CODEWITHSUNDEEP
posixsh
user1011471
user1011471

Reputation: 1130

POSIX syntax difference (colon) in defaulting a string?

foo=${foo:-default}

bar=${bar-default}

Is there a functional difference between including the colon : and excluding it?

Upvotes: 1

Views: 56

Answers (1)

Benjamin W.
Benjamin W.

Reputation: 52451

See the spec:

$ myvar=
$ echo "${myvar:-default}"
default
$ echo "${myvar-default}"

$ unset myvar
$ echo "${myvar-default}"
default

With the colon, the check is for "if unset or null, use default". Without the colon, it's just "if unset, use default".

The relevant paragraph (emphasis mine):

In the parameter expansions shown previously, use of the in the format shall result in a test for a parameter that is unset or null; omission of the shall result in a test for a parameter that is only unset.

Same goes for Bash, by the way. Quoting from the manual:

When not performing substring expansion, using the form described below (e.g., :-), Bash tests for a parameter that is unset or null. Omitting the colon results in a test only for a parameter that is unset. Put another way, if the colon is included, the operator tests for both parameter's existence and that its value is not null; if the colon is omitted, the operator tests only for existence.

Upvotes: 4

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