Decision making

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 nothing but comparison between two values.
For compression you can use test or [ expr ] statements or even exist status can be also used.
An expression is nothing but combination of values, relational operator (such as etc) and mathematical operators (such as etc ). http://siber.cankaya.edu.tr/SystemsProgramming/cfiles/showfileshowfile
```
#!/bin/sh
#
#Script to print file
#
if cat $1
then
        echo -e "\n\nFile $1, found and successfully echoed"
fi
```

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 ]
```
Following script determine whether given argument number is positive. http://siber.cankaya.edu.tr/SystemsProgramming/cfiles/ispositiveispositive
```
#!/bin/sh
#
# Script to see whether argument is positive
#
if test $1 -gt 0
then
        echo "$1 number is positive"
fi
```

test or [ expr ] works with

Integer ( Number without decimal point)
File types
Character strings

**Table 3.2:** Operator in Shell Script
Mathematical	Operator in Shell
-eq	is equal to ; if test 5 -eq 6 ; if [ 5 -eq 6 ]
-ne	is not equal to ; if test 5 -ne 6; if [ 5 -ne 6 ]
-lt	is less than ; if test 5 -lt 6; if [ 5 -lt 6 ]
-le	is less than or equal to ; if test 5 -le 6; if [ 5 -le 6 ]
-gt	is greater than ; if test 5 -gt 6; if [ 5 -gt 6 ]
-ge	is greater than or equal to ; if test 5 -ge 6; if [ 5 -ge 6 ]
String Comparisons	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
File and directory	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	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

Script to see whether argument is positive or negative; http://siber.cankaya.edu.tr/SystemsProgramming/cfiles/isnumpnisnumpn

#!/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

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. Syntax:

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

http://siber.cankaya.edu.tr/SystemsProgramming/cfiles/nestedifnestedif

#!/bin/sh
#
# Script to see nesting of ifs
#
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

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 condition,condition1,condition2 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: http://siber.cankaya.edu.tr/SystemsProgramming/cfiles/elfelf

#!/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

Integer comparison is expected.