Bash programming(!) language

bash-programming-showcase.bash

#!/bin/bash

# This file is supposed to give you a quick idea about what Bash can do beyond its regular job as a shell or scripting language.

# Bash is more than a scripting language. It's a half-decent programming language. It's snail slow, but half-decent otherwise.
# While it's very archane for such usage, it can be a good substitute for Perl and Python in a pinch.
# These things won't work on cut down shells (e.g. sh, ash, dash), and may explode on ZSH, which has its own,
# somewhat friendlier(!), more modern(!) ways of doing these things.

# Remember kids: always quote your variables in Bash! You're almost certainly expecting them to expand
# to one token, not several and, if you don't quote them, get surprised by spaces in variable values.

declare -A arr=(
	[summary]="Bash 4 supports associative arrays!"
	[text2]="Text 2"
	[text3]="Text 3"
)

echo "Associative array access: ${arr[summary]}"

# Pointers!
# Bash also has "pointers", but it calls them "indirect expansion"

indirect="arr[text2]"
echo "Indirect value: ${!indirect}"

read -r "$indirect" <<<"Updated indirect value"
echo "Updated: ${!indirect}"

# print $arr
declare -p arr

# Performant way to build a string:
# Append to an array of strings then join it together
# Appending to a string to obtain a new one gets slow fast

declare -a items=()
for ((i=0; i<9; i++)); do
	# Yes, we expand the array only to set it back in with one more element, but that doesn't have a noticeable
	# performance impact, and it's miles faster than doing the same thing with strings
	items=("${items[@]}" "item $i")
done

# Joining arrays is archane. We abuse parameter expansion.
# Use use IFS to define the "joining character", but that also means the string can only be one character long.
# I'm using `read` and `Process Substitution` (see man bash) in order to avoid this other pattern:
# OldIFS="$IFS"; IFS=";"; string="${items[*]}"; IFS="$OldIFS"

read -r string < <(IFS=";"; echo "${items[*]}")
echo "Joined string: $string"

# Personal preferences:
# - Use the .bash file extension to make it clear that the script is not meant for non-Bash shells and most like will not work
# - tabs for indentation (I use 8-space tabs, but if you use 4, it's only cosmetic for display, so it's also good)