Page 444 - DCAP103_Principle of operating system

Unit 14: Case Study of Linux Operating System

the file system is implemented. Some of these ideas derive from MULTICS, and many of them Notes
have been copied by MSDOS, Windows, and other systems, but others are unique to UNIX-
based systems. With minimal mechanism and a very limited number of system calls, Linux
nevertheless provides a powerful and elegant file system.

14.6.1 Fundamental Concepts of Linux File System
The initial Linux file system was the MINIX 1 file system. However, due to the fact that it
limited file names to 14 characters (in order to be compatible with UNIX Version 7) and
its maximum file size was 64 MB (which was overkill on the 10 MB hard disks of its era),
there was interest in better file systems almost from the beginning of the Linux development,
which began about 5 years after MINIX 1 was released. The first improvement was the ext
file system, which allowed file names of 255 characters and files of 2 GB, but it was slower
than the MINIX 1 file system, so the search continued for a while. Eventually, the ext2 file
system was invented with long file names, long files, and better performance, and that has
become the main file system. However, Linux supports several dozens of file systems using
the Virtual File System (VFS) layer (described in the next section).
When Linux is linked, a choice is offered of which file systems should be built into the kernel.
Other ones can be dynamically loaded as modules during execution, if need be. A Linux file is
a sequence of 0 or more bytes containing arbitrary information. No distinction is made between
ASCII files, binary files, or any other kinds of files. The meaning of the bits in a file is entirely
up to the file’s owner. The system does not care. File names are limited to 255 characters, and
all the ASCII characters except NUL are allowed in file names, so a file name consisting of
three carriage returns is a legal file name (but not an especially convenient one). By convention,
many programs expect file names to consist of a base name and an extension, separated by a
dot (which counts as a character). Thus prog.c is typically a C program, prog.f90 is typically a
FORTRAN 90 program, and prog.o is usually an object file (compiler output). These conventions
are not enforced by the operating system but some compilers and other programs expect them.
Extensions may be of any length and files may have multiple extensions, as in prog.java.gz,
which is probably a gzip compressed Java program.
Files can be grouped together in directories for convenience. Directories are stored as files, and
to a large extent can be treated like files. Directories can contain subdirectories, leading to a
hierarchical file system. The root directory is called / and usually contains several subdirectories.
The / character is also used to separate directory names, so that the name /usr/ast/x denotes the
file x located in the directory ast, which itself is in the /usr directory. Some of the major directories
near the top of the tree are shown in Figure 14.23. There are two ways to specify file names in
Linux, both to the shell and when opening a file from within a program. The first way is using
an absolute path, which means telling how to get to the file starting at the root directory. An
example of an absolute path is /usr/ast/books/mos3/chap-10. This tells the system to look in
the root directory for a directory called usr, then look there for another directory, ast. In turn,
this directory contains a directory books.

Figure 14.23: Some Important Directories Found in Most Linux Systems

Directory Contents

bin Binary (executable) programs
dev Special files for I/O devices

etc Miscellaneous system files

lib Libraries

usr User directories

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