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.

test x"$var" != x

Only needed for ancient Bourne shells and not so ancient pdksh or ash versions. Don't use this in modern scripts.

test -n "$var"

POSIX sh plus some older Bourne shells which lacked [.

[ -n "$var" ]

POSIX sh and most Bourne shells including Heirloom. Best overall choice for new scripts requiring POSIX compatibility.

test "$var"

POSIX sh. POSIX specifies that for one argument, non-null is always true while null is always false.

[ "$var" ]

POSIX sh. As above. Still very portable.

[[ -n $var ]]

Here and below is Bash/Ksh/Zsh only. This is compatible with some buggy shell versions which didn't process the implicit -n correctly.

[[ $var ]]

Also works in Bash, Ksh and Zsh since version 5.0.6. Same as above, only more ambiguous.

(( ${#var} ))

About as portable as the above though hits some bugs in some versions of Ksh when the variable contains invalid characters. Almost always slower. Don't use this to test for empty strings.

case "$var" in "")

Extremely portable. You'll probably want to use that if you want to check for different values including the empty string.

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 ]]
[[ ! $var ]] # Also won't work in Zsh prior to 5.0.6. If such portability is needed, adjust solutions on this page to use -z or -n explicitly.
case "$var" in "") ;; *)...

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.

# Bourne / POSIX:
${var+':'} false
case ${var+y} in y)...
[ -n "${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+_} ]]
(( ${var+1} ))

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 and bash (since 4.3) support array indices with the -v test. Bash 4.2 did not. This is the "best" but also least portable method to test for an unset variable.

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.

declare -F f >/dev/null       # Bash only - declare outputs "f" and returns 0 if defined, returns non-zero otherwise.
typeset -f f >/dev/null       # Bash/Ksh - typeset outputs the entire function and returns 0 if defined, returns non-zero otherwise.
[[ $(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

BashFAQ/083 (last edited 2016-07-24 18:05:27 by atlantic850)