Unix file systems - an introduction

Kim Oldfield

About this talk

This talk is an introduction only
If you don't understand something please ask questions
Available from http://www.luv.asn.au/node.php?id=16

Basic concepts

All the files are in one tree, there is no concept of "drives" (a:, c:)
Separate devices are mounted in the one file system tree,
eg floppy disk may be accessed at /mnt/floppy, a cdrom on /cdrom
/etc/fstab shows where file systems are mounted
file and directory names can contain any character except "/".
Use " and ' to protect file names with spaces.
eg ls "file name with spaces"
files names are case sensitive,
eg Main.c and main.c are two different files

Shell Commands

Most commands default to using the current directory.

pwd	show the current directory, eg "pwd" may print "/home/kim"
cd	change current directory, with no arguments will change to your home directory
ls	shows files in current directory
ls foo	shows files in directory "foo" or the file "foo"
ls *.html	shows files in the current directory ending in .html
ls -l	shows details of files
ls -a	show all files, including files beginning with "."
tree -d	show directory structure
less	display a file on screen
man	where to find help on any command
mount	show all file systems currently mounted, or add a file system to the tree, eg "mount -t vfat /dev/fd0 /floppy
umount	un mount a file system, eg "umount /dev/fd0" or "umount /floppy"
id	show which groups you belong to
chmod	change permissions on files
df -h	show the free space on all file systems
du -h	show how much disk space each direcotry uses

Special directories

.	current directory, eg /home/kim/./ is /home/kim/, eg `ls .` is the same as `ls`
..	parent directory, eg /home/kim/../ is /home, eg if you are in /home/kim `ls ..` will show you /home
.abc	files beginning with "." are not show by default, use ls -a to see
/	paths starting with / are absolute, and do not depend on the current directory
./foo	paths which do not start with / are relative, eg ../foo.py, or subdir/bar.py

Permissions: users and groups

> ls -al
drwxr-xr-x  5 kim   kim    8192 Feb 26 19:34 .
drwxrwsr-x  4 root  staff  4096 Nov 22 11:26 ..
-rw-r--r--  1 kim   kim    6684 Feb 26 19:32 unix-file-system.html
-rwxrwxr-x  1 kim   users  4072 Feb 26 19:32 helloworld

TUUUGGGOOO LL user  group  size date   time  file-name

T	Type, "d" is a directory, "-" is a file
UUU	Permissions for "user"
GGG	Permissions for "group"
OOO	Permissions for someone who is not the user of in the group
LL	number of hard links to the file

Permissions can be changed using chmod.

File system types

The following file system types are used by mount and fstab.
They are listed in /etc/fstab or can be specified by mount with -t.

ext2	default file system for many Linux distributions
ext3	ext2 + journaling, which means that no fsck is required after a power failure
reiserfs, xfs	newer file systems, similar to ext2, but improve on various aspects (journaling, speed, size, efficiency)
iso9660	used by most cdroms
proc	special virtual file system, none of the files actually exist, but they are created by the kernel on the fly, eg cat /proc/filesystems contains a list of supported file system types on the current system
devfs	virtual file system for devices (see below)
msdos	dos file system, with 8.3 file names only
vfat	win95, 98, 2K, XP file system, supports long file names
ntfs	read-only support for NTFS (as used by NT, 2K, XP)

Devices

The device files live in /dev.
Two types, character and block.
Each has a major and minor number

selected device files
/dev/null	throws away all input
/dev/psaux	ps/2 mouse
/dev/hda	all of the first IDE disk (master device on first IDE channel) typically this is your hard disk
/dev/hda1	the first partition on /dev/hda
/dev/hda2	the second partition on /dev/hda
/dev/hda5	the extended partition, partitions 6+ are contained within this
/dev/hdb	the slave device on the first IDE controller
/dev/hdc	the master device on the second IDE controller (typically your CDROM)

Layout

/home	user home directories, eg /home/kim
/etc	configuration files
/usr	applications and data, eg /usr/bin
/var	data which changes, eg /var/spool/mail
/dev	devices, special files
/lost+found	where files are recovered to by fsck after file system corruption. normally empty
/tmp	temporary files, often cleared during bootup
/proc	system information, eg /proc/cpuinfo
/bin	programs required during bootup
/sbin	programs required during bootup which would only be run the the superuser (root)

Advanced concepts (if time)

file types and permissions
- execute on directories
- suid and sgid files
- sgid directories
- stick bit (tmp)
links
- symbolic: ln -s can point to anywhere
- hard: only on same file system

Questions?

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