What does ${MY_VAR:-1} mean in bash?

Question

I saw this mentioned in some documentation. But it doesn't seem to do anything...

~ $ echo $DYNO
run.9917
~ $ echo ${DYNO}
run.9917
~ $ echo ${DYNO:-1}
run.9917

What does the :-1 do?

unfortunately it's closed before i post my answer. here is a good reference for it: http://tldp.org/LDP/abs/html/refcards.html#AEN22728 — Jason Hu, Mar 21 '15 at 20:39

score 2 · Answer 1 · answered Mar 21 '15 at 20:24

2

${DYNO:-1} means if DYNO exists and isn’t null, return its value; otherwise return 1. It's the :- between the variable's name and the default value inside the {} that make this happen.

answered Mar 21 '15 at 20:24

Paul Evans

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note: `:` optional here. this `${DYNO-1}` work too. – Roman Kiselenko Mar 21 '15 at 20:27
@Зелёный thanks, didn't know that! Just tried it out and works fine. – Paul Evans Mar 21 '15 at 20:28
not for all but for some case of variable substitution this works. +1 for you. – Roman Kiselenko Mar 21 '15 at 20:30
2

@Зелёный that's not all true. `:-` means not defined and not null, while `-` means not defined **only**. have a look http://www.grymoire.com/Unix/Sh.html#uh-32 – Jason Hu Mar 21 '15 at 20:33
@HuStmpHrrr Interesting, so we get some new behavior. And I see that's similarly true for `${var=val}`, `${var+val}` and `${var?message}`. – Paul Evans Mar 21 '15 at 20:38
@HuStmpHrrr the colon (`:`) is actually optional. If the colon is ommited, then change "exists and isn't null" to "exists" as i know. Except `${var:offset:length}` – Roman Kiselenko Mar 21 '15 at 20:38
@Зелёный that's not optional. if you have `VAR=''`, `${VAR:-foo}` will be `foo` while `${VAR-foo}` will remain empty string. – Jason Hu Mar 21 '15 at 20:41
@PaulEvans exactly. see this: http://tldp.org/LDP/abs/html/refcards.html#AEN22728. – Jason Hu Mar 21 '15 at 20:41

score 1 · Answer 2 · answered Mar 21 '15 at 20:33

This is covered in the Shell Parameter Section of the Bash Reference Manual.

"${var:-1}" means expand the parameter named var if it's defined, and if not, expand 1 instead.

Other, similar expansions:

"${var: -1}" means expand the substring of var from the last character.
"${var:=1}" means assign 1 to var if it's not defined and then, either way, expand the parameter.

Examples:

$ x=hi y=
$ echo "${x:-1}" "${x: -1}" "${y:-2}"
hi i 2

Roman Kiselenko · Accepted Answer · 2015-03-21T20:28:24.370

0

${var:-word} If var is null or unset, word is substituted for var. The value of var does not change.

$> echo "${bar:-1}"
$> 1
$> bar=3
$> echo "${bar:-1}"
$> 3
$> echo "${bar-1}"
$> 3

edited Mar 21 '15 at 20:28

answered Mar 21 '15 at 20:20

Roman Kiselenko

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Thanks. Wanna add some examples? – John Bachir Mar 21 '15 at 20:22
http://stackoverflow.com/questions/10390406/usage-of-in-bash – robertjlooby Mar 21 '15 at 20:22
@JohnBachir. In short, `${var:-word}` allows `word` to serve as the **default value** for `var` in case `var` is **(1)** not already set, or **(2)** empty/null. There are other variations that change the circumstances under which `word` is used as the value for `var`. – David C. Rankin Mar 21 '15 at 20:32

What does ${MY_VAR:-1} mean in bash?

3 Answers3