Bash Array value in default value assignment

Question

I'm trying to assign an array of three values to a variable if it hasn't been assigned yet with the line

: ${SEAFILE_MYSQL_DB_NAMES:=(ccnet-db seafile-db seahub-db)}

Unfortunately, echoing ${SEAFILE_MYSQL_DB_NAMES[@]} results in (ccnet-db seafile-db seahub-db) and ${SEAFILE_MYSQL_DB_NAMES[2]} prints nothing. It seems, the value has been interpreted as a string and not as an array. Is there any way I can make my script assign an array this way?

This problem occurs on a Debian Jessie with bash 4.3.30 (in a docker container if that matters). Interestingly, the same code works on Ubuntu 16.04 with bash version 4.3.42, where it is handled as an array as I would expect it.

Are you sure this works on your Ubuntu machine? I just tried it on my debian unstable machine with `GNU bash, version 4.3.42(1)-release` and it did not work the way you said you expect/saw. That all said I'm not sure that I would even expect this to work in the first place. Array assignment isn't the same as the `:=`/`=` expansion. — Etan Reisner, Apr 27 '16 at 16:54
@EtanReisner You're right, must have made a mistake when testing it, it behaves the same on the Ubuntu machine. What do you mean exactly with "Array assignment isn't the same as the `:=/=` expansion"? I would assume that assigning a value in the default value construct acts the same as when simplly assigning it the normal way (i.e. `SEAFILE_MYSQL_DB_NAMES=(ccnet-db seafile-db seahub-db)` results in an array variable) — RikuXan, Apr 27 '16 at 17:02
The `:=` expansion is not the same as a simple assignment. There's no reason to assume that they have to work the same way. Normal array assignment is a specific "compound assignment" nothing says that the `:=` expansion needs to support the same compound assignment. It certainly *could* work that way but I don't think it is a necessary assumption/extension that it would. — Etan Reisner, Apr 27 '16 at 17:13

score 2 · Accepted Answer · edited May 23 '17 at 12:01

2

How about doing it in several stages? First declare the fallback array, then check if SEAFILE_MYSQL_DB_NAMES is set, and assign if needed.

DBS=(ccnet-db seafile-db seahub-db)
[[ -v SEAFILE_MYSQL_DB_NAMES ]] || read -ra SEAFILE_MYSQL_DB_NAMES <<< ${DBS[@]}

Based on this answer.

edited May 23 '17 at 12:01

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answered Apr 27 '16 at 17:13

dimid

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This doesn't work—`SEAFILE_MYSQL_DB_NAMES` is set to `DBS` and not to `(ccnet-db seafile-db seahub-db)`. – kgadek Dec 27 '16 at 16:18
oops, that still returns a string instead of an array, I've used a different approach now. – dimid Dec 27 '16 at 22:26
Use quotes on variable expansion to prevent word splitting and pathname expansion. – heemayl Dec 28 '16 at 04:49
@heemayl, but the array *should* be split by spaces, no? What do you mean? – dimid Dec 28 '16 at 06:53

Bash Array value in default value assignment

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