Unix – Decision Making

While writing a shell script, there may be a situation when you need to adopt one path out of the given two paths. So you need to make use of conditional statements that allow your program to make correct decisions and perform right actions.

Unix Shell supports conditional statements which are used to perform different actions based on different conditions. Here we will explain following two decision making statements:

• The if...else statements
• The case...esac statement

The if...else statements:

If else statements are useful decision making statements which can be used to select an option from a given set of options.

Unix Shell supports following forms of if..else statement:

if...fi statement

The if...fi statement is the fundamental control statement that allows Shell to make decisions and execute statements conditionally.

Syntax:


Here Shell expression is evaluated. If the resulting value is true, given statement(s) are executed. If expression is false then no statement would be not executed. Most of the times you will use comparison operators while making decisions.

Give you attention on the spaces between braces and expression. This space is mandatory otherwise you would get syntax error.

If expression is a shell command then it would be assumed true if it return 0 after its execution. If it is a boolean expression then it would be true if it returns true.

Example:


This will produce following result:


if...else...fi statement

The if...else...fi statement is the next form of control statement that allows Shell to execute statements in more controlled way and making decision between two choices.

Syntax:


Here Shell expression is evaluated. If the resulting value is true, given statement(s) are executed. If expression is false then no statement would be not executed.

Example:

If we take above example then it can be written in better way using if...else statement as follows:


This will produce following result:


if...elif...else...fi statement

The if...elif...fi statement is the one level advance form of control statement that allows Shell to make correct decision out of several conditions.

Syntax:


There is nothing special about this code. It is just a series of if statements, where each if is part of theelse clause of the previous statement. Here statement(s) are executed based on the true condition, if non of the condition is true then else block is executed.

Example:


This will produce following result:


Most of the if statements check relations using relational operators discussed in previous chapter.

The case...esac Statement:

You can use multiple if...elif statements to perform a multiway branch. However, this is not always the best solution, especially when all of the branches depend on the value of a single variable.

Unix Shell supports case...esac statement which handles exactly this situation, and it does so more efficiently than repeated if...elif statements.

There is only one form of case...esac statement which is detailed here:

case...esac statement

You can use multiple if...elif statements to perform a multiway branch. However, this is not always the best solution, especially when all of the branches depend on the value of a single variable.

Shell support case...esac statement which handles exactly this situation, and it does so more efficiently than repeated if...elif statements.

Syntax:

The basic syntax of the case...esac statement is to give an expression to evaluate and several different statements to execute based on the value of the expression.

The interpreter checks each case against the value of the expression until a match is found. If nothing matches, a default condition will be used.


Here the string word is compared against every pattern until a match is found. The statement(s) following the matching pattern executes. If no matches are found, the case statement exits without performing any action.

There is no maximum number of patterns, but the minimum is one.

When statement(s) part executes, the command ;; indicates that program flow should jump to the end of the entire case statement. This is similar to break in the C programming language.

Example:


This will produce following result:


A good use for a case statement is the evaluation of command line arguments as follows:


Here is a sample run of this program:


Unix Shell's case...esac is very similar to switch...case statement we have in other programming languages like C or C++ and PERL etc.

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