Osp2.pdf

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 isexecuted.
Syntax:
if
conditi
onthen
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
oneintegers" exit 1
fi
if test $1
-gt 0then
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
1 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
2 will print Name of script
3 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).

$ vi nestedif.sh
osch=0
echo "1. Unix (Sun Os)"
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
statementof the body of an else statement. This is called the nesting of ifs.
echo "2. Linux (Red Hat)"
echo -n "Select your os choice [1 or
2]? "read osch

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)
2]? 2You Pick up Linux
(Red Hat)
$ ./nestedif.sh
1. Unix (Sun Os)
2. Linux (Red Hat)
2]? 3
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

What you don't like Unix/Linux OS.
Note that Second if-else constuct is nested in the first else statement. If
thecondition 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
elsestatement isexecuted.
You can use the nested ifs as follows also:
Syntax:
if
conditi
onthen
if
conditi
onthen
.....
else
..
do this
....
else
fi
...
.....
do this
do this
f
Multilevel if-then-else
Syntax:
if
conditi
onthen
condition is zero (true - 0)
execute all commands up to elif
statementelif condition1
then

condition1 is zero (true - 0)
execute all commands up to elif
statementelif condition2
then
else
condition2 is zero (true - 0)
execute all commands up to elif statement
None of the above condtion,condtion1,condtion2 are true (i.e.
all of the above nonzero or false)
execute all commands up to fi
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 aloop."
Bash supports:
 for loop
 while loop
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

$ cat > mtable
#!/bin/sh
#
#Script to test for loop
#
#
if [ $# -eq 0 ]
then
echo "Error - Number missing form command line argument"
$ cat > testfor
for i in 1 2 3 4 5
Before try to understand above syntax try the following script:
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
makeyou idea more clear try following script:
echo "Syntax : $0 number"
echo "Use to print multiplication table for given
number"exit 1
f
i
n
=
$
1

for i in 1 2 3 4 5 6 7 8
9 10do
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
producemultiplication 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
argumentSyntax : ./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 userknow what use of the script is and how to use the
script.
Note that to terminate our script we used 'exit 1' command which
takes 1as 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
anddone until expr2 is TRUE
Done
In above syntax BEFORE the first iteration, expr1 is evaluated.
This isusually used to initialize variables for the loop.
All the statements between do and done is executed repeatedly UNTIL the
valueof expr2 is TRUE.
AFTER each iteration of the loop, expr3 is evaluated. This is usually
use toincrement 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 tozero.
Second expression is condition i.e. all statements between do and done
executedas long as expression 2 (i.e continue as long as the value of variable
i is less thanor equel to 5) is TRUE.
Last expression i++ increments the value of i by 1 i.e. it's equivalent to i = i
+ 1statement.

$ vi nestedfor.sh
for (( i = 1; i <= 5; i++ ))### Outer for loop ###
do
for (( j = 1 ; j <= 5; j++ )) ### Inner for loop ###
do
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
followingshell script.
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.
while loop
Syntax:
while [
condition ]do
command3
command1
command2
..
....
done

$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"
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:
echo " Use to print multiplication table for given
number"exit 1
f
i
n
=
$
1
i
=
1
while [ $i -le 10 ]
do
echo "$n * $i = `expr $i *
$n`"i=`expr $i + 1`
done
The case Statement
The case statement is good alternative to multilevel if-then-else-fi
statement. Itenable you to match several values against one variable. It’s
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.
Theshell 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. writescript 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

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