document updated 2 days ago, on Nov 4, 2025

Bash

How does Bash determine true/false?

'if' by itself

Documentation:

Bash manual: 'if'

Without using test, Bash only considers a program's exit status. Zero ⇒ true, non-zero ⇒ false.

For example:

if true;   then echo "true!";   else echo "false";   fi

if false;   then echo "true!";   else echo "false";   fi

# Negation works here too.
if ! true;   then echo "true!";   else echo "false";   fi


# Is 47 an odd number?
if perl -e 'my $bool = shift() % 2;  exit ! $bool' 47; then
    echo "That's an odd number"
else
    echo "That's an even number"
fi

test, AKA [

Test is both an standalone tool /usr/bin/test, as well as a Bash built-in. It's also known as [.

Documentation:

Bash manual: 'test'
manpage for 'test'

[ "foobar" = "qux" ];   echo $?     # A return value of 0 means true, 1 means false. This is false.

[ "$foo" ];   echo $?               # True if $foo evaluates to the empty string.

[ 1 ];   echo $?                    # True.

[ 0 ];   echo $?                    # True because "0" is a non-empty string.

[ false ];   echo $?                # True because this is interpreted as the string "false", not a command to be run.

[ "" ];   echo $?                   # False.

[ ];   echo $?                      # False.

extended test command, AKA [[

Documentation:

Bash manual: Conditional Expressions
Bash manual: the conditional construct [[ (the =~ operator is discussed further down in this section)
Bash manual: Pattern Matching
When == is used, then Bash manual: extglob matching
Extended Globbing in Bash | Linux Journal

# Are we currently running under Cygwin?
if [[ "$(uname)" =~ ^CYGWIN_ ]]; then
                  ## ^^ Don't surround this with quotes, otherwise it will be interpreted as a literal string instead of a regex.

arithmetic command, AKA ((

Documentation:

Bash manual: Shell Arithmetic

(TODO)