---
title: Bash
subtitle: best practices
---
(Associative) arrays
--------------------

### Declaration

`"${#xs[@]}"` doesn't work with `nounset` if `xs` wasn't defined, i.e. was
declared with either of

```bash
local -a xs
declare -a xs
local -A xs
declare -A xs
```

Therefore, if you want to extract the length of an array, append `=()` to the
statements above.

```bash
local -a xs=()
declare -a xs=()
```

And now `"${#xs[@]}"` works with `nounset`.
It doesn't affect expansion (see below) though.

### Expansion

#### Do

```bash
func ${arr[@]+"${arr[@]}"}
```

#### Don't

```bash
func "${arr[@]}"                # Doesn't work with `nounset`.
func "${arr[@]+"${arr[@]}"}"    # Doesn't work properly with `declare -a arr=('')`.
```

### `unset`

#### Do

```bash
unset -v 'arr[x]'
unset -v 'arr[$i]'
```

#### Don't

```bash
unset -v arr[x]         # May break due to globbing.
unset -v arr[$i]        # The same as above + a possible problem with quotation.
unset -v 'arr["x"]'     # Doesn't work for some reason.
unset -v 'arr["]"]'     # The same as above; just highlighting the problem with funny characters in array indices.
unset -v 'arr["$i"]'    # Also rejected.

# An insightful discussion on the topic: https://lists.gnu.org/archive/html/help-bash/2016-09/msg00020.html.
```

`errexit`
---------

I hate this feature, and I especially hate people who prefer "standards" over
useful behaviour.

### Command substitution

#### Do

```bash
shopt -s inherit_errexit    # Without this, bar will be executed w/ errexit disabled!

bar() {
    false
    echo 'should never see this' >&2
}

bar_output="$( bar )"
foo "$bar_output"
```

#### Don't

```bash
bar() {
    false
    echo 'should never see this' >&2
}

foo "$( bar )"    # Even with errexit, foo will still get executed.
                  # More than that, the script will print 'should never see this'!
```

### Process substitution

#### Do

```bash
output="$( command )"

while IFS= read -r line; do
    process_line "$line"
done <<< "$output"
```

#### Don't

```bash
# This causes some bash insanity where you cannot change directories or set
# variables inside a loop: http://mywiki.wooledge.org/BashFAQ/024
command | while IFS= read -r line; do
    process_line "$line"
done

# errexit doesn't work here no matter what:
while IFS= read -r line; do
    process_line "$line"
done < <( command )
echo 'should never see this'
```

### Functions

#### Do

```bash
foo() {
    false
    echo 'should never see this' >&2
}

foo
echo ok
```

#### Don't

```bash
if foo; then
    echo ok       # foo will still print 'should never see this'.
fi

foo && echo ok    # Same here.
foo || echo ok
```