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?

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
– Etan Reisner, Commented Apr 27, 2016 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
– RikuXan, Commented Apr 27, 2016 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
– Etan Reisner, Commented Apr 27, 2016 at 17:13
There are no array values in bash; x=(1 2 3) is just special syntax for repeated assignments to indexed names, not the assignment of a single special value to the name x. — chepner
– chepner, Commented Jan 19, 2023 at 18:18

Community · Accepted Answer · 2017-05-23 12:01:49Z

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, 2017 at 12:01

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answered Apr 27, 2016 at 17:13

dimid

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4 Comments

kgadek Over a year ago

This doesn't work—SEAFILE_MYSQL_DB_NAMES is set to DBS and not to (ccnet-db seafile-db seahub-db).

dimid Over a year ago

oops, that still returns a string instead of an array, I've used a different approach now.

heemayl Over a year ago

Use quotes on variable expansion to prevent word splitting and pathname expansion.

dimid Over a year ago

@heemayl, but the array should be split by spaces, no? What do you mean?

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