C151 Multi-User Operating Systems

C151 Multi-User Operating Systems Scripts and Shell Programming

Shell Script ------------ ------------ ------------ ------------ ------------ Shell interpreter A file containing list of commands Usually it has the extension .sh. Each line (command) is interpreted and executed by a shell program (interpreter). We will discuss bash shell. Shell script

Steps to Run a Shell Script • Create a file (say file_name.sh)using your favorite editor • Make the file executable chmod 700 file_name.sh • Run the script file_name.sh

First Line • The first line should look like this #!/bin/bash • This line tells: • Which shell (interpreter) should be used to execute the script: bash • Where to find the shell (interpreter): /bin/bash

Comments • Comments: any line starting with a # will be considered as comments • Exception: The very first one • Example: #!/bin/bash #This is my first shell script pwd ls –l

Combining Commands • Several commands on the same line: separated with ; • Example:cd.. ; make ; cdsrc

Output Use echo command #!/bin/bash #This is my first shell script echo “This is my current working directory” pwd echo “These are the files in my current working directory” ls –l

See This Script #!/bin/bash echo $0 echo $1 echo $2 echo $# echo $* test.sh

See this script #!/bin/bash echo $0 echo $1 echo $2 echo $# echo $* test.sh

Command Line Parameters • The command parameters are arguments that can be given in the command line when we run the script. • $0is the name of the script file. • $1is the first argument in the command line, $2is the second, etc. • $# is the number of arguments. • $* contains all the arguments.

Variables • The name of a variable is a placeholder for its value, the data it holds. • If v is the name of a variable, then $v is a reference to its value • What result will be displayed for the following script? #!/bin/bash v=23 echo v echo $v

Variables • The name of a variable is a placeholder for its value, the data it holds. • If v is the name of a variable, then $v is a reference to its value • What result will be displayed for the following script? #!/bin/bash v=23 echo v echo $v v 23

Assignment operator = • Rule: • No space permitted on either side of = sign.

Variable Rule • Assign a value to name of a variable • Read variable value through reference of the variable • Example: • Name of the variable: num • Value of the variable $num num 23

Task 1 • Create two variables: a, b • Assign a with value 100 • Assign b with value 200 • Disply the values of a and b • Reassign b with the value of a • Display the values of a and b

Script for Task 1 #!/bin/bash a=100 b=200 echo $a echo $b b=$a echo $a echo $b

Script for Task 1 #!/bin/bash a=100 b=200 echo $a echo $b b=$a echo $a echo $b 100 200 100 100

Number Operations • All computations in a shell script are done with integers. • A number operation must be placed within the following syntax: $[expression] • For example a=$[2+3] echo $a (5 will be displayed)

Number Operations v1=100 echo $v1 (100) v2=$v1/2 echo $v2 (?)

Number Operation v1=100 echo $v1 (100) v2=$v1/2 echo $v2 (100/2)

Number Operation v1=100 echo $v1 (100) v2=$[$v1/2] echo $v2 (50)

Arithmetic Operators • + plus • - minus • * multiplication • / division • ** exponentiation • % modulo

Task 2 • Create two variables a and b, and assign each with a value. • Calculate value a plus value b and assign it to variable c • Calculate value a minus value b and assign it to variable d • Calculate value c multiply value d and assign it to variable e • Calculate value e divided by 2 and assign it to variable f • Display value of f

Script for Task 2 #!/bin/bash a=5 b=3 c=$[$a+$b] d=$[$a-$b] e=$[$c*$d] f=$[$e/2] echo $f

Script for Task 2 #!/bin/bash a=5 b=3 c=$[$a+$b] d=$[$a-$b] e=$[$c*$d] f=$[$e/2] echo $f 8

Input • Use readcommand read variable_name

Input Example #!/bin/bash echo “Enter your name:” read name echo “Enter your age:” read age echo “The age of $name is $age”

Integer Comparison • Test if two integer variables are equal or not if [ $a -eq $b ] • -Note: must have a space before and after brackets and operators

Integer Comparison • -eq is equal to • -ne is not equal to • -gt is greater than • -ge is greater than or equal to • -lt is less than • -le is less than or equal to

If Statement if [ condition ] then command(s) • elif[ condition ] • then • command(s) • else command(s) fi if [ condition ] then command(s) else command(s) fi You can have more elif [] then statements

Task 3 • Ask user to enter two integer numbers and display which number is greater

Script for Task 3 #!/bin/bash echo “Enter number 1:” read a echo “Enter number 2:” read b if [ $a -ge $b ] then echo “$a is greater than or equal to $b” else echo “$a is less than $b” fi

String Comparison • == is equal to • if [ “$a” == “$b” ] • != is not equal to • if [ “$a” != “$b” ] • Note: • Must have a space before and after [ and ]. • Must have spaces before and after operators == and != • Must use double quote for string variables • You can also use double quote for number variables. But, it is not required.

Task 4 • Ask user to enter two strings and display whether they are equal or not

Script for Task 4 #!/bin/bash echo “Enter string 1:” read a echo “Enter string 2:” read b if [ “$a” == “$b” ] then echo "$a is same as $b" else echo "$a is different from $b" fi

Logical Operators • &&AND • || OR • Example: #!/bin/bash a=100 b=200 if ([ $a -gt 0 ] && [ $b -gt 0]) then echo "both $a and $b are positive" fi

While Loop while [ condition ]do command(s)... done • Note: keep working in loop as long as condition is true (terminate if condition is false)

Task 5 • Ask the user to continually enter an integer number and show the summation of all the numbers the user has entered so far. • The program terminates when user enters word “end” • No input error checking is required

Scripts for Task 5 #!/bin/bash sum=0 echo "Enter a number(end to exit)" read n while [ "$n" != "end" ] do sum=$[$sum+$n] echo “The sum of the numbers you entered so far is $sum" echo "Enter a number(end to exit)" read n done

Until Loop until [ condition ]do command(s)... done • Keep in loop as long as condition is false (terminate when condition is true) • Condition is opposite to while loop

Scripts for Task 5 (using until loop) #!/bin/bash sum=0 echo "Enter a number(end to exit)" read n until [ "$n" == "end" ] do sum=$[$sum+$n] echo “The sum of the numbers you entered so far is $sum" echo "Enter a number(end to exit)" read n done

A Note about Conditions in Brackets • Always keep spaces between the brackets and the actual check/comparison • Always keep spaces between comparison operators and variable (data) if [$a -lt 0 ] while [ $a –gt $b ] until [ “$a” == “$b” ]

For Loop • for loop is used to extract items from a list forarg in $list_variabledo command(s)... done

For Loop Example #!/bin/bash fruits=“apples oranges pears bananas” for fruit in $fruits #fruits is considered as a list (items are separated by spaces) do echo $fruit # Each fruit on a separate line. done • Result • apples • oranges • pears • bananas

For Loop Example #!/bin/bash fruits=“apples oranges pears bananas” for fruit in “$fruits” #fruits is considered as a single item do echo $fruit done Result: apples oranges pears bananas

When to use double quote on string variable? • Should use double quote: • Single value string (no need to separate words) • Should not use double quote: • String is a list of word and we need to separate words from the list

Command Substitution • Assigns the output of a command to a variable variable_name=$(command) • Example #!/bin/bash cw=$(pwd) echo $cw

Redirect Output to Files • We can redirect the output of Shell Script from Screen to a File echo …….. >> file_name

C151 Multi-User Operating Systems