Bash read input parameters

08 May 2021

Normally a bash script has some input parameters used to processing data and there are at least two ways to read them:

Read by name

In this way you make explicit the parameter name with its value.
Example:

./print_phrase_by_name.sh --first-word "Hello" --second-word "World!"

Bash script:

#!/bin/bash

FIRST_WORD=""
SECOND_WORD=""

while [[ ${#} -gt 0 ]]; do
	case "${1}" in
		--first-word)
			if [ ${#2} -ne 0 ]; then
				FIRST_WORD="${2}"
				shift 2
			fi
			;;
		--second-word)
			if [ ${#2} -ne 0 ]; then
				SECOND_WORD="${2}"
				shift 2
			fi
			;;
		--help)
			echo "usage: print_phrase_by_name.sh [--help] [--first-word <word>] [--second-word <word>]"
			echo "OPTIONS"
			echo "    --first-word"
			echo "        First word to print."
			echo ""
			echo "    --second-word"
			echo "        Second word to print."
			echo ""
			echo "    --help"
			echo "        Read this manual."
			exit 0
			;;
		*)
			echo "Invalid options found ${1}, please read --help for more information."
			exit 1
			;;
	esac
done

# TODO check FIRST_WORD and SECOND_WORD are not empty
echo "${FIRST_WORD} ${SECOND_WORD}"

exit 0

Read by position

For me read by position is not the first choice because it obfuscated the meaning of the input parameter.
The advantage of this approach compared to by name is more concise.
Example:

./print_phrase_by_pos.sh "Hello" "World!"

Bash script:

#!/bin/bash

FIRST_WORD="${1}"
SECOND_WORD="${2}"

# TODO check FIRST_WORD and SECOND_WORD are not empty
echo "${FIRST_WORD} ${SECOND_WORD}"

exit 0

Example in a containerized environment

Copy previous example in /tmp/print_phrase_by_pos.sh and /tmp/print_phrase_by_name.sh
Then give them execution permission (like chmod +x) for both scripts.

docker run -it --mount 'type=bind,source=/tmp/,target=/tmp/' -w /tmp ubuntu /bin/bash

From inside the container:

# ./print_phrase_by_name.sh --first-word "Hello" --second-word "World!"
Hello World!

# ./print_phrase_by_name.sh --help                                     
usage: print_phrase_by_name.sh [--help] [--first-word <word>] [--second-word <word>]
OPTIONS
    --first-word
        First word to print.

    --second-word
        Second word to print.

    --help
        Read this manual.

# ./print_phrase_by_pos.sh "Hello" "World!"
Hello World!

References