Decision Making

As in popular programming languages, the shell also supports logical decision making.

The basic conditional decision making construct is:

if [ expression ]; then

code if 'expression' is true

fi

NAME="John"
if [ "$NAME" = "John" ]; then
  echo "True - my name is indeed John"
fi

It can be expanded with 'else'

NAME="Bill"
if [ "$NAME" = "John" ]; then
  echo "True - my name is indeed John"
else
  echo "False"
  echo "You must mistaken me for $NAME"
fi

It can be expanded with 'elif' (else-if)

NAME="George"
if [ "$NAME" = "John" ]; then
  echo "John Lennon"
elif [ "$NAME" = "George" ]; then
  echo "George Harrison"
else
  echo "This leaves us with Paul and Ringo"
fi

The expression used by the conditional construct is evaluated to either true or false. The expression can be a single string or variable. A empty string or a string consisting of spaces or an undefined variable name, are evaluated as false. The expression can be a logical combination of comparisons: negation is denoted by !, logical AND (conjunction) is denoted by &&, and logical OR (disjunction) is denoted by ||. Conditional expressions should be surrounded by double brackets [[ ]].

Types of numeric comparisons

comparison    Evaluated to true when
$a -lt $b    $a < $b
$a -gt $b    $a > $b
$a -le $b    $a <= $b
$a -ge $b    $a >= $b
$a -eq $b    $a is equal to $b
$a -ne $b    $a is not equal to $b

Types of string comparisons

comparison    Evaluated to true when
"$a" = "$b"     $a is the same as $b
"$a" == "$b"    $a is the same as $b
"$a" != "$b"    $a is different from $b
-z "$a"         $a is empty

• note1: whitespace around = is required

• note2: use "" around string variables to avoid shell expansion of special characters as *

Logical combinations

if [[ $VAR_A[0] -eq 1 && ($VAR_B = "bee" || $VAR_T = "tee") ]] ; then
    command...
fi

case structure

case "$variable" in
    "$condition1" )
        command...
    ;;
    "$condition2" )
        command...
    ;;
esac

simple case bash structure

mycase=1
case $mycase in
    1) echo "You selected bash";;
    2) echo "You selected perl";;
    3) echo "You selected phyton";;
    4) echo "You selected c++";;
    5) exit
esac

Exercise

Change the variables in the first section, so that each if statement resolves as True.