[WIP] The standard 'md5sum' implemented as a Bash function for (older?) macOS

md5sum

#!/usr/bin/env bash
# h/t: https://stackoverflow.com/a/28776166
if ( return 0 2>/dev/null ); then sourced=1; else sourced=0; fi

# fail on unset variables unless sourced from another script
if (( !sourced )); then set -u; fi

# set TRACE=1 in the environment to enable execution tracing
if (( ${TRACE:-} )); then set -x; fi

_md5sum() {
    if [[ $(uname -s) == Darwin ]]; then
        # && ! command -v md5sum &>/dev/null
        if [[ ${1:-} == '-c' ]]; then
            # and when 'md5sum' fails it looks like this, so emulate that
            #   $ md5sum /etc/passwd | tr 1-4 6-9 | md5sum -c -
            #   /etc/passwd: FAILED
            #   md5sum: WARNING: 1 of 1 computed checksums did NOT match
            # otherwise
            #   $ md5sum /etc/passwd | md5sum -c -
            #   /etc/passwd: OK
            shift
            local failed=0 total=0
            while read sum filename; do
                if [[ $($FUNCNAME $filename) == $sum ]]; then
                    echo "$filename: OK"
                else
                    echo "$filename: FAILED"
                    failed=$(( failed + 1 ))
                fi
                total=$(( total + 1 ))
            done < <( cat ${@:+"$@"} )
            # useless use of cat for compatibility with the real 'md5sum',
            # which already knows how to read from stdin if any argument is '-'

            if (( failed )); then
                echo "md5sum: WARNING: $(( failed )) of $(( total )) checksums did NOT match"
                return 1
            fi
        else
            # recent *BSD have a '-n' which puts the output in "normal" form;
            # (older?) macOS doesn't have this flag
            #
            # so tranform from md5's format, which is:
            #   $ md5 /etc/passwd
            #   MD5 (/etc/passwd) = 578778de897a870096cd897e6f967b5d
            #   $ echo HI | md5
            #   39d2b1ae41c609d04d9bbcde036940d8
            #
            # if no filenames args are passed, 'md5sum's behavior is to just
            # sit there and wait for stdin
            if [[ $# -gt 0 ]]; then
                for file in "$@"; do
                    md5 "$file" | sed -E 's/.*\((.+)\) = ([[:xdigit:]]+)/\2  \1/'
                done
            else
                # reading from stdin
                echo "$(md5)  -"
            fi
        fi
    else
        md5sum ${@:+"$@"}
    fi
}   # _md5sum

# if we were sourced, do nothing; otherwise run with arguments
if (( !sourced )); then
    _md5sum ${@:+"$@"}
fi

•Makefile

SHELL = bash
DIM := $(shell tput dim)
RESET := $(shell tput sgr0)
## tell 'md5sum' that it's under test
#TESTING = 1
#export TESTING

check: test
test: have-md5-in-path have-md5sum-in-path md5sum \
	read-from-stdin read-filename read-multiple-filenames \
	check-from-stdin check-from-filename check-from-multiple-filenames

read-from-stdin:
	@echo -e "\n$(DIM)## $@$(RESET)" >&2
	diff -u <(echo HI | command md5sum) <(echo HI | ./md5sum)

read-filename: /etc/passwd
	@echo -e "\n$(DIM)## $@$(RESET)" >&2
	diff -u <(command md5sum $<) <(./md5sum $<)

read-multiple-filenames: /etc/passwd /etc/group /etc/hosts /etc/aliases
	@echo -e "\n$(DIM)## $@$(RESET)" >&2
	diff -u <(command md5sum $^) <(./md5sum $^)

check-from-stdin:
	@echo -e "\n$(DIM)## $@$(RESET)" >&2
	diff -u <(echo HI | command md5sum | command md5sum -c) <(echo HI | command md5sum | ./md5sum -c)

check-from-filename: MD5SUMS
	@echo -e "\n$(DIM)## $@$(RESET)" >&2
	diff -u <(command md5sum -c $<) <(./md5sum -c $<)

check-from-multiple-filenames: MD5SUMS MD5SUMS2
	@echo -e "\n$(DIM)## $@$(RESET)" >&2
	diff -u <(command md5sum -c $^) <(./md5sum -c $^)

MD5SUMS:
	md5sum * > $@

MD5SUMS2: MD5SUMS
	# `tail -r` is macOS/BSD thing; otherwise use `tac`
	if [[ $$(uname -s) == Darwin ]]; then \
		tail -r $< > $@; \
	else \
		tac $< > $@; \
	fi

clean:
	-rm MD5SUMS*

	
have-md5-in-path:
ifeq ($(shell command -v md5),)
	$(error Need 'md5' in the search path to run tests)
endif

have-md5sum-in-path:
ifeq ($(shell command -v md5sum),)
	$(error Need 'md5sum' in the search path to run tests)
endif

This is a work-in-progress.

macOS doesn't have the standard md5sum utility, except if you install it from Homebrew or MacPorts (package: md5sha1sum).

This bit of shell script implements the standard behavior of md5sum, including reading and writing from a pipe, in Bash script, wrapped around the *BSD md5 utility, which macOS does have.

Testing

It works for some of the common use cases, but others not. Running make -f •Makefile after cloning the gist as a Git repo locally will run the tests.