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---
title: Bash
subtitle: best practices
layout: plain
links:
- {rel: stylesheet, href: 'assets/css/bash.css'}
features:
- title: Script header
topics:
- do:
- |
#!/usr/bin/env bash
set -o errexit -o nounset -o pipefail
shopt -s inherit_errexit lastpipe
dont:
- |
#!/bin/sh -e
- title: Arrays
topics:
- title: Declaration
do:
- |
local -a xs=()
declare -a xs=()
local -A xs=()
declare -A xs=()
dont:
- |
local -a xs
declare -a xs
local -A xs
declare -A xs
# Doesn't work with nounset:
echo "${#xs[@]}"
- title: Expansion
do:
- |
func ${arr[@]+"${arr[@]}"}
dont:
- |
# Doesn't work with nounset:
func "${arr[@]}"
- |
# Expands to 0 arguments instead of 1:
declare -a arr=('')
func "${arr[@]+"${arr[@]}"}"
- title: unset
do:
- |
unset -v 'arr[x]'
unset -v 'arr[$i]'
dont:
- |
# May break due to globbing:
unset -v arr[x]
# In addition, possible quoting problem:
unset -v arr[$i]
# Doesn't work for some reason:
unset -v 'arr["x"]'
unset -v 'arr["]"]'
# Also rejected:
unset -v 'arr["$i"]'
# An insightful discussion on the topic:
# https://lists.gnu.org/archive/html/help-bash/2016-09/msg00020.html
- title: errexit
topics:
- title: Command substitution
do:
- |
shopt -s inherit_errexit
foo() { echo foo ; }
bar() { false ; echo bar >&2 ; }
output="$( bar )"
foo "$output"
# If inherit_errexit is unavailable, you can do
#output="$( set -e; bar )"
dont:
- |
foo() { echo foo ; }
bar() { false ; echo bar >&2 ; }
# This will print both "foo" and "bar":
foo "$( bar )"
# This will also print "foo":
foo "$( false )"
- |
foo() { echo foo ; }
bar() { false ; echo bar >&2 ; }
# This will still print both "foo" and "bar".
output="$( bar )"
foo "$output"
# This won't print anything.
output="$( false )"
foo "$output"
- title: Process substitution
do:
- |
shopt -s lastpipe
result=()
cmd | while IFS= read -r line; do
result+=("$( process_line "$line" )")
done
dont:
- |
# Without lastpipe, the loop is executed is a subshell,
# and the array will be empty:
result=()
cmd | while IFS= read -r line; do
result+=("$( process_line "$line" )")
done
- |
# errexit doesn't work for <( cmd ) no matter what:
while IFS= read -r line; do
process_line "$line"
done < <( cmd )
# This will be printed even if cmd fails:
echo 'should never see this'
- |
# This breaks if $output contains the \0 byte:
output="$( cmd )"
while IFS= read -r line; do
process_line "$line"
done <<< "$output"
- title: Functions
do:
- |
foo() { false ; echo foo >&2 ; }
foo
echo ok
dont:
- |
foo() { false ; echo foo >&2 ; }
# This will print "foo" no matter what.
if foo; then
echo ok
fi
# Same below.
foo && echo ok
foo || echo fail
# It currently appears to be completely impossible to
# execute a function inside a conditional with errexit
# enabled. Therefore, you should try to avoid this
# whenever possible.
---
{% for feature in page.features %}
<h2>{{ feature.title }}</h2>
{% for topic in feature.topics %}
{% if topic.title %}
<h3>{{ topic.title }}</h3>
{% endif %}
<div class="row">
<div class="col-md-6">
{% for guide in topic.do %}
<div class="pre_container pre_do">
{% highlight bash %}{{ guide }}{% endhighlight %}
<div class="pre_mark"><span class="glyphicon glyphicon-ok"></span></div>
</div>
{% endfor %}
</div>
<div class="col-md-6">
{% for guide in topic.dont %}
<div class="pre_container pre_dont">
{% highlight bash %}{{ guide }}{% endhighlight %}
<div class="pre_mark"><span class="glyphicon glyphicon-remove"></span></div>
</div>
{% endfor %}
</div>
</div>
{% endfor %}
{% endfor %}
|
