Why should eval be avoided in Bash, and what should I use instead?

Question

Time and time again, I see Bash answers on Stack Overflow using eval and the answers get bashed, pun intended, for the use of such an \"evil\" construct. Why is

Answer 1

How to make eval safe

eval can be safely used - but all of its arguments need to be quoted first. Here's how:

This function which will do it for you:

function token_quote {
  local quoted=()
  for token; do
    quoted+=( "$(printf '%q' "$token")" )
  done
  printf '%s\n' "${quoted[*]}"
}

Example usage:

Given some untrusted user input:

% input="Trying to hack you; date"

Construct a command to eval:

% cmd=(echo "User gave:" "$input")

Eval it, with seemingly correct quoting:

% eval "$(echo "${cmd[@]}")"
User gave: Trying to hack you
Thu Sep 27 20:41:31 +07 2018

Note you were hacked. date was executed rather than being printed literally.

Instead with token_quote():

% eval "$(token_quote "${cmd[@]}")"
User gave: Trying to hack you; date
%

eval isn't evil - it's just misunderstood :)