CMT3321 (02/03) 
(Linux study guide 4 – Shell scripting)
For Lab. 4, 5



Lecturer : Mr. Lee Bo-sing (Joe)
Agenda
?	Programming basics
?	Shell script basics
?	Shell script content
?	Shell quoting 
?	Shell variable
?	Relational condition
?	Logical operation
?	Arithmetic statement
?	Flow control statements
?	Std & file I/O statement
?	Sed & awk introduction  ??



Programming basics (1)
?	What does a program/script do:
–	Input data (from keyboard or file)
–	Process input data based on specification
–	Output processed result (to monitor or file)
?	2 types of programs :
–	Compiled program 
?	Need to compile before running,  eg. C program
?	Executes faster
–	Interpreted program
?	Don't need to compile. An interpreter (shell) is responsible for interpret/translate it during 
running.  eg  shell script
?	Executes slower
?	Shell script is just a kind of interpreted program
Programming basics (2)
?	2 types of programming bugs :
–	Syntax bug     
?	mainly due to unfamiliar with the programming language syntax & linux 
environment. Lab 4, 5 helps to develop your ability of shell scripting 
language syntax.
?	Read the error messages carefully. It usually provides useful hints.
–	Logical bug    
?	most difficult to discover
?	Logical thinking error
?	Careless human mistake
Programming basics (3)
?	General procedures for interpreted program :
–	Understand specification of your program from end user or your boss
–	Design and write down your programming specification using any pseudo-
code or flow-chart
–	Edit source code of your program using any editor such as pico or vi or any 
editor you get used to
–	chmod your source code to executable in unix environment. If you just type 
the name of script file, make sure that current directory (".") is included in the 
variable $PATH. Or, type a relative or full pathname of your script file.
–	Run and check for any conflict with specification.
–	If conflict exists, modify your script file, debug and re-run. Repeat step 5, 6 
until specification is fulfilled
Shell script basics (1)
?	How does UNIX know this is shell script?
–	Kernel check the first 2 byte of line of script file  for  #!
–	Kernel then check whole first line for #!/bin/ksh or #!/bin/sh  or 
#!/bin/csh  or #!/bin/bash to determine which shell interpreter to 
use.
–	Note that syntax of different shell varies. For consistency, I use K-
shell for teaching. ??
?	How to make shell script executable
–	In unix prompt, chmod 700  script_filename
–	Current Directory (".") is  included in $PATH
Shell script basics (2)
?	How to run shell script?
–	use shell specified in your script to interpret your script   (better  as 
portable)
?	Just type pathname of script
–	Or use your current shell to interpret your script
?	For ksh or sh script,     .   pathname_of_script  (eg.  .   ~/.test.ksh)
?	For csh script  source Pathname_of_script  (eg.  source   ~/.test.csh)
?	Exit status :
?	0 => successful
?	1 => fail
Shell script content (1)
?	A script can include any of  followings :
–	First line MUST be #!/bin/??
–	Comments prefixed by #
–	Any shell built-in commands (mkdir, cd, echo, eval, etc.)
–	Any UNIX external utilities or executable complied programs (cp, tar, grep, 
awk, etc.)
–	Shell variable assignment statement (var_name="value"; variable is declared 
and assigned at the same time)
–	Read shell variable content by including $VAR_NAME in script statement.

Shell script content (2)
–	Arithmetic or relational statement
–	Control flow statement :
?	Conditional expression (if, then, else, elseif)
?	Looping expression
?	Case  statement
?	Test statement
–	Std I/O statement :
–	File I/O statement
–	Shell function definition & calling

Shell quoting (1)
?	Some characters have special meaning to shell and need to be quoted when that 
special meaning is not wanted, eg. You want print $.
?	Special characters includes :
–	; & ( ) |  ^  < >  ?  * [ ] \ $ ` " '  {  }  newline  tab  space
?	3 types of quoting :
–	Quote single character by backslash (\) eg. Echo  \$.   Output is $
–	Quote a string by a single quote ('   '). All special meaning of quoted special 
characters is lost and these characters will be printed.
–	Quote a string by a double quote ("  "). Similar to single quote but some 
special characters such as $, `, \ , still have their special meanings.
Shell quoting (2)
?	When to use quoting :
?	Pass special meaning to the called program . eg.   find  .    -name  
"*.rpm"
?	A string with white space
?	Eg. echo  "There are space in the sentence"
?	Intend to print out special character.
?	Eg. echo  "dollar sign is  \$.  Back quote is \`.  Backslash is \\"

shell variable (1)
?	Declaration and assignment within one statement
–	In assignment statement, the variable is not prefixed with dollar sign.  White 
space (space, tab) is not allowed on either side of the equal sign (=) 
–	Eg.  VAR  =  abc      (wrong)
–	Eg.   VAR=abc       (correct)
–	However, to access a variable, the variable must be prefixed with dollar sign. 
–	Eg. ${VAR-NAME}   (better) or   $VAR-NAME
–	Eg.  echo   "VAR_NAME is  ${VAR_NAME}
?	Variable or environment variable defined in a script is not available after the 
script's execution completes.
?	Variables defined in sub-shell are localized and parent shell cannot access these 
variables
?	Read and run file var1.ksh

shell variable (2)
?	Content of  an variable is manipulated as string. Even when a 
variable contains a number, it is stored as a string of numeric 
character.
?	Content can be 
–	single, double quoted for string
–	backslash quoted for single character with special meaning , eg. $ ? & ' ' " " ( )  
[ ]  |  < > `
?	Eg.  VAR1=abc
?	Eg.  VAR2='this is a variable'
?	Eg.  VAR3="today is `date` and VAR1 is $VAR1"
?	Eg.  VAR4="\$VAR1"
?	Read and run file var2.ksh

shell variable (3)
?	Special predefined variables :
–	$?   =>  exit status of last executed command
–	$$   =>  process ID of current command
–	$0, $1, $2, $3, $4, …..
–	e.g. Command-name   par1   par2  par3   par4
–	$0  equals to command-name
–	$1  equals to par1
–	$2  equals to par2
–	$3  equals to par3
–	$4  equals to par4
–	$#  equals to number of  parameters, i.e. 4
?	Read and run file var3.ksh


Relational condition
?	  [  expr1  -eq   expr2  ]
–	Is equal to
?	  [  expr1  -ne   expr2  ]
–	Is not equal to
?	  [  expr1  -lt   expr2   ]
–	Is less than
?	  [  expr1  -le   expr2  ]
–	Is less than or equal to 
?	  [  expr1  -gt   expr2  ]
–	Is greater than 
?	  [  expr1  -ge   expr2  ]
–	Is greater than  or equal to 
?	Refer to file if1.ksh, if2.ksh

Logical operation of conditions
?	 AND of 2 or more conditions :
–	Condition-1   &&    condition-2
–	[  expr1  -lt   expr2  ]   &&  [  expr2 -gt   expr3  ]
?	OR  of  2 or more conditions :
–	Condition-1   ||    condition-2
–	[  expr1  -lt   expr2  ]   ||  [  expr2 -gt   expr3  ]
?	Logical NOT of  a condition :
?	!  Condition
?	!   [  expr1  -lt   expr2  ]   &&  [  expr2 -gt   expr3  ]
?	Refer to file if1.ksh, if2.ksh


arithmetic
?	Works for  +  -  *  /   %  and any combination
?	Syntax : 
–	Let  var-name=3+4-3*2/2
?	Read and run file arith.ksh
Shell flow control statement
?	Conditional expression (if, then, else, elseif)
?	Case  statement 
?	While loop  statement
?	For loop  statement

 Conditional statement (1)
?	if   condition1
    then
           command_block1
    fi
?	if   condition1
      then
           command_block1 
      else
           command_block2  
      fi
?	Read and run file if1.ksh
 Conditional statement (2)
?	if   condition1
      then
            command_block1 
      elif  condition2
            command_block2  
      else
            command_block3
      fi
?	Read and run file if2.ksh

Case statement
?	case   string  in
          pattern1)  command_block1  ;;
          pattern2)  command_block2  ;;
              
           patterni)   command_blocki  ;;
            *)            command_block   ;;
     esac
?	Read and run file case.ksh

While & for loop statement
?	While loop :
–	while  condition
    do 
          command_block
    done
?	For loop :
–	for   name   in   words
do
Command_block
done
?	Read and run file loop.ksh 

Std I/O
?	Read data from stdin (keyboard) and assign this data to a 
variable by :
–	read   VAR_NAME
?	Print(or write) to stdout (monitor) by :
–	echo  "anything you want to print, can include variable content, space, 
execution result of a command"

file I/O (1) –write to a file
?	3 steps :
?	Open a file and assign a file descriptor "n" to this file.  This file is created if 
not existed. If already created, all original content will be erased.
?	exec   n>filename        (n >= 3)
–	Redirect any std output of a command to this open file (file descriptor n) using 
>
?	echo  "message"    1>&n
?	Close the file
?	exec   n>&-
?	Above 3 steps can be simplified into :
?	echo  "message"   >   filename
file I/O (2) - Append to an existed file
?	3 steps :
?	Open the file and assign a file descriptor to this file. 
?	exec   n>>filename       (n >= 3)
–	Redirect any stdout of a command to this open file (file descriptor n) using >>
?	echo  "message"    1>>&n
?	Close the file
?	exec   n>&-
?	Above 3 steps can be simplified into :
?	echo "message"   >>  filename
?	Read & run file1.ksh

file I/O (3) –read from an existed file
?	3 steps :
–	Open existed file for reading and assign a file descriptor to this file. 
?	exec   n<filename       (n >= 3)
?	Redirect any std input of a command to read from this open file 
(file descriptor n) 
?	while    read     LINE    0<&n
do
        ……..
done
–	Close the file
?	exec  n<&-
?	Read & run file2.ksh



Sed
?	Function : works on stdin or file. Very useful tool
–	to replace any src_string from stdin to  dest_string and print to 
monitor  by  "s"
?	eg.  cat  file |  sed  -e  "s/src_str/dest_str/g"
?	s stands for substitute, 
?	g stands for global, 
–	if g is not given, only first src_str is replaced by dest_str. 
–	If g is given, all src_src of each line are replaced by dest_str
?	No limitation to length of src_str and dest_str
–	To delete particular target_str (just specify dest_str as  nothing  by  
"s"
–	Eg.  cat  file  | sed  -e  "s/target_str//g"


Sed
–	To delete any line containing any target_str  by "d"
–	cat  filename |  sed  -e  "/target_str/d"
–	d stands for delete
–	To delete any empty line   by "d"
–	cat  filename | sed  -e  "/^$/d"
–	To delete particular line(s)  by "d"
–	cat  filename | sed  -e  "1,4d"        (line 1 to 4)
–	cat  filename | sed  -e   "1d"            (line 1 only)
?	cat  filename | sed –e "$d"             (last line)
?	cat  filename | sed  -e   "2,$d"                   (2nd to last line)
?	To select particular line(s)  by  "p"  & "-n"
?	cat  filename  |  sed  -n  "1,4p"                 (line 1 to 4)
?	cat  filename  |  sed  -n  "4p"                 (line 4 only)

Sed
?	More precisely to specify the src_str, target_str by :
–	Eg.  cat  file  | sed   -e   "s/^src_str/dest_str/g"
–	^src_str  stands for only src_str at the beginning of each line
–	Eg.  cat  file  | sed  -e  "s/src_str\$/dest_str/g"
–	src_str\$  stands for any src_str at the end of each line    (\ is used as 
quote  special meaning of $ to shell)
?	More on sed from 
http://www.ptug.org/sed/sedfaq.htm

awk
?	Actually a complete programming language. You can make 
use its column selection feature to write your script
–	Similar to cut
–	Eg. To select field 1,2,3 (field starts counting at 1) , separated by 
":", from a file
–	cat  filename | awk  -F:   '{print  $1,$2,$7}'
–	-F:  specify : as field separator,  output field 1, 2, 7
–	More on awk from http://www.ssc.com/ssc/eap/

 

Open 
Discussions
Exercise 1
?	Run & understand each of demo. Files :
?	var1.ksh, var2.ksh,  var3.ksh
?	if1.ksh, if2.ksh
?	arith.ksh
?	loop.ksh
?	case.ksh
?	file1.ksh  file2.ksh



Exercise 2
?	Write a script which display "Good morning", "good 
afternoon" or "good evening" , depending on the time 
of running the script, to monitor when it runs.



Exercise 3
?	Write a script which 
–	a. prompt to read your  name & age
?	Enter your name: Joe
?	Enter your age: 25
–	b. write the read name to first line of file, named my_info.
–	c. change your age by "age*2-3" and write to second line of 
my_info.
?	Content of my_info :
–	my name is Joe
–	my modified age is 72


Exercise 4
?	Write a script which read a number in units of 
seconds and convert it hour:minute:second and print 
to monitor
?	Your script must prompt for re-input if negative value 
is input
–	Enter number of seconds :  12345
–	Result :
–	12345 seconds is  3:25:45



Exercise 5
?	Write a script calculate, which accepts 4 arguments 
a, b, c, d and print out "a*20-b*2+c/d" to monitor
–	$  calculate  2   12   5  2
–	result of  "2*20 – 12*2 + 5/2" is  18