UNIX Redirection, C Basics

Prof. Brian D. Davison

Computer Science & Engineering, Lehigh University

Announcements

• Short Quiz #2 today
• Reminder: Programming assignment #1 is due Monday. Questions?
• New: Homework #2 is posted, due Wednesday.

Redirection

• One of the great features of UNIX is the ability to connect multiple programs that read from stdin and stdout.
• cat, for example, writes to standard out
• In UNIX, stdin and stdout (and stderr) can be redirected.
  • "ls -l > ls-l.txt" will take the output of "ls -l" and write it to a file called ls-l.txt
  • "more < file1" will use file1 as the standard input to more
  • "prog1 >> file1" will append prog1 stdout to file1 (not replace)
  • "prog1 2> errorfile" will only write stderr messages to errorfile
  • "prog1 &> bothfile" will send both stdout and stderr to bothfile
  • "sort < numfile > sortednums" redirects both stdin and stdout

Pipes

• Programs can be chained together using "pipes"
  • "cat longfile | wc" sends the output of "cat longfile" to the input of "wc"
  • "cat numfile | sort > sortednums" sends the output of "cat numfile" to the input of "sort", whose output is written to "sortednums"
• A more complex (realistic) example:

  cat airweb-access_log airweb-access_log.2005Jan* | grep -v airweb | 
     cut -d' ' -f11 | sort | uniq -c | sort -nr | more

  Takes web server logs with entries such as:

  192.114.107.4 - - [23/Jan/2005:09:20:10 -0500] "GET / HTTP/1.1" 200 3515 
  "http://www2005.org/tutorials/accept.html" "Mozilla/4.0 (compatible; MSIE 
  6.0; Windows NT 5.0; .NET CLR 1.0.3705)"

  and generates reports like:
  

     1410 "-"
       24 "http://www.cse.lehigh.edu/~brian/"
       21 "http://www2005.org/tutorials/"
       18 "http://timconverse.com/blog/"
       17 "http://www2005.org/tutorials/workshops.html"
       16 "http://nike.psu.edu/dbevents/"
        8 "http://www.kdnuggets.com/news/2004/n24/28i.html"
        8 "http://blog.searchenginewatch.com/blog/050121-124259"
  

C versus C++

• C++ is mostly a superset of C
• So what are the major additions?

Major C differences from C++

• No classes or objects -- all code is in functions
• C structures cannot have methods
• I/O in C is based on library functions
• No function overloading
• No new or delete (use library functions instead)
• No reference variables (aliases)

Other Differences

• Variables must be declared at the beginning of a block, typically at the beginning of the function
• No bool datatype
• No << and >> operators for I/O
• Cannot substitute and, or, and not for boolean operators &&, ||, and !
• Different approach to strings

Basic C Data Types

• Pretty much the same as in C++
  • int (%d, -40), short int, long int, long long int, unsigned int
  • float (%f, 3.14), double, long double
  • char (%c, 'a'), unsigned char, signed char
  
  
• Variable names are also similar to C++
  • made of letters, digits, underscore
  • cannot start with a number

Operators

• Arithemetic operators
  • Typical binary operators: + - * /
  • Unary + and -
  • Modulus % (e.g., 25 % 4 = 1)
• Logical (Boolean) operators
  • &&, ||, !
• Bitwise operators
  • &, |, ^, ~, <<, >>
• Relational operators
  • <, <=, >, =>, ==, !=
• Conditional operator

  expression ? expression1 : expression2 ;

Assignment Operators

• Assignment = (e.g., a = 4;)
• Modify and assign
• Increment ++ and Decrement -- (e.g., a++; --b;)
• Combinations *=, /=, %=, +=, -=, &=, ^=, |=, <<=, >>=

  a += 4;
  b <<= 1;
  

Type Conversions

• C permits (and performs) many kinds of type conversions
• -Wall should warn you about them if they are not explicitly cast
• When a float converted to an int, the value is truncated
• Integer arithmetic generates integer results (even when the result has a fractional value)
• If any value in an expression is of type float (or double, etc.) then the result will also be of that type
• Use of the type cast operator explicitly tells the compiler to do the conversion (and has higher precedence than anything but unary +/-)

  (int) 29.55 + (int) 21.99 is the same as 29 + 21 in C
  

Control Flow

• Loops

while (expression)
   statement

for (expr1; expr2; expr3)
   statement

do
   statement
while (expression);


The break statement drops control out of the innermost loop while continue moves on to the next iteration.
• Conditionals

if (expression)
   statement1
else
   statement2

switch (expression) {
   case const-expression: statements
   case const-expression: statements
   default: statements
}