Flow Control in shell script - if and case statements

Shell script usually consist of sequence of commands that starts at the first line and continues line by line until it reaches the end. Most programs do much more than this. They make decisions and perform different actions depending on conditions.

The shell provides several commands that we can use to control the flow of execution in our program. In this lesson, we will look at the following: 

test

if
case
exit

 
test

The test command is used most often with the if command to perform true/false decisions. The command is unusual in that it has two different syntactic forms: # First form

test expression
# Second form
[ expression ]

The test command works simply. If the given expression is true, test exits with a status of zero; otherwise it exits with a status of 1.

Common numerical test conditions
-gt   Greater than
-lt    Less than
-ge  Greater than or equal to
-le    Less than or equal to
-eq   Equal to
-ne   Not equal to

-a AND
-o OR

Files and strings can also be tested

Expression	Description
-d file	True if file is a directory.
-e file	True if file exists.
-f file	True if file exists and is a regular file.
-L file	True if file is a symbolic link.
-r file	True if file is a file readable by you.
-w file	True if file is a file writable by you.
-x file	True if file is a file executable by you.
file1 -nt file2	True if file1 is newer than (according to modification time) file2
file1 -ot file2	True if file1 is older than file2
-z string	True if string is empty.
-n string	True if string is not empty.
string1 = string2	True if string1 equals string2.
string1 != string2	True if string1 does not equal string2.

if

The first command we will look at is if. The if command is fairly simple on the surface; it makes a decision based on the exit status of a command. The if command's syntax looks like this:

The if statement has the following syntax:

simple if
if [condition ]
then
#commands
fi

if else statement
if [ condition ]
then
#commands
else
#commands
fi

if elif statement
if [ condition ]
then
commands
elif [ condition ]
#commands
else
#commands
fi
multiple conditions are specified with AND ( -a) and OR(-o)

Example: Even or odd ( using simple if)

echo "Enter a number"
read n
r=`expr $n % 2`
if [ $r -eq 0 ]
then
    echo "$n is even"
else
    echo "$n is odd"
fi

Example: Biggest of 2 numbers ( using if elif)
echo "Enter two numbers in single line separate with space"
read a b
if [ $a -gt $b ]
then
    echo "$a is bigger"
elif [ $b -gt $a ]
then
    echo "$b is bigger"
else
    echo "equal"
fi

 
Biggest of 3 numbers using and (-a) operator
echo "enter three numbers.."
read a b c
if [ $a -gt $b -a $a -gt $c ]
then
    echo "$a is big"
elif [ $b -gt $a -a $b -gt $c ]
then
    echo "$b is big"
else
    echo "$c is big"
fi

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. It is easier to read and write. 

Syntax

The syntax is as follows:

case $variable-name in
pattern1)
command1
...
....
commandN
;;
pattern2)
command1
...
....
commandN
;;
patternN)
command1
...
....
commandN
;;
*)
esac
OR
case $variable-name in
pattern1|pattern2|pattern3)
command1
...
....
commandN
;;
pattern4|pattern5|pattern6)
command1
...
....
commandN
;;
pattern7|pattern8|patternN)
command1
...
....
commandN
;;
*)
esac

The case statement allows you to easily check pattern (conditions) and then process a command-line if that condition evaluates to true.

In other words the $variable-name is compared against the patterns until a match is found.

*) acts as default and it is executed if no match is found.

The pattern can include wildcards.

You must include ;; at the end of each command. The shell executes all the statements up to the two semicolons that are next to each other.

The esac is always required to indicate end of case statement.

Example:Read a day number and print the day

echo "Enter the day number.."

read d

case $d in

1) echo "sunday";;

2)echo "monday";;

3)echo "tuesday";;

4)echo "wednesday";;

5)echo "thursday";;

6)echo "friday";;

7)echo "saturday";;

*)echo "Invalid choice..enter 1-7";;

esac

Example:Enter a char and check for vowel

echo "Enter a character.."

read c

case $c in

    a|A|e|E|i|I|o|O|u|U) echo "vowel";;

    *)echo "not a vowel character";;

esac

exit

In order to be good script writers, we must set the exit status when our scripts finish. To do this, use the exit command. The exit command causes the script to terminate immediately and set the exit status to whatever value is given as an argument. 

For example:

exit 0 exits your script and sets the exit status to 0 (success), whereas
exit 1 exits your script and sets the exit status to 1 (failure).