if [ "$1" ]
then
  echo "True"
fi

# Check if $1 is empty or non-empty
if [ -n "$1" ]
then
  echo "True, $1 is a non-empty value"
fi

# Check instead if $1 is true or false, as in Java
[ "$1" = "true" ]

# Check instead if $1 is non-zero or zero, as in C
[ "$1" -ne 0 ]

# Check instead if $1 is defined (even if just assigned the empty string) or undefined
[ "${1+x}" = "x" ]

[ "$var" ] is equivalent to [ -n "$var" ] and checks whether the string is non-empty.

Users more familiar with other languages are therefore often surprised to learn that [ "$var" ] is true when var=false or var=0.

Adding the explicit -n helps clarify that this is a string comparison, and not related to any concept of boolean values or comparisons as it is in most languages.

If you are familiar with the semantics of [, you can ignore this suggestion with no ill effects.

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