# Decision Making

> Source: https://learn-bash.net/decision-making/
> Part of Learn Bash, free to read.

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**

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

It can be expanded with ‘else’

```bash
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)

```bash
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

```bash
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

```bash
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

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

### case structure

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

### simple case bash structure

Note in this case $case is variable and does not have to be named case - this is just an example

```bash
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
```