Syntax: if expr1 ?then? body1 elseif expr2 ?then? body2 elseif ... ?else? ?bodyN?

Tests a chain of one or more expressions, and executes the matching body, which must be a script. Returns the result of the last command executed in the selected body.

Both the then and else keywords are optional. The standard TCL convention is to always omit the then keywords and to always include the else keyword when there's an else clause.

if {$x > 0} {
    puts "positive"
}

if {$x < 0} {
    puts "negative"
} else {
    puts "non-negative"
}

if {$x > 0} {
    puts "positive"
} elseif {$x < 0} {
    puts "negative"
} else {
    puts "zero"
}

set value [if {$x > 0} {
    expr {$x + $y}   
} else {
    expr {$x - $y}   
}]