Shell

1
Shell

• Youngwoo Park
• (ywpark_at_core.kaist.ac.kr, 5425)
• September 10th, 2009
• Computer Engineering Research Laboratory
• Korea Advanced Institute of Science and Technology

2
Shell ?

• Shell An interpreter that accept input from the
  user and executes the command that are given
• Interface between user and the OS
• Shell is not a OS component
• Shell is application for OS

3
Which shell?

• Command line shell (CLI)
• bash, tcsh, ksh, csh (Linux),
• cmd (windows)
• Graphical user interface (GUI)
• Gnome, KDE (Linux),
• Windows, Mac

4
What does the shell do?

• Command processing
• The basic role of shell
• I/O redirection
• Change the source and destination of I/O
• Piping
• Piping two command
• Environment control
• Control execution path, shell prompt, alias
• Job control
• Foreground, Background job
• Shell scripts(programming)

5
Command line format

• Command -option arguments
• Ls h myfile
• tokens

6
Command processing

• Read what you type at the prompt
• Parse it (command/options/args )
• Use space/tab/cr for parsing
• Optionally expands(transforms) it
• Execute the command
• Program Execution or Emitting error message

7
Command processing example

• cp i /home/ywpark/myfile yourfile
• cp command ? copy file
• -ioption ? prompt before overwrite
• /home/ywpark/myfile argument ? source file name
• yourfile argument ? destination file name
• ls h myfile gt filename
• ls command ? list-up file name and file
  characteristics
• -h option ? print sizes in human readable format
  
• myfile argument (need expansion) ? print file
  whose names start as myfile
• gt redirection ? redirect I/O
• filename argument for redirection ? redirected
  file I/O name

8
Expansion

• Filename expansion
• stands in any 0 or more characters
• ls /home/ywpark/ list-up all files in
  /home/ywpark directory
• ? stands in any single character
• ls ?o list-up file go, do, ho
• 1-6 stands in for one of 1,2,3,4,5,6
• ls lab1-6 list-up file lab1, lab2, lab3, lab4,
  lab5, lab6
• oa stands in for any char except o or a
• ls .oa list-up file .b, .c, .d, , .n,
  .p,
• / stands in for your home directory
• ls /bin list-up file in bin of your home
  directory
• ywpark stands in for ywparks home directory
• Ls ywpark/bin list-up file in bin of ywparks
  home directory

9
Filename expansion example
10
Transformation example

• Transformation Use metacharacters ()ltgt
  
• It is very useful for shell programming

Ref.) homepage.smc.edu/morgan_david/linux/a01-shel
l.pdf
11
Transformation example
Ref.) homepage.smc.edu/morgan_david/linux/a01-shel
l.pdf
12
IO redirection

• Four I/O candidate
• Standard input, Standard output, Standard error,
  File
• Default get an input from standard input device,
  put an output to standard output device, put an
  error message to standard error device
• Redirection
• Use file for input lt,
• Use file for output gt, 2gt (stderr), gt
  (stdoutstderr), gtgt(append to the file)
• Example
• cat source gtgt results
• Append the contents of source to file results
• cp abc 2gt err
• Print error message to err file

13
I/O redirection example
14
Pipe

• Allow multiple command to be connected
• Send the output from one command to the input of
  the next
• Example
• ls -h grep abc

15
Pipe example
16
Environment control

• Variables
• Environment variables
• Get passed to child processes
• Local variables
• Dont get passed
• Environment control
• Setup variables
• SHELL decide shell
• PATH command search path
• PS1 shell prompt setting
• Execute built-in commands after login
• alias set up the aliases

17
(No Transcript)
18
Job control

• Foreground job
• A program which has control of the terminal
• Background job
• Runs concurrently with the parent shell and does
  not take control of the keyboard
• Job control command
• jobs list-up active jobs
• bg resume job in the background
• fg resume job in the foreground
• kill kill the job(process)
• execute job in the background
• User can interact with shell after background job
  is executed

19
Job control example

• Example
• ls h
• sort bigfile

20
Shell script?

• File containing a list of shell command
• A great way to automate a repeated sequence of
  commands
• Generate Shell script
• Make and edit script file
• File should be start with !/bin/sh
• absolute path to the shell program
• Not the same on every machine
• Add execution mode for shell script
• Chmod x scriptname

21
Command and variable

• Command
• You can run any program in a shell by calling it
  as you would on the command line
• There are also some built-in command
• Echo, test, read, exit, wait shift
• Refer to bash manual (man bash)
• Variable
• Local variable spaces matter
• namevalue (Assignment)
• name (Replaced by value of name)
• Single value example
• bindir /usr/bin
• List of values (separated by spaces)
• searchdirstests HOME/test2 .

22
String replacement function

• String replacement (positional parameters)
• Variables are placeholders where we will
  substitute the value of the variable
• Function

iters1 2 3 4 for i in iters do echo i done
For i in 1 2 3 4 do echo i done
Function functionname shell commands
23
Example

• Variable example
• String replacement example

24
Example

• Variable function example

script ascript   function afunc echo in
function 0 1 2 var1in function echo
var1 var1 var1outside function echo
var1 var1 echo 0 1 2 afunct funcarg1
funcarg2 echo var1 var1 echo 0 1 2
script ascript   function afunc echo in
function 0 1 2 var1in function echo
var1 var1 var1outside function echo
var1 var1 echo 0 1 2 afunct funcarg1
funcarg2 echo var1 var1 echo 0 1 2
25
Flow control

• Bash support flow control
• If/else, for, while, until, case, select

for x 1 to 10 do begin statements end ------
--------------- for dir in PATH do ls ld
dir done
case expression in pattern1) statements patt
ern2)_ statements .. esac
if/else if condition then statements elif
condition then statements else statements f
i
while condition do statements done
select name in list do statements that can use
name done
until command do statements done
26
Flow control example
!/bin/bash renames.sh ?? ?? ??? ????
criteria1 re_match2 replace3 for i in
( ls criteria ) do srci tgt(echo i
sed -e "s/re_match/replace/") mv src tgt
done
27
Reference

• man bash (man page for bash)
• http//en.wikipedia.org/wiki/Bourne-Again_shell
• http//tldp.org/LDP/Bash-Beginners-Guide/html/
• www.cs.toronto.edu/krueger/.../f05/.../Week2-Shel
  l-1up.pdf 
• homepage.smc.edu/morgan_david/linux/a01-shell.pdf