Bash
getopts in Bash: Parse Command-Line Options Properly
A tested getopts skeleton for Bash scripts: flags, options with values, required options, error handling, and a manual loop for --long options.
Topic
input-validation
Bash
in_array() in Bash: Check if an Array Contains a Value
A correct in_array() for Bash, an associative-array set for many lookups, and a demonstration of why [[ "${arr[*]}" =~ value ]] gives false positives.
Bash
yesno() in Bash: Ask a Yes/No Question Safely
A yes/no confirmation prompt for Bash scripts that accepts any case, has an optional default and handles cron, pipes and EOF without looping forever.
Bash
is_num() in Bash: Check if a Value Is a Number
Check that input is a whole number, a signed integer or a decimal before doing arithmetic, and avoid the octal, empty-string and injection problems in (( )).