Check if a program exists from a bash script.Thanks to twitter.com/joshnesbitt and twitter.com/mheap for the help with detecting npm packages.

is_installed.sh

#!/bin/bash

# Functions ==============================================

# return 1 if global command line program installed, else 0
# example
# echo "node: $(program_is_installed node)"
function program_is_installed {
  # set to 1 initially
  local return_=1
  # set to 0 if not found
  type $1 >/dev/null 2>&1 || { local return_=0; }
  # return value
  echo "$return_"
}

# return 1 if local npm package is installed at ./node_modules, else 0
# example
# echo "gruntacular : $(npm_package_is_installed gruntacular)"
function npm_package_is_installed {
  # set to 1 initially
  local return_=1
  # set to 0 if not found
  ls node_modules | grep $1 >/dev/null 2>&1 || { local return_=0; }
  # return value
  echo "$return_"
}

# display a message in red with a cross by it
# example
# echo echo_fail "No"
function echo_fail {
  # echo first argument in red
  printf "\e[31m✘ ${1}"
  # reset colours back to normal
  printf "\033\e[0m"
}

# display a message in green with a tick by it
# example
# echo echo_fail "Yes"
function echo_pass {
  # echo first argument in green
  printf "\e[32m✔ ${1}"
  # reset colours back to normal
  printf "\033\e[0m"
}

# echo pass or fail
# example
# echo echo_if 1 "Passed"
# echo echo_if 0 "Failed"
function echo_if {
  if [ $1 == 1 ]; then
    echo_pass $2
  else
    echo_fail $2
  fi
}

# ============================================== Functions

# command line programs
echo "node    $(echo_if $(program_is_installed node))"
echo "gulp    $(echo_if $(program_is_installed gulp))"
echo "webpack $(echo_if $(program_is_installed webpack))"
echo "eslint  $(echo_if $(program_is_installed eslint))"
echo "tsc     $(echo_if $(program_is_installed tsc))"
echo "brew    $(echo_if $(program_is_installed brew))"
echo "gem     $(echo_if $(program_is_installed gem))"

# local npm packages
echo "lodash  $(echo_if $(npm_package_is_installed lodash))"
echo "react   $(echo_if $(npm_package_is_installed react))"
echo "angular $(echo_if $(npm_package_is_installed angular))"

Maybe you can write the functions a little more compact. For example, the code snippet for program_is_installed:

program_is_installed(){
	type $1 >/dev/null 2>&1 && echo "1" || echo "0"
}