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Eugen Konkov
Eugen Konkov

Reputation: 25207

What is the difference between `${var:-value}` and `${var:=value}`?

I do not understand the difference between ${var:-value} and ${var:=value}.
It seems that both commands in shell work identically.

What is the difference?

Upvotes: 0

Views: 280

Answers (2)

Thomas Dickey
Thomas Dickey

Reputation: 54563

Both do something if the variable is unset or null (an empty value).

If you happen to be assigning the expression to something, or using it in a here-document, both may appear to be doing the same thing, since the unset/null var is replaced by $value.

But the difference is that the latter one also assigns $value to var, so that it can be used in subsequent shell expressions.

That is often used in this idiom:

: ${var:=value}

to assign the default value where none exists.

Upvotes: 1

glenn jackman
glenn jackman

Reputation: 247012

If var is unset or null:

  • ${var:-value} will expand to "value"
  • ${var:=value} will expand to "value" and set var=value

Demo

$ var=

$ echo "${var:-y}"; echo ">$var<"
y
><

$ echo "${var:=y}"; echo ">$var<"
y
>y<

Doc: https://www.gnu.org/software/bash/manual/bashref.html#Shell-Parameter-Expansion

Upvotes: 2

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