dirname.sh

#!/bin/sh

strip_trailing_slashes() {
  printf "$1" | sed 's/[/]*$//'
}

# If empty arg, return .
# If nothing but slashes, return /
# If no slashes after stripping all trailing slashes, return .
# Otherwise, return everything up until last path component
dirname() {
  if [ -z "$1" ]; then printf '.'; return; fi
  if ! $(printf "$1" | grep -q '[^/]'); then printf '/'; return; fi

  local s="$(strip_trailing_slashes $1)"
  if ! $(printf "$s" | grep -q '/'); then printf '.'; fi

  printf "$(strip_trailing_slashes $(printf $s | sed 's/[^/]*$//'))"
}

dirname "$@"

dirname_test.sh

#!/bin/sh

# fail and pass counters
FC=0
PC=0

# params: expected, actual, error_message
fail() {
  FC=$(expr $FC + 1)
  printf "   [FAIL] expected: \'$1\', actual: \'$2\'"
  if ! [ -z "$3" ]; then printf " ($3)"; fi
  printf "\n"
}

# params expect, actual, error_message
assert() {
  if ! [ "x$1" = "x$2" ]; then fail "$1" "$2" "$3"; return 1; fi
  PC=$(expr $PC + 1)
}

# params expected, actual, error_message
assert_fail() {
  if [ "x$1" = "x$2" ]; then fail "$1" "$2" "$3"; return 1; fi
  PC=$(expr $PC + 1)
}

# https://stackoverflow.com/a/36692524/758334
oneliner() {
  printf "$1" | sed -n -e '1p' | sed  -e 's#//*#/#g' -e 's#\(.\)/$#\1#' -e 's#^[^/]*$#.#' -e 's#\(.\)/[^/]*$#\1#'
}

# test input, expect
T() {
  printf "=> test: input \'$1\', expect: \'$2\'\n" 
  assert "$2" "$(dirname.sh $1)"

  # test against `dirname` for compatibility
  assert "$(dirname $1)" "$(dirname.sh $1)" "dirname compatibility"

  # test against oneliner
  assert "$2" "$(oneliner $1)" "oneliner compatibility"
}

for t in '""' '.' '.foo' './foo' 'foo' 'foo/' 'foo///' '..' '..foo' '..foo//' '../'
do
  T "$t" '.'
done

for t in '/' '//' '///'
do
  T "$t" '/'
done

for t in 'a/b/c' 'a/b/c/' 'a/b/c///'
do
  T "$t" 'a/b'
done

for t in '/a/b/c' '/a/b/c/' '/a/b/c///'
do
  T "$t" '/a/b'
done

for t in '//a/b/c' '//a/b/c//'
do
  T "$t" '//a/b'
done


printf "==========================================\n"
if [ "$FC" -gt 0 ]; then
  printf "FAIL ($FC tests failed, $PC tests passed)\n"
  printf "==========================================\n"
  exit 1
else
  printf "SUCCESS: ($PC tests passed)\n"
  printf "==========================================\n"
  exit 0
fi