在shell中判断某个可执行程序是否存在

$ command -v foo >/dev/null 2>&1 || { echo >&2 "I require foo but it's not installed.  Aborting."; exit 1; }
$ type foo >/dev/null 2>&1 || { echo >&2 "I require foo but it's not installed.  Aborting."; exit 1; }
$ hash foo 2>/dev/null || { echo >&2 "I require foo but it's not installed.  Aborting."; exit 1; }

if which brew 2>/dev/null; then
  echo "brew exists!"
else
  echo "nope, no brew installed."
fi

详见：https://segmentfault.com/q/1010000000156870