Shell script which prints error message when package not found

Question

I'm writing a shell script, and I need to check for some dependencies being installed before executing anything. I found I can use which <package> to see if it is installed or not. The problem is that when that dependency is not found, it throws the following error into console's output:

which: no abc in (/home/pace/.emacs.d/bin:/usr/local/bin:/home/pace/.emacs.d/bin:/usr/local/bin:/home/pace/.local/bin:/usr/local/sbin:/usr/local/bin:/usr/bin:...)

I want to avoid having such output, as I already have error messages shown when something fails. How can I avoid which from writing anything?

function is_installed() {
  if [[ ! $(which $1) ]]
  then
    echo "[ERROR]: $1 $2"
    exit 1
  fi
}

Answer 1

Well, there might be better ways to do what you're trying to do (I'm not certain of the "best" way), but you can redirect stderr and stdout to hide the results from the output:

function is_installed() {
  if [[ ! $(which $1 > /dev/null 2>&1 ) ]]
  then
    echo "[ERROR]: $1 $2"
    exit 1
  fi
}

(recent versions of bash support using >& /dev/null too to do both at once, but the above is slightly more portable)

EDIT -- try this instead

function is_installed() {
  which $1 > /dev/null 2>&1
  if [ $? = 1 ] ; then
    echo "[ERROR]: $1 $2"
    exit 1
  fi
}