Why does shell ignore quoting characters in arguments passed to it through variables? [duplicate]

Answer 1

When you put quote characters into variables, they just become plain literals (see http://mywiki.wooledge.org/BashFAQ/050; thanks @tripleee for pointing out this link)

Instead, try using an array to pass your arguments:

argumentString=(-ir 'hello world')
grep "${argumentString[@]}" .