On Wed, Aug 04, 2010 at 02:02:39AM -0700, Linda Walsh wrote: > Other string operators? What other ones operate this way? > > This is what I'm referring to: > > a="hi" > if [[ "hi" == "$a" ]]; then echo "this matches"; fi > if [[ 'hi' == '$a' ]] then echo "this doesn't"; fi
You are misunderstanding the role of quotes. Double quotes and single quotes have some things in common: * They cause the content inside of them to be treated as a single word, instead of potentially multiple words. * They cause shell/pattern metacharacters such as space, asterisk, parentheses, braces, brackets, etc. to be treated as literal. And one thing different: * Double quotes permit `...` and $ substitutions to occur. Single do not. In your examples, '$a' is matched as the literal string "dollar-sign a". "$a" is matched as a *string* (not a pattern) whose value is the content of the variable a. The example you did NOT show is: if [[ "hi" == $a ]]; then ... In that one, the $a is matched as a *pattern* (not a string) whose value is the content of the variable a. That is the significance of the double quotes, or their lack. In action: imadev:~$ a='*a' imadev:~$ if [[ "baa" = "$a" ]]; then echo yes; else echo no; fi no imadev:~$ if [[ "baa" = $a ]]; then echo yes; else echo no; fi yes > I would prefer this work: > > a="h." > > if [[ "hi" =~ "$a" ]];.... Quoting the right hand side makes bash treat the contents of a as a string rather than a pattern. If you want the contents treated as a pattern (or in this case, an ERE) then remove the quotes. a='^h.$' if [[ "hi" =~ $a ]]; ... In fact, putting the ERE-pattern you want to match against into a variable and then using =~ $variable *is* the recommended practice, if you need to use =~ at all. It is the only way to get consistent results among all the different releases of bash that have =~, since the behavior changed multiple times during the 3.x series. In this particular case, I would use a glob rather than an ERE, because "h?" is much simpler than "^h.$".
