Programming Languages

1
Programming Languages

• BASH
• Jing Chan

2
Introduction

• It is acronym for Bourne-again shell.
• Created in 1987 by Brian Fox
• Shell scripting language written for the GNU
  Project

3
Where to run Bash

• Default shell of most of Linux systems, and also
  Mac OS X
• Unix machine or Windows machine
• Unix-like operating system such as Cygwin

4
Outline

• Variables
• Tokens
• Basic syntax and examples
• Comparison (.cpp and .sh run in both cygwin and
  linux)
• Why use bash
• Why not use bash
• Other considerations
• Reference
• Q A

5
HelloWorld

• Java
• class HelloWorld
• public static void main (String args)
• System.out.println("Hello World!")
• Bash
• !/bin/bash
• echo Hello World

6
Variables

• A variable in bash can contain a number, a
  character, a string of characters
• Not necessary to declare a variable
• Assign a value to its reference
• Example
• STRING"Hello World"
• echo STRING

7
Variables (cont)

• Local variables
• Create local variables by using keyword local
• Example
• !/bin/bash
• HELLOHello
• function hello local HELLOWorld
• echo HELLO echo HELLO

8
Tokens in BASH

• Backslash \
• Single quote
• Double quote
• Pipe
• Backtick

9
Token (Backslash)

• creating directory names, file names and etc.
• Example
• mkdir foo\ bar
• //creating a directory called foo bar
• rm rf foo\ bar
• //remove

10
Backslash (cont)

• Escape Sequence break down a long string
• Example
• STRINGABCDEFGHIGK\
• LMNOPQRSTUV
• //STRING equals to both two lines

11
Tokens (Single Quote)

• Enclosing characters in single quotes preserves
  the literal value of each character within the
  quotes.
• A single quote cant be used between single
  quotes, even when preceded by a backslash.

12
Example

• value5
• echo I have value dollars
• Output
• I have value dollars
• value will not be interpreted

13
Token (Double Quote)

• Quoting characters
• Groping the space separated works together
• Example cat conference agenda.txt"
• Interpreted the string
• Example
• value5
• echo I have value dollars
• I have 5 dollars //output

14
Quotes

• What if echo without any quotes
• echo '"This text is surrounded by double
  quotes."'

15
Token (Pipe)

• Allows you use the output of a program as the
  input of another one
• -program1 program2
• Examples
• ls grep test
• ls -l sed -e "s/aeio/u/g"

16
Backtick

• It is quotation of commands
• What is the difference between
• echo date
• and
• echo date

17
For Loop

• for i in seq 1 10
• do
• echo i
• done
• //print the sequence from 1 to 10
• What if we take out the ?

18
While Loop

• Structure
• While CONTRO-COMMAND
• do
• CONSEQUENT-COMMANDS
• done
• CONTRO-COMMAND can be any commands which exit
  with a success or failure status.
• CONSEQUENT-COMMANDS can be any programs, scripts
  or shell construct.

19
While Loop

• i"0"
• while i -lt 10
• do
• xterm
• ii1
• done

20
Comparison

• Created 4 files
• 2 .sh and 2 .cpp
• -test.sh and test.cpp
• -test2.sh and test2.cpp
• Run in Cygwin Bash shell and SSH
• Guess which one is the most expensive to run

21
test.sh

• !/usr/bin/bash
• echo Bash Program Begins...
• BEGINTIME(date s)
• n0
• for i in seq 1 100
• do
• for j in seq 1 10
• do
• for k in seq 1 100
• do
• n(( n i j k ))
• done
• done
• done
• echo n
• ENDTIME(date s)
• DIFF(( ENDTIME - BEGINTIME ))
• echo "It took DIFF seconds"

22
test.cpp

• int main(int argc, char argv)
• printf("C Program Starts...\n")
• time_t begintime
• time_t endtime
• int difference
• int n 0
• begintime time(NULL)
• for (int i1 ilt 100 i )
• for (int j1 jlt10 j )
• for (int k1 klt100 k)
• n nijk

23
Result of the comparison
Windows Linux
test.sh gt 30 seconds 3 seconds
test.cpp 0 seconds 0 seconds
24
Another example

• Basically, convert all the lower cases characters
  in a file into upper cases, and the output will
  be in a output file
• test2.sh and test2.cpp
• Run in Cygwin and SSH

25
test2.sh

• !/usr/bin/bash
• Changes a file to all uppercase.
• E_BADARGS65
• if -z "1" Standard check for command line
  arg.
• then
• echo "Usage basename 0 sourcefile destfile"
• exit E_BADARGS
• fi
• if -z "2"
• then
• echo "Usage basename 0 sourcefile destfile"
• exit E_BADARGS
• fi
• echo Bash Program Begins...

26
test2.cpp

• int main(int argc, char argv)
• if (argv1"" argv2"")
• printf ("Usage s sourcefile destfile",
  argv0)
• exit(1)
• printf("C Program Starts...\n")
• time_t begintime
• time_t endtime
• int difference
• string line
• ifstream file_op
• file_op.open(argv1,iosin)

27
Result of the 2nd comparison(coding time is not
included)
Windows Linux
test.sh 0 seconds 3 seconds
test.cpp 0 seconds 0 seconds
28

• Can we make it faster?

29

• cat test.txt tr a-z A-Z gt testA.out

• !/usr/bin/bash
• Changes a file to all uppercase
• E_BADARGS65
• if -z "1" Standard check for command line
  arg.
• then
• echo "Usage basename 0 sourcefile destfile"
• exit E_BADARGS
• fi
• if -z "2"
• then
• echo "Usage basename 0 sourcefile destfile"
• exit E_BADARGS
• fi
• echo Bash Program Begins...
• BEGINTIME(date s)

30
Why use bash

• Any idea?
• Because bash is already running, any additional
  bash scripts that you run are inherently
  memory-efficient because they share memory with
  any already-running bash processes.
• System administration is easier to use the
  existing tools available in bash than to write a
  new program every time.

31
When not to use bash

• While there are many small tasks to implement
• So just put all the tasks in one program instead
  of using bash

32

• If you have a company that produces software,
  will you use bash?
• Why/ Why not?

33
Considerations of using Bash

• Security issue
• -Is the source code readable?

34
References

• http//en.wikipedia.org/wiki/Bash
• http//www.faqs.org/docs/Linux-HOWTO/Bash-Prog-Int
  ro-HOWTO.htmlss2.1
• https//wiki.ubuntu.com/Spec/EnhancedBash
• http//tiswww.case.edu/php/chet/bash/bash.html
• http//www.gnu.org/software/bash/bash.html
• http//www.dsj.net/compedge/shellbasics1.html

35

• Questions?