document updated a month ago, on Jan 17, 2025

Bash

How does Bash determine true/false?

'if' by itself

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

# is the given file a Perl script?
if perl -ne 'exit 0 if /^#!.*perl\b/; exit 1' $1; then
    echo "yes, $1 is a Perl script"
fi

test AKA [

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

[ "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 [[

(TODO)

arithmetic command AKA ((

(TODO)