr passwd
---ia--------- passwd

$ rm passwd
rm: remove write-protected regular file `passwd'? y
rm: cannot remove `passwd': Operation not permitted

• modify permissions on file

$ sudo chattr -ia passwd
$ rm passwd
rm: remove regular file `passwd'? y