Expect Script Tutorial: Expressions, If Conditions, For Loop, and While Loop Examples

Expect scripting language is easy to learn. It expects specific string, and sends (or responds) strings accordingly.

If you are new to expect, read our 6 expect script examples (including hello world example) to get a jump start.

This article explains the following in the expect scripting language.

• Expressions – arithmetic operation
• if construct in expect
• looping constructs

Define Expect Variables – set command

In expect, you can set the variable values using set command as shown below,

# To set the variable with numeric values
set var1 10

# To set the variable with string literal
set name "john"

# To set the variable value which includes output of another command
set proc_id "process id : [pid]"

Note: For expect command line arguments, read 6 Expect Script Command Line Argument Examples.

Expect Expressions – expr command

To evaluate the expressions, use the expr command, which executes the given expression and returns the result. Expect expressions are similar to the C expressions. Some of the valid expressions are listed below.

# To add two simple numerical values
set sum "[expr 1 + 1]"

# To multiple the value of variables
set mul "[expr $sum * $sum]"

# To evaluate conditions and returns 1 or 0 to indicate success or failure accordingly.
set ret "[expr (1+1) == 2]"

# Conditions may contain the command return values.
set ret [expr [pid] == 0]

Expect Conditional Expressions – If command

If command is used for conditional flow of execution of statements as shown in the example below.

if { $count < 0} {
    puts "True : $count\n";
} else {
    puts "False : $count\n";
}

Just like any other programming language, you can use the elseif command in expect as shown below.

if { $count < 0} {
    puts "Success Condition1 : $count\n";
} elseif { $count == 0 } {
    puts "Success Condition2 : $count\n";
} else {
    puts "False : $count\n";
}

Expect Looping Constructs

Expect For Loop Examples:

As we know, for loop is used to do repeated execution of expression until certain condition.

General for loop construct :

for {initialization} {conditions} {incrementation or decrementation} {
...
}

Expect for loop example :

for {set i 1} {$i < $no} {incr i 1} {
 set $total [expr $total * $i ]
}
puts "$total";

Note: You should place the loop open brace in the same line as it contains “for” keyword.

Expect While Loop Examples:

set count 5;
while {$count > 0 } {
puts "count : $count\n";
set count [expr $count-1];
}