if-statements.bash

#!/usr/bin/env bash
#
# shellcheck disable=SC2016
#
# The point of this script is to show the difference between the different kinds of if
# and test statements, and how they behave differently when dealing with different kinds
# variables.
#
########################################################################################
####[ Setup ]###########################################################################


a=true
b=false
c="cat"
#d="does_not_exist" <- DO NOT UNCOMMENT THIS LINE

echo "
----------------------------------
| Value of variables:            |
| a=true                         |
| b=false                        |
| c='cat'                        |
| # d is a non-existent variable |
----------------------------------
"


####[ Testing 'If Statements' ]#########################################################


###
### [ Section 1: If Statements ]
###

# True if ==> $a is true
# Produce ==> true
printf 'if "$a"'
if "$a"; then echo ' => true'
else          echo ' => false'
fi

# True if ==> $b is true
# Produce ==> false
printf 'if "$b"'
if "$b"; then echo ' => true'
else          echo ' => false'
fi

# True if ==> DOES NOT WORK...
# Produce ==> DOES NOT WORK ...
#printf 'if "$c"'
#if "$c"; then echo ' => true'
#else          echo ' => false'
#fi

# True if ==> COMMAND NOT FOUND
# Produce ==> COMMAND NOT FOUND
#printf 'if "$d"'
#if "$d"; then echo ' => true'
#else          echo ' => false'
#fi

###
### [ Section 2: If Statements With Brackets ]
###

# True if ==> $a exists
# Produce ==> true
printf 'if [[ $a ]]'
if [[ $a ]]; then echo ' => true'
else              echo ' => false'
fi

# True if ==> $b exists
# Produce ==> true
printf 'if [[ $b ]]'
if [[ $b ]]; then echo ' => true'
else              echo ' => false'
fi

# True if ==> $c exists
# Produce ==> true
printf 'if [[ $c ]]'
if [[ $c ]]; then echo ' => true'
else              echo ' => false'
fi

# True if ==> $d exists
# Produce ==> false
printf 'if [[ $d ]]'
if [[ $d ]]; then echo ' => true'
else              echo ' => false'
fi

###
### [ Section 3: If Statements With Brackets and Equals ]
###

# True if ==> $a is true
# Produce ==> true
printf 'if [[ $a = true ]]'
if [[ $a = true ]]; then echo ' => true'
else                     echo ' => false'
fi

# True if ==> $b is true
# Produce ==> false
printf 'if [[ $b = true ]]'
if [[ $b = true ]]; then echo ' => true'
else                     echo ' => false'
fi

# True if ==> $c is true
# Produce ==> false
printf 'if [[ $c = true ]]'
if [[ $c = true ]]; then echo ' => true'
else                     echo ' => false'
fi

# True if ==> $d is true
# Produce ==> false
printf 'if [[ $d = true ]]'
if [[ $d = true ]]; then echo ' => true'
else                     echo ' => false'
fi

###
### [ Section 4: && Statements ]
###

# True if ==> $a is true
# Produce ==> true
printf '"$a" &&'
"$a" && echo ' => true' || echo ' => false'

# True if ==> $b is true
# Produce ==> false
printf '"$b" &&'
"$b" && echo ' => true' || echo ' => false'

# True if ==> DOES NOT WORK...
# Produce ==> DOES NOT WORK...
#printf '"$c" &&'
#"$c" && echo ' => true' || echo ' => false'

# True if ==> COMMAND NOT FOUND
# Produce ==> COMMAND NOT FOUND
#printf '"$d" &&'
#"$d" && echo ' => true' || echo ' => false'

###
### [ Section 5: && Statements With Brackets ]
###

# True if ==> $a exists
# Produce ==> true
printf '[[ $a ]] &&'
[[ $a ]] && echo ' => true' || echo ' => false'

# True if ==> $b exists
# Produce ==> true
printf '[[ $b ]] &&'
[[ $b ]] && echo ' => true' || echo ' => false'

# True if ==> $c exists
# Produce ==> true
printf '[[ $c ]] &&'
[[ $c ]] && echo ' => true' || echo ' => false'

# True if ==> $c exists
# Produce ==> false
printf '[[ $d ]] &&'
[[ $d ]] && echo ' => true' || echo ' => false'

###
### [ Section 6: && Statements With Brackets and Equals ]
###

# True if ==> $a is true
# Produce ==> true
printf '[[ $a = true ]] &&'
[[ $a = true ]] && echo ' => true' || echo ' => false'

# True if ==> $b is true
# Produce ==> false
printf '[[ $b = true ]] &&'
[[ $b = true ]] && echo ' => true' || echo ' => false'

# True if ==> $c is true
# Produce ==> false
printf '[[ $c = true ]] &&'
[[ $c = true ]] && echo ' => true' || echo ' => false'

# True if ==> $d is true
# Produce ==> false
printf '[[ $d = true ]] &&'
[[ $d = true ]] && echo ' => true' || echo ' => false'