command -v brew >/dev/null && brew install jq

  hash foo &>/dev/null || { echo "foo command not found blabla..." >&2; exit 1; } 

command: command [-pVv] command [arg ...] Execute a simple command or display information about commands.

    Runs COMMAND with ARGS suppressing  shell function lookup, or display
    information about the specified COMMANDs.  Can be used to invoke commands
    on disk when a function with the same name exists.
    
    Options:
      -p    use a default value for PATH that is guaranteed to find all of
            the standard utilities
      -v    print a description of COMMAND similar to the `type' builtin
      -V    print a more verbose description of each COMMAND
    
    Exit Status:
    Returns exit status of COMMAND, or failure if COMMAND is not found.

I never knew about help to describe a builtin shell command. normally searching for something like "command" in the bash man page would be very tedious.

also: help echo

`which ls` and `help ls` will show you, if it is similar on your system.

"man echo" on linux describes /usr/bin/echo "help echo" describes the builtin.

on the othe hand, "man command" on mac os x gives a huge manpage of builtins (still hard to search for the common word "command")

  test (type brew 2>/dev/null); and brew # blahblah

  type -p brew >/dev/null ^&1; and brew

  type -p brew &>/dev/null && brew

With a proper dependency graph, the tasks installing things would depend on the task `brew-installed`.

This, of course, is leaving alone the point that installing stuff on startup is weird, doubly so with brew which is pretty slow.

  if which brew >/dev/null 2>&1 ; then
      brew install jq
  fi

  brew not found
  /dev/null not found

  if which jq &>/dev/null; then
    brew install jq
  fi