Bash boolean examples

bash-booleans.sh

#!/usr/bin/env bash

# Various methods of using Booleans in Bash

# Credit for most of these techniques comes from:
# http://stackoverflow.com/questions/2953646/

###########################################################
# Using the builtin 'true' and 'false' commands
###########################################################

# Most elegant solution, but has some pitfalls.
# The empty string and unset variables evaluate to true.
# Also opens up the possibility of shell injection.

foo=true
bar=false

if $foo && ! $bar
then
    echo "True"
else
    echo "False"
fi

if $bar || ! $foo
then
    echo "True"
else
    echo "False"
fi

###########################################################
# Using strings and checking for string equality
###########################################################

# Not as elegant, but safest and most POSIX compatible option.
# Can be any string, doesn't have to be true or false.

foo="true"
bar="false"

# POSIX compatible
if [ "$foo" = true -a ! "$bar" = true ]
then
    echo "True"
else
    echo "False"
fi

# Bash-ism
if [[ $bar == true || $foo != true ]]
then
    echo "True"
else
    echo "False"
fi

###########################################################
# Using 0 and 1 and evaluating arithmetic expressions
###########################################################

# Works, but can become very difficult to keep
# track of whether 0 or 1 is true or false.
# Similar to how C handles Booleans.

foo=1  # true
bar=0  # false

if ((foo && ! bar))
then
    echo "True"
else
    echo "False"
fi

if ((bar || ! foo))
then
    echo "True"
else
    echo "False"
fi