Using the array name without an index gets the first array element in Bash, ok?

Question

GNU Bash 4.4 seems forgiving when accessing the first element or only element of an array by using just the array name or accessing a variable with array syntax. Both ${var[0]} and ${var[@]} return "Value" when the variable var='Value'.

I can simplify my code by using one of the arrays as a variable when only a variable is needed. For this package I'm more concerned with it not braking with the next Bash update than with portability.

Is it safe to write code that accesses the first element of an array using variable syntax?

declare -a foo='Value'
declare -p var
declare -a var=([0]="Value")
echo ${foo}
Value

Better use standard "${var[0]}" to access first element.

anubhava
– anubhava

2018-03-26 21:50:09 +00:00
Commented Mar 26, 2018 at 21:50 — anubhava
– anubhava, Commented Mar 26, 2018 at 21:50

codeforester · Accepted Answer · 2018-03-26 23:04:18Z

6

This is a documented behavior:

Referencing an array variable without a subscript is equivalent to referencing with a subscript of 0.

However, I recommend you still use the subscript, to make your code clearer.

edited Mar 26, 2018 at 23:04

codeforester

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answered Mar 26, 2018 at 21:50

Barmar

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Comments

Robin A. Meade · Accepted Answer · 2022-03-05 20:02:30Z

0

I confirmed that it is documented as far back as bash 2.0 (Dec 31, 1996):

curl -sL https://ftpmirror.gnu.org/bash/bash-doc-2.0.tar.gz \
| tar -xz 'bash-doc-2.0/doc/bashref.html' --to-stdout \
| grep -F --after-context=1 'without a subscript'

Result:

Referencing an array variable without a subscript is equivalent to
referencing element zero.

I couldn't find it documented in the bash 1.14 documentation (Aug 29, 1996) found at https://ftpmirror.gnu.org/bash/. The behavior might exist in v1.14 (I didn't test) but it isn't documented as far as I could tell.

edited Mar 5, 2022 at 20:02

answered Mar 5, 2022 at 19:48

Robin A. Meade

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