UNIX File System Hierarchy

Computer History Notes - Herong's Tutorial Notes

∟1970 - UNIX Operating System Developed by AT&T Bell Labs

∟UNIX File System Hierarchy

This section provides a quick introduction of UNIX file system hierarchy.

On a UNIX system, almost all resources are represented as files in a single file system hierarchy.

For example, the first floppy disk drive is represented as a device file: /dev/fd0. Data can be read from and write to a floppy disk directly through /dev/fd0. But if a floppy disk has file system structure, it can also be mounted to the file system hierarchy.

The "df" command displays usage information of physical storage devices that are mounted on the current file system hierarchy:

#df
Filesystem   1024-blocks     Used   Available   Capacity   Mounted on
/dev/hda1         198275   134358       53672        71%   /
/dev/hda3          92333        1       87564         0%   /mnt/backup
/dev/df0            1676      960         716        57%   /mnt/floppy

The output tells us that 3 storage devices with file systems are mounted on the file system hierarchy:

/dev/hda1 - Represents the first partition of the hard disk. Files in on /dev/hda1 is mounted at /.
/dev/hda3 - Represents the third partition of the hard disk. Files in on /dev/hda3 is mounted at /mnt/backup.
/dev/df0 - Represents the floppy disk in the first drive. Files in on /dev/df0 is mounted at /mnt/floppy.

UNIX system also has a very interesting special file called /dev/null, representing an empty file for reading and a black hole for writing. /dev/null is very useful for truncating log files that are currently in use. Here is an example of commands to rotate the mail.log file once a day on a Solaris system:

#vi /usr/lib/newsyslog
   cd /var/log
   test -f mail.log.2 && mv mail.log.2 mail.log.3
   test -f mail.log.1 && mv mail.log.1 mail.log.2
   test -f mail.log.0 && mv mail.log.0 mail.log.1
   mv mail.log mail.log.0
   cp /dev/null /var/log/mail.log
   chmod 644 /var/log/mail.log
   kill -HUP `cat /etc/syslog.pid`

#crontab -e root
   59 23 * * 6 /usr/lib/newsyslog