Diff for "BashFAQ/083"

How do I determine whether a variable is already defined? Or a function?

There are several ways to test for a defined or non-empty variable or function depending upon the requirements.

"Declared", "Defined", and "Undefined"

Like in many languages, declaration and definition can be independent steps. To declare an object is to give it a name and datatype, whereas a definition associates it with a value, which may be empty. In Unix shell terminology, "unset", "set", and "null" are often used to mean "undefined", "defined", and "empty" respectively. Since "string" is very nearly the only common datatype, "null" usually means the empty string. With arrays, things become less consistent between shells.

Testing for a defined, non-empty variable / parameter.

Since the most common goal is to distinguish between an empty and non-empty variable or parameter, we'll discuss it first. These are the most common solutions arranged from most to least portable.

To check the converse, whether a variable is empty (unset or zero-length), invert the above logic.

test x"$var" = x
test -z "$var"
[ -z "$var" ]
test ! "$var"
[ ! "$var" ]
[[ -z $var ]]
[[ ! $var ]] # Also won't work in Zsh in any emulate mode. If such portability is needed, adjust solutions on this page to use -z or -n explicitly.

Testing that a variable has been declared

Distinguishing between a variable that is undefined and one that is defined but empty is somewhat trickier, but possible.

# POSIX
${var+:} false
! ${var+false}
[ -n "${var+_}" ]
[ "${var+_}" ]

Which of the above performs best varies from shell to shell. If POSIX isn't a requirement, the following [[ solution usually performs best.

# Bash/ksh
[[ ${var+_} ]]

Another method that's occasionally seen is to check the status of typeset -p. The above method should be preferred over this.

# Bash/ksh/zsh
typeset -p var >/dev/null 2>&1
# returns 0 if var exists, error otherwise

Bash 4.2 adds a -v test:

# Bash 4.2 / ksh93
if [[ -v var ]]; then echo "var is defined"; fi

ksh93 additionally supports array indices with the -v test, while Bash so far does not. This should be the preferred method when targeting ksh93 specifically.

Testing that a function has been defined

For determining whether a function with a given name is already defined, there are several answers, all of which require Bash (or at least, non-Bourne) commands. Testing that a function is defined should rarely be necessary.

typeset -f f >/dev/null        # Bash/Ksh - typeset returns false if undefined, otherwise true.
[[ $(typeset -f f) ]]          # Bash/Ksh - typeset prints nothing if undefined, otherwise outputs the function's definition.
[[ $(type -t f) == function ]] # Bash-only - "type" outputs "function" if defined. In ksh (and mksh), the "type" alias for "whence -v" differs.

# Bash/Ksh. Workaround for the above, but the first two are preferable.
isFunction() [[ $(type ${BASH_VERSION:+-t} "$1") == ${KSH_VERSION:+"$1 is a "}function ]]; isFunction f