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|
---
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
```
|
