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bashshelldouble-quotes

failed to avoid “[: too many arguments”

I know that MyVar="a b" ; [ $MyVar = "a b" ] will signal an error because the resultant expansion is [ a b = "a b" ].

I tried to avoid that case:

$ MyVar="\"a b\""
$ [ $MyVar = "a b" ]
-bash: [: too many arguments

Why did I fail? Didn't it expand to [ "a b" = "a b" ]?

I know there is a [[ ... ]] form; I know I can write "$MyVar".

I am asking why is my approach ineffective.

Solution

  • why is my approach ineffective.

    That's because $MyVar without quotes will be split into "a and b" (two strings) and the == operator only works with one argument on the left hand side and one argument on the right hand side.

    Note that parameter expansion is a one-pass thing. It expands the variable $MyVar into "a and b" and does then not reevaluate the result. The order in which the expansions occur is listed in Shell expansion:

    The order of expansions is: brace expansion; tilde expansion, parameter and variable expansion, arithmetic expansion, and command substitution (done in a left-to-right fashion); word splitting; and filename expansion.