Shell script. Shell Scripts. A shell script contains a sequence of commands in a text file. Shell is an command language interpreter.

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2 Shell Scripts A shell script contains a sequence of commands in a text file. Shell is an command language interpreter. Shell executes commands read from a file. Shell is a powerful programming available belong the Bourne shell. It Used also an interactive Linux shell The speed is compared to a C program/perl program.

3 kernel kernel is the heart of the operating system It is the low level core of the system. It act as the interface between applications and H/W.

4 Type of Shell The shell is sits on the interface between users and kernel. It is a command interpreter which also has programming capability. Shell Name Developed By Where Remark Bourne Shell Stephen Bourne Bell Labs Default shell KSH (Korn Shell) David Korn AT & T Bells Labs Middle between bourne & C shell. CSH (C Shell) Bill Joy University of California The C shell s similar to C language. Bash (Bourne again Shell) Brian Fox and Chet Ramey Free Software Foundation Most common Shell in Linux

5 Which shell am i running [sdbt ~]$ echo $SHELL /bin/bash [sdbt ~]$ which bash /bin/bash [sdbt ~]$ whatis bash bash (1) - GNU Bourne-Again SHell bash [builtins] (1) - bash built-in commands, see bash(1) bash [sh] (1) - GNU Bourne-Again SHell [sdbt ~]$ whereis bash bash: /bin/bash /usr/share/man/man1/bash.1.gz

6 [sdbt ~]$ whatis bash bash (1) - GNU Bourne-Again SHell bash [builtins] (1) - bash built-in commands, see bash(1) bash [sh] (1) - GNU Bourne-Again SHell [sdbt ~]$ whereis bash bash: /bin/bash /usr/share/man/man1/bash.1.gz

7 Variable System variable User defined variable System varaibes created and maintained itself by linux eg echo $SHELL User defined variables created and maintained by user eg variable= value. a=10

8 system variables System Variable Value Meaning BASH /bin/bash Shell name BASH_VERSION 4.1.2(1)-release Shell version name COLUMNS 80 Number of columns for our screen HOME /home/sdbt Default home directory LINES 24 Number of lines for screen LOGNAME Sdbt Logging name OSTYPE linux-gnu O/S type PS1 \W]\$ Prompt Settings PWD /home/sdbt Current working directory SHELL /bin/bash Shell name

9 comment # This symbol comment the line Double quotes Enclose in double quotes removed meaning of that characters(except \ and $). Single quotes Enclosed in single quotes remains unchanged. Back quote To execute command.

10 echo shell variable [sdbt ~]$ echo $# 0 #Number of command line arguments [sdbt ~]$ echo $? 0 #exit status [sdbt ~]$ echo $- himbh #option supplied to shell [sdbt ~]$ echo $$ #PID of shell

11 How to write a shell? [sdbt ~]$ cat > sdbt.sh echo "Welcome to SDBT File name end.sh extension How to execute a shell? [sdbt ~]$ sh sdbt.sh Welcome to SDBT [sdbt ~]$. sdbt.sh Welcome to SDBT [sdbt ~]$ chmod u+x sdbt.sh [sdbt ~]$./sdbt.sh Welcome to SDBT Execute sh filename Execute. Filename Execute./ filename

12 Operators There are different type of operators used in shell script. They are Arithmetic Operator Relational Operator Logical Operator special Operator Bitwise Operator

13 Operators Arithmetic Operators Arithmetic Operators + Addition - Substraction * Multiplication / Division % Modules = Assignment == Equal!= Not equal Description *Note substitution used in \ (back slash) as \* for multiplication.

14 - Operators Arithmetic Operators Basic linux command to add value [sdbt ~]$ expr linux command used in shell [sdbt ~]$ cat add_sample.sh val=`expr ` echo "the total count is $val" val=`expr ` Variable=back quotes closed linux command Executing arthemetic shell [sdbt ~]$ sh add_sample.sh the total count is 30

15 method written in three different variable parsing method Type of method Dynamic static Argument parsing Description The value get dynamicaly by script The value stored in side the schell The value passed while runnig the schell.

16 Dynamic Method When executing it ask input value dynamically store it for responsible variable.

17 Static Method When executing it used input value which store directly to variable.

18 Argument Parsing When executing add the input value it replace responsible variable.

19 Shell Basic linux command to add value [sdbt ~]$ x`expr linux command used in shell [sdbt ~]$ cat add_sample.sh val=`expr ` echo "the total count is $val" Executing arthemetic shell [sdbt ~]$ sh add_sample.sh the total count is 30 val=`expr ` Variable=back quotes closed linux command #Expr linux command use to arithmetic function #echo use to print option. # $ symbol assign to priefix with variable to print output

20 - Operators Relational Operators Relational Operators -gt -lt -eq -ne -ge -le Description Greather than Lesser than Equal Not equal Greater than equal too Lesser than equal too. *Note [ 10 gt 5 ] or (( 10 > 5))

21 Conditional The conditional if statement is present in present in shell. if (condition1) then option1 else option2 fi if Condition true option1 false else option2 fi

22 If..else..fi

23 Shell Script - Operators Logical Operators Logical operators Description! Not -o Or -a and

24 Conditional The conditional if statement is present in present in shell. if (condition1) then option1 elsif(condition2) then option2 else option3 fi if elsif false false Condition 1 Condition 2 true option2 true option1 else option3 fi

25 if..elsif..else..fi

26 Shell Script - Operators Special Operators Special Operators Description -d Checks the content is a directory -f Checks the content is a file. -s Checks if file has size greater than 0 -e Checks if file exists -r Checks if file is readable -w Checks if file is writable -x Checks if file is executable -n Checks if the string not null -z Checks if the string is null.

27 Special characters

28 For loop A bash programming language statement which allows code to be repeatedly executed. initialize For false true Body of loop increment

29 For loop

30 While loop interpreter continues executing the code in the while loop portion of the script as long as the condition is true. condition false true command

31 While loop

32 Until loop An interpreter continues executing the code in the until loop portion of the script as long as the condition is false.

33 case..esac It is alternative to multilevel if-thenelse-fi statement. It enable us to match several values against one variable. It is easier to read and write. case1 true command1 false case2 true command2 false case3 true command3

34 Case..esac

35 Other Operators Bitwise Operators Name Description & Ampersand execute more than one command in the background ; semicolon Execute several command in single execution. && And execute the second command only. Or execute second command only after failed first command.! Not execute all except the condition provided. Pipe first command acts as a input of the second command. {} Command Combination Second command depends upon the execution of the first command. () Precedence execute command in precedence order. \ Concatenation concatenate large commands over several lines.

36 Function Manage memory, i/o devices, allocates the time between user and process, inter process communication, sets process priority. [sdbt ~]$ cat function.sh #!/bin/bash function s () { if [[ "$1" == 'yes' ]] ; then local sdbt fi sdbt=database } [sdbt ~]$. function.sh db database sdbt=db s yes echo "$sdbt" s echo "$sdbt"

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