Shell Script Template

• Must not run only on Bash (thus use shebang #!/bin/sh)
• Must quit on errors and uninitialized variables (set -ue)
• Must print a usage when used wrong (e.g. wrong parameters)
• Must have a header, which explains the purpose of the script

#!/bin/sh -ue
#
# Script which reads variables from a .config file (key=value format)
#
# Author: Karl Krach
# Date:   2018/01/25
#
# Depends on: sh, cut, sed, tr
#

# Defaults
DUMMY_NUMBER=7
LOWERCASE="false"
KEEPNEWLINE="false"

print_usage() {
	echo "USAGE: $(basename $0) [-l] [-d NUMBER] [--keep-newline] CONFIGFILE VARIABLE"
	echo
	echo "Reads variables from a .config file (key=value format)"
	echo
	echo "  -h, --help           shows this output"
	echo "  -d, --dummy          dummy parameter (default: $DUMMY_NUMBER)"
	echo "  -l, --to-lowercase   converts the output to lowercase (default: $LOWERCASE)"
	echo "  --keep-newline       does not replace the linefeed by spaces (default: $KEEPNEWLINE)"
	echo "  CONFIGFILE           the config file to read from"
	echo "  VARIABLE             the variable to find"
	echo
}


while [ $# -gt 0 ] ; do
	case "$1" in
		-h|--help)
			print_usage
			exit 0
			;;
		-d|--dummy)
			shift
			DUMMY_NUMBER="$1"
			shift
			;;
		-l|--to-lowercase)
			LOWERCASE="true"
			shift
			;;
		--keep-newline)
			KEEPNEWLINE="true"
			shift
			;;
		--)
			# end of flags, e.g. if parameter shall start with minus
			shift
			break
			;;
		-*)
			echo "ERROR: Unknown parameter '$1'!"
			echo
			print_usage
			exit 1
			;;
		*)
			break
			;;
	esac
done

if [ $# -lt 2 ] ; then
	echo "ERROR: Parameter missing!"
	echo
	print_usage
	exit 1
fi

CONFIGFILE=$1
VARIABLE=$2

if [ ! -e "$CONFIGFILE" ] ; then
	echo "ERROR: File '$CONFIGFILE' does not exist!"
	echo
	print_usage
	exit 1
fi

grep "^$VARIABLE=" $CONFIGFILE | sed 's/^[^=]*=\(.*\)/\1/' | sed 's/^"\(.*\)"$/\1/' | \
 ( [ "$LOWERCASE"   = "true" ] && tr '[:upper:]' '[:lower:]' || cat ) | \
 ( [ "$KEEPNEWLINE" = "true" ] && cat || tr -d '\n' )