UNIX file system : An Overview

A file is nothing more than a collection of data. Here describes a set of commands.

The command ‘od’ prints a visible representation of a file. The ‘-c’ option is for interpreting bytes as characters. ‘-b’ for printing the bytes as octal numbers.

$od -c file_name
$od -b file_name

The ‘file’ command guesses the type of the file. ‘od’ command with no option dumps the file in 16 bit words.

$od file_name
$file file_name

The command ‘du’ (disc usage) is to display the disc usage that consumed by the files in that directory. The -a option is for displaying all including the files in a directory. To search a specific file, the output is piped through grep.

$du
$du -a
$du -a | grep file_name

A directory consists of 16 byte chunks, the last 14 bytes of which hold the file name padded with ascii NULs. The first two of which tell the system where the administrative information resides.

The ‘find’ command finds the file specified.

$find file_name

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Programming in C: Remember these points on constants.

A ‘long’ constant is written like 1234567697L or 567874338l. Big integer type constant is also taken as long. ‘unsigned’ constants written with a terminal u or U. ‘unsigned’ long type constants written with the terminal ul or UL. Floating point constants contain decimal point or an exponent (1e-1) or both. They are declared as ‘double’ type. The value of an integer can be specified as octal or hexadecimal. A leading zero on an integer constant means octal and a leading 0x or 0X means hexadecimal.

decimal : 31 octal : 031 hex : 0x1F

Character constants :

It is an integer. eg.’x’. A constant expression is an expression that involves only constants. They are used in any place where a constant occurs. A string constant or a string literal is a sequence of zero or more characters surrounded by double quotes.

In switch each case is labeled by integer valued constants or constant expressions. In the while and do loops ‘continue’ means that the test part is executed immediately. In for loop the control passes to the increment step. The continue statement applies only to loops not to ‘switch’.

Programming in C : Remember these points on operators.

The cast operator has same high degree of precedence as other unary operators. The increment and decrement operators can only be applied to variables. An expression like ‘(exp) ++’ is illegal. Bitwise shift operators can be used with constants or variables.

Var >> n

Here ‘Var’ is shifted n positions to the right. ‘n’ must be a positive value. Bitwise operators are using only on integral operands.