Build bash scripts with `source` files into a single script.

build.sh

#!/usr/bin/env bash
#
# https://gist.github.com/joehillen/30f08738c1c3c0ca3e4c754ad33ad2ff
#
# This script inlines 'source' files.
#
# For long scripts, it is nice to be able to break them into multiple files
# to make them easier to work with but still release as a single script.
#
# Inspired by https://stackoverflow.com/a/37533160/334632 with the following enhancements:
# - supports sourcing files with quotes, spaces, and ~.
# - supports sources in sources
# - inline scripts from $PATH
# - errors on (mutual) recursion
# - preserves indentation
# - omits shebangs in sourced files
#
# This will NOT work with variables in source paths.
#
# WARNING MacOS users: Requires a modern version of Bash

set -e
set -o pipefail

declare -A sourced

function _inline_sources {
  while IFS='' read -r line; do
    if [[ $line =~ ^([[:space:]]*)(source|\.)[[:space:]]+(.+) ]]; then
      indent=${BASH_REMATCH[1]}
      source_command=${BASH_REMATCH[2]}
      file=${BASH_REMATCH[3]}
      echo "${indent}# $source_command $file"
      if [[ $file != */* ]]; then
        fp=$(type -p "$file" || :) # look in $PATH
      fi
      if [[ -z $fp ]]; then
        fp=$(eval realpath "$file") # resolve links, relative paths, ~, quotes, and escapes.
      fi

      # fail if we've already sourced this file
      if [[ ${sourced[$fp]} = 1 ]]; then
        echo "ERROR: Recursion detected: $fp"
        exit 1
      fi

      sourced["$fp"]=1
      _inline_sources "$fp" | sed -e "/^#!.*/d;s/^/${indent}/" # remove shebang and add indentation
      sourced["$fp"]=0
      continue
    fi

    echo "$line"
  done < "$1"
}

_inline_sources "$1"

the script from https://stackoverflow.com/a/37533160 (just wrapped it into a function like yours)

#!/usr/bin/env bash

set -e
set -o pipefail

declare -A sourced

function _inline_sources {

    while read line; do
        if [[ "$line" =~ (\.|source)\s+.+ ]]; then
            file="$(echo $line | cut -d' ' -f2)"
            echo "$(cat $file)"
        else
          echo "$line"
        fi
    done < "$1"
}

_inline_sources "$1"

sadly your script includes the first sourced file for all further sources.

.
├── bin
│   └── build.sh
├── lib
│   ├── example.sh
│   ├── header.sh
│   └── lib-wget.sh
└── test.sh

test.sh

#!/bin/ash
source lib/header.sh

echo "my script start"

source lib/lib-wget.sh
source lib/example.sh
echo do something