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Chapter 3

Introduction

Making decision is important part in ONCE life as well as in computers logical driven program. In fact logic is not LOGIC until you use decision making. This chapter introduces to the bashs structured language constricts such as:

Is there any difference making decision in Real life and with Computers? Well real life decision are quit complicated to all of us and computers even don't have that much power to understand our real life decisions. What computer know is 0 (zero) and 1 that is Yes or No. To make this idea clear, lets play some game (WOW!) with bc - Linux calculator program.
$ bc
After this command bc is started and waiting for your commands, i.e. give it some calculation as follows type 5 + 2 as:
5 + 2
7
7 is response of bc i.e. addition of 5 + 2 you can even try
5 - 2
5 / 2
See what happened if you type 5 > 2 as follows
5 > 2
1
1 (One?) is response of bc, How? bc compare 5 with 2 as, Is 5 is greater then 2, (If I ask same question to you, your answer will be YES), bc gives this 'YES' answer by showing 1 value. Now try
5 < 2
0
0 (Zero) indicates the false i.e. Is 5 is less than 2?, Your answer will be no which is indicated by bc by showing 0 (Zero). Remember in bc, relational expression always returns true (1) or false (0 - zero).

Try following in bc to clear your Idea and not down bc's response
5 > 12
5 == 10
5 != 2
5 == 5
12 < 2

Expression Meaning to us Your Answer BC's Response
5 > 12      Is 5 greater than 12     NO 0
5 == 10     Is 5 is equal to 10        NO 0
5 != 2 Is 5 is NOT equal to 2   YES 1
5 == 5 Is 5 is equal to 5          YES 1
1 < 2 Is 1 is less than 2 Yes 1

It means when ever there is any type of comparison in Linux Shell It gives only two answer one is YES and NO is other.

In Linux Shell Value  Meaning Example
Zero Value (0) Yes/True  0
NON-ZERO Value No/False -1, 32, 55 anything but not zero

Remember both bc and Linux Shell uses different ways to show True/False values

Value Shown in bc as Shown in Linux Shell as
True/Yes 1 0
False/No 0 Non - zero value

 

if condition

if condition which is used for decision making in shell script, If given condition is true then command1 is executed.
Syntax:

        if condition
        then
                command1 if condition is true or if exit status
                of condition is 0 (zero)
                ...
                ...
        fi

Condition is defined as:
" Condition is nothing but comparison between two values."

For compression you can use test or [ expr ] statements or even exist status can be also used.

Expreession is defined as:
" An expression is nothing but combination of values, relational operator (such as >,<, <> etc) and mathematical operators (such as +, -, / etc )."

Following are all examples of expression:
5 > 2
3 + 6
3 * 65
a < b
c > 5
c > 5 + 30 -1

Type following commands (assumes you have file called foo)
$ cat foo
$ echo $?

The cat command return zero(0) i.e. exit status, on successful, this can be used, in if condition as follows, Write shell script as

$ cat > showfile
#!/bin/sh
#
#Script to print file
#
if cat $1
then
echo -e "\n\nFile $1, found and successfully echoed"
fi

Run above script as:
$ chmod 755 showfile
$./showfile foo
Shell script name is showfile ($0) and foo is argument (which is $1).Then shell compare it as follows:
if cat $1 which is expanded to if cat foo.

Detailed explanation
if cat command finds foo file and if its successfully shown on screen, it means our cat command is successful and its exist status is 0 (indicates success), So our if condition is also true and hence statement echo -e "\n\nFile $1, found and successfully echoed" is proceed by shell. Now if cat command is not successful then it returns non-zero value (indicates some sort of failure) and this statement echo -e "\n\nFile $1, found and successfully echoed" is skipped by our shell.

Exercise
Write shell script as follows:

cat > trmif
#
# Script to test rm command and exist status
#
if rm $1
then
echo "$1 file deleted"
fi

Press Ctrl + d to save
$ chmod 755 trmif

Answer the following question in referance to above script:
(A) foo file exists on your disk and you give command, $ ./trmfi foo what will be output?
(B) If bar file not present on your disk and you give command, $ ./trmfi bar what will be output?
(C) And if you type $ ./trmfi What will be output?

For Answer click here.

 

test command or [ expr ]

test command or [ expr ] is used to see if an expression is true, and if it is true it return zero(0), otherwise returns nonzero for false.
Syntax:
test expression OR [ expression ]

Example:
Following script determine whether given argument number is positive.

$ cat > ispostive
#!/bin/sh
#
# Script to see whether argument is positive
#
if test $1 -gt 0
then
echo "$1 number is positive"
fi

Run it as follows
$ chmod 755 ispostive

$ ispostive 5
5 number is positive

$ispostive -45
Nothing is printed

$ispostive
./ispostive: test: -gt: unary operator expected

Detailed explanation
The line, if test $1 -gt 0 , test to see if first command line argument($1) is greater than 0. If it is true(0) then test will return 0 and output will printed as 5 number is positive but for -45 argument there is no output because our condition is not true(0) (no -45 is not greater than 0) hence echo statement is skipped. And for last statement we have not supplied any argument hence error ./ispostive: test: -gt: unary operator expected, is generated by shell , to avoid such error we can test whether command line argument is supplied or not.

test or [ expr ] works with
1.Integer ( Number without decimal point)
2.File types
3.Character strings

For Mathematics, use following operator in Shell Script

Mathematical Operator in  Shell Script  Meaning Normal Arithmetical/ Mathematical Statements But in Shell
      For test statement with if command For [ expr ] statement with if command
-eq is equal to 5 == 6 if test 5 -eq 6 if [ 5 -eq 6 ]
-ne is not equal to 5 != 6 if test 5 -ne 6 if [ 5 -ne 6 ]
-lt is less than 5 < 6 if test 5 -lt 6 if [ 5 -lt 6 ]
-le is less than or equal to 5 <= 6 if test 5 -le 6 if [ 5 -le 6 ]
-gt is greater than 5 > 6 if test 5 -gt 6 if [ 5 -gt 6 ]
-ge is greater than or equal to 5 >= 6 if test 5 -ge 6 if [ 5 -ge 6 ]

NOTE: == is equal, != is not equal.

For string Comparisons use

Operator Meaning
string1 = string2 string1 is equal to string2
string1 != string2 string1 is NOT equal to string2
string1 string1 is NOT NULL or not defined 
-n string1 string1 is NOT NULL and does exist
-z string1 string1 is NULL and does exist

Shell also test for file and directory types

Test Meaning
-s file    Non empty file
-f file    Is File exist or normal file and not a directory 
-d dir     Is Directory exist and not a file
-w file   Is writeable file
-r file    Is read-only file
-x file    Is file is executable

Logical Operators

Logical operators are used to combine two or more condition at a time

Operator            Meaning
! expression Logical NOT
expression1  -a  expression2 Logical AND
expression1  -o  expression2 Logical OR

 

if...else...fi

If given condition is true then command1 is executed otherwise command2 is executed.
Syntax:

           if condition
           then
                       condition is zero (true - 0)
                       execute all commands up to else statement

           else
                       if condition is not true then
                       execute all commands up to fi
           fi

For e.g. Write Script as follows:

$ vi isnump_n
#!/bin/sh
#
# Script to see whether argument is positive or negative
#
if [ $# -eq 0 ]
then
echo "$0 : You must give/supply one integers"
exit 1
fi


if test $1 -gt 0
then
echo "$1 number is positive"
else
echo "$1 number is negative"
fi

Try it as follows:
$ chmod 755 isnump_n

$ isnump_n 5

5 number is positive

$ isnump_n -45

-45 number is negative

$ isnump_n

./ispos_n : You must give/supply one integers

$ isnump_n 0

0 number is negative

Detailed explanation
First script checks whether command line argument is given or not, if not given then it print error message as " ./ispos_n : You must give/supply one integers". if statement checks whether number of argument ($#) passed to script is not equal (-eq) to 0, if we passed any argument to script then this if statement is false and if no command line argument is given then this if statement is true. The echo command i.e.
echo "$0 : You must give/supply one integers"
         |              |
         |              |
        1             2
1 will print Name of script
2 will print this error message
And finally statement exit 1 causes normal program termination with exit status 1 (nonzero means script is not successfully run).

The last sample run $ isnump_n 0 , gives output as " 0 number is negative", because given argument is not > 0, hence condition is false and it's taken as negative number. To avoid this replace second if statement with if test $1 -ge 0.

Nested if-else-fi

You can write the entire if-else construct within either the body of the if statement of the body of an else statement. This is called the nesting of ifs.

$ vi nestedif.sh
osch=0

echo "1. Unix (Sun Os)"
echo "2. Linux (Red Hat)"
echo -n "Select your os choice [1 or 2]? "
read osch

if [ $osch -eq 1 ] ; then

     echo "You Pick up Unix (Sun Os)"

else #### nested if i.e. if within if ######
            
       if [ $osch -eq 2 ] ; then
             echo "You Pick up Linux (Red Hat)"
       else
             echo "What you don't like Unix/Linux OS."
       fi
fi

Run the above shell script as follows:
$ chmod +x nestedif.sh
$ ./nestedif.sh

1. Unix (Sun Os)
2. Linux (Red Hat)
Select you os choice [1 or 2]? 1
You Pick up Unix (Sun Os)

$ ./nestedif.sh
1. Unix (Sun Os)
2. Linux (Red Hat)
Select you os choice [1 or 2]? 2
You Pick up Linux (Red Hat)

$ ./nestedif.sh
1. Unix (Sun Os)
2. Linux (Red Hat)
Select you os choice [1 or 2]? 3
What you don't like Unix/Linux OS.

Note that Second if-else constuct is nested in the first else statement. If the condition in the first if statement is false the the condition in the second if statement is checked. If it is false as well the final else statement is executed.

You can use the nested ifs as follows also:
Syntax:

        if condition
        then
                if condition
                then
                        .....
                        ..
                        do this
                else
                        ....
                        ..
                        do this
                fi
        else
                ...
                .....
                do this
        fi

 

Multilevel if-then-else

Syntax:

           if condition
           then
                       condition is zero (true - 0)
                       execute all commands up to elif statement
           elif condition1 

then condition1 is zero (true - 0) execute all commands up to elif statement elif condition2 then condition2 is zero (true - 0) execute all commands up to elif statement else None of the above condtion,condtion1,condtion2 are true (i.e. all of the above nonzero or false) execute all commands up to fi fi

For multilevel if-then-else statement try the following script:

$ cat > elf
#
#!/bin/sh
# Script to test if..elif...else
#
if [ $1 -gt 0 ]; then
  echo "$1 is positive"
elif [ $1 -lt 0 ]
then
  echo "$1 is negative"
elif [ $1 -eq 0 ]
then
  echo "$1 is zero"
else
  echo "Opps! $1 is not number, give number"
fi

Try above script as follows:
$ chmod 755 elf
$ ./elf 1
$ ./elf -2
$ ./elf 0
$ ./elf a
Here o/p for last sample run:
./elf: [: -gt: unary operator expected
./elf: [: -lt: unary operator expected
./elf: [: -eq: unary operator expected
Opps! a is not number, give number
Above program gives error for last run, here integer comparison is expected therefore error like " ./elf: [: -gt: unary operator expected" occurs, but still our program notify this error to user by providing message " Opps! a is not number, give number".

 

Loops in Shell Scripts

Loop defined as:
" Computer can repeat particular instruction again and again, until particular condition satisfies. A group of instruction that is executed repeatedly is called a loop."

Bash supports:

Note that in each and every loop,

(a) First, the variable used in loop condition must be initialized, then execution of the loop begins.

(b) A test (condition) is made at the beginning of each iteration.

(c) The body of loop ends with a statement that modifies the value of the test (condition) variable.

 

for Loop

Syntax:

            for { variable name } in { list }
            do
                     execute one for each item in the list until the list is
                     not finished (And repeat all statement between do and done)
            done

Before try to understand above syntax try the following script:

$ cat > testfor
for i in 1 2 3 4 5
do
echo "Welcome $i times"
done

Run it above script as follows:
$ chmod +x testfor
$ ./testfor

The for loop first creates i variable and assigned a number to i from the list of number from 1 to 5, The shell execute echo statement for each assignment of i. (This is usually know as iteration) This process will continue until all the items in the list were not finished, because of this it will repeat 5 echo statements. To make you idea more clear try following script:

$ cat > mtable
#!/bin/sh
#
#Script to test for loop
#
#
if [ $# -eq 0 ]
then
echo "Error - Number missing form command line argument"
echo "Syntax : $0 number"
echo "Use to print multiplication table for given number"
exit 1
fi
n=$1
for i in 1 2 3 4 5 6 7 8 9 10
do
echo "$n * $i = `expr $i \* $n`"
done

Save above script and run it as:
$ chmod 755 mtable
$ ./mtable 7
$ ./mtable
For first run, above script print multiplication table of given number where i = 1,2 ... 10 is multiply by given n (here command line argument 7) in order to produce multiplication table as
7 * 1 = 7
7 * 2 = 14
...
..
7 * 10 = 70
And for second test run, it will print message -
Error - Number missing form command line argument
Syntax : ./mtable number
Use to print multiplication table for given number

This happened because we have not supplied given number for which we want multiplication table, Hence script is showing Error message, Syntax and usage of our script. This is good idea if our program takes some argument, let the user know what is use of the script and how to used the script.
Note that to terminate our script we used 'exit 1' command which takes 1 as argument (1 indicates error and therefore script is terminated)

Even you can use following syntax:

Syntax:

         for (( expr1; expr2; expr3 ))

do
..... ... repeat all statements between do and done until expr2 is TRUE Done

In above syntax BEFORE the first iteration, expr1 is evaluated. This is usually used to initialize variables for the loop. 
All the statements between do and done is executed repeatedly UNTIL the value of expr2 is TRUE.
AFTER each iteration of the loop, expr3 is evaluated. This is usually use to increment a loop counter.

$ cat > for2
for ((  i = 0 ;  i <= 5;  i++  ))
do
  echo "Welcome $i times"
done

Run the above script as follows:
$ chmod +x for2
$ ./for2

Welcome 0 times
Welcome 1 times
Welcome 2 times
Welcome 3 times
Welcome 4 times
Welcome 5 times

In above example, first expression (i = 0), is used to set the value variable i to zero.
Second expression is condition i.e. all statements between do and done executed as long as expression 2 (i.e continue as long as the value of variable i is less than or equel to 5) is TRUE.
Last expression i++ increments the value of i by 1 i.e. it's equivalent to i = i + 1 statement.

Nesting of for Loop

As you see the if statement can nested, similarly loop statement can be nested. You can nest the for loop. To understand the nesting of for loop see the following shell script.

$ vi nestedfor.sh
for (( i = 1; i <= 5; i++ ))      ### Outer for loop ###
do

    for (( j = 1 ; j <= 5; j++ )) ### Inner for loop ###
    do
          echo -n "$i "
    done

  echo "" #### print the new line ###

done

Run the above script as follows:
$ chmod +x nestedfor.sh
$ ./nestefor.sh

1 1 1 1 1
2 2 2 2 2
3 3 3 3 3
4 4 4 4 4
5 5 5 5 5

Here, for each value of i the inner loop is cycled through 5 times, with the varible j taking values from 1 to 5. The inner for loop terminates when the value of j exceeds 5, and the outer loop terminets when the value of i exceeds 5.

Following script is quite intresting, it prints the chess board on screen.

$ vi chessboard
for (( i = 1; i <= 9; i++ )) ### Outer for loop ###
do
   for (( j = 1 ; j <= 9; j++ )) ### Inner for loop ###
   do
        tot=`expr $i + $j`
        tmp=`expr $tot % 2`
        if [ $tmp -eq 0 ]; then
            echo -e -n "\033[47m "
        else
            echo -e -n "\033[40m "
        fi
  done
 echo -e -n "\033[40m" #### set back background colour to black
 echo "" #### print the new line ###
done

Run the above script as follows:
$ chmod +x chessboard
$ ./chessboard

On my terminal above script produec the output as follows:

chessboard shell script output i.e. chess board

Above shell script cab be explained as follows:

Command(s)/Statements
Explanation
for (( i = 1; i <= 9; i++ ))
do
Begin the outer loop which runs 9 times., and the outer loop terminets when the value of i exceeds 9
for (( j = 1 ; j <= 9; j++ ))
do

Begins the inner loop, for each value of i the inner loop is cycled through 9 times, with the varible j taking values from 1 to 9. The inner for loop terminates when the value of j exceeds 9.

tot=`expr $i + $j`
tmp=`expr $tot % 2`
See for even and odd number positions using these statements.

if [ $tmp -eq 0 ]; then
    echo -e -n "\033[47m "
else
    echo -e -n "\033[40m "
fi

If even number posiotion print the white colour block (using echo -e -n "\033[47m " statement); otherwise for odd postion print the black colour box (using echo -e -n "\033[40m " statement). This statements are responsible to print entier chess board on screen with alternet colours.

done

End of inner loop

echo -e -n "\033[40m"

Make sure its black background as we always have on our terminals.
echo "" Print the blank line
done End of outer loop and shell scripts get terminted by printing the chess board.

Exercise
Try to understand the shell scripts (for loops) shown in exercise chapter.

 

while loop

Syntax:

           while [ condition ]
           do
                 command1
                 command2
                 command3
                 ..
                 ....
            done

Loop is executed as long as given condition is true. For e.g.. Above for loop program (shown in last section of for loop) can be written using while loop as:

$cat > nt1
#!/bin/sh
#
#Script to test while statement
#
#
if [ $# -eq 0 ]
then
   echo "Error - Number missing form command line argument"
   echo "Syntax : $0 number"
   echo " Use to print multiplication table for given number"
exit 1
fi
n=$1
i=1
while [ $i -le 10 ]
do
  echo "$n * $i = `expr $i \* $n`"
  i=`expr $i + 1`
done

Save it and try as
$ chmod 755 nt1
$./nt1 7
Above loop can be explained as follows:

 

n=$1 Set the value of command line argument to variable n. (Here it's set to 7 )
i=1 Set variable i to 1
while [ $i -le 10 ]                   This is our loop condition, here if value of i is less than 10 then, shell execute all statements between do and done
do  Start loop
echo "$n  *  $i = `expr $i  \*  $n`" Print multiplication table as
7 * 1 = 7
7 * 2 = 14
....
7 * 10 = 70, Here each time value of variable n is multiply be i.
i=`expr $i + 1` Increment i by 1 and store result to i.  ( i.e. i=i+1)
Caution: If you ignore (remove) this statement  than our loop become infinite loop because value of variable i always remain less than 10 and program will only output
7 * 1 = 7
...
...
E (infinite times)
done                                   

Loop stops here if i is not less than 10 i.e. condition of loop is not true. Hence
loop is terminated.

 

The case Statement

The case statement is good alternative to Multilevel if-then-else-fi statement. It enable you to match several values against one variable. Its easier to read and write.
Syntax:

           case  $variable-name  in
                pattern1)   command
                                ...
                                ..
                                command;;
                pattern2)   command
                                ...
                                ..
                                command;;
                patternN)   command
                                ...
                                ..
                                command;;
                *)             command
                                ...
                                ..
                                command;;
           esac

The $variable-name is compared against the patterns until a match is found. The shell then executes all the statements up to the two semicolons that are next to each other. The default is *) and its executed if no match is found. For e.g. write script as follows:

$ cat > car
#
# if no vehicle name is given
# i.e. -z $1 is defined and it is NULL
#
# if no command line arg

if [ -z $1 ]
then
  rental="*** Unknown vehicle ***"
elif [ -n $1 ]
then
# otherwise make first arg as rental
  rental=$1
fi

case $rental in
   "car") echo "For $rental Rs.20 per k/m";;
   "van") echo "For $rental Rs.10 per k/m";;
   "jeep") echo "For $rental Rs.5 per k/m";;
   "bicycle") echo "For $rental 20 paisa per k/m";;
   *) echo "Sorry, I can not gat a $rental for you";;
esac

Save it by pressing CTRL+D and run it as follows:
$ chmod +x car
$ car van
$ car car
$ car Maruti-800

First script will check, that if $1(first command line argument) is given or not, if NOT given set value of rental variable to "*** Unknown vehicle ***",if command line arg is supplied/given set value of rental variable to given value (command line arg). The $rental is compared against the patterns until a match is found.
For first test run its match with van and it will show output " For van Rs.10 per k/m."
For second test run it print, " For car Rs.20 per k/m".
And for last run, there is no match for Maruti-800, hence default i.e. *) is executed and it prints, " Sorry, I can not gat a Maruti-800 for you".
Note that esac is always required to indicate end of case statement.

See the one more example of case statement in chapter 4 of section shift command.

 

How to de-bug the shell script?

While programming shell sometimes you need to find the errors (bugs) in shell script and correct the errors (remove errors - debug). For this purpose you can use -v and -x option with sh or bash command to debug the shell script. General syntax is as follows:
Syntax:
sh   option   { shell-script-name }
OR
bash   option   { shell-script-name }
Option can be
-v Print shell input lines as they are read.
-x After expanding each simple-command, bash displays the expanded value of PS4 system variable, followed by the command and its expanded arguments.

Example:

$ cat > dsh3.sh
#
# Script to show debug of shell
#
tot=`expr $1 + $2`
echo $tot

Press ctrl + d to save, and run it as
$ chmod 755 dsh3.sh
$ ./dsh3.sh 4 5

9
$ sh -x dsh3.sh 4 5
#
# Script to show debug of shell
#
tot=`expr $1 + $2`
expr $1 + $2
++ expr 4 + 5
+ tot=9
echo $tot
+ echo 9
9

See the above output, -x shows the exact values of variables (or statements are shown on screen with values).

$ sh -v dsh3.sh 4 5

Use -v option to debug complex shell script.