[linux] eval command in Bash and its typical uses

I like the "evaluating your expression one additional time before execution" answer, and would like to clarify with another example.

var="\"par1 par2\""
echo $var # prints nicely "par1 par2"

function cntpars() {
  echo "  > Count: $#"
  echo "  > Pars : $*"
  echo "  > par1 : $1"
  echo "  > par2 : $2"

  if [[ $# = 1 && $1 = "par1 par2" ]]; then
    echo "  > PASS"
  else
    echo "  > FAIL"
    return 1
  fi
}

# Option 1: Will Pass
echo "eval \"cntpars \$var\""
eval "cntpars $var"

# Option 2: Will Fail, with curious results
echo "cntpars \$var"
cntpars $var

The Curious results in Option 2 are that we would have passed 2 parameters as follows:

  • First Parameter: "value
  • Second Parameter: content"

How is that for counter intuitive? The additional eval will fix that.

Adapted from https://stackoverflow.com/a/40646371/744133

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