Why doesn't alias foo='echo "This is a quote: \'"' work?

Question

In order to remind myself when I try to use shopt in Zsh instead of setopt, I created the following alias, testing it first at a shell prompt:

$ alias shopt='echo "You\'re looking for setopt. This is Z shell, man, not Bash."'

Despite the outer single quotes matching and the inner double quotes matching, and the apostrophe being escaped, I was prompted to finish closing the quotes with:

dquote > _

What's going on?

It appeared that the escaping was being ignored, or that it needed to be double-escaped because of multiple levels of interpretation... So, just to test this theory, I tried double-escaping it (and triple-escaping it, and so on) all the way up until:

alias shopt='echo "You\\\\\\\\\\\\\\\\\\\\\\'re looking for setopt. This is Z shell, man, not Bash." '

and never saw any different behavior. This makes no sense to me. What kind of weird voodoo is preventing the shell from behaving as I expect?

The practical solution is to not use quotes for echo, since it doesn't really need any, and to use double quotes for alias, and to escape the apostrophe so it is ignored when the text is echoed. Then all of the practical problems go away.

Can you help me? I need resolution to this perplexing problem.

Answer 1

This is zsh, man, not fish.

In zsh, like in every Bourne-like shell (and also csh), single quotes are strong quotes, there is no escaping within them (except by using the rcquotes options as hinted by @JdeBP where zsh emulates rc quotes¹). You cannot have a single quote inside a single-quoted string, you need to first close the single quoted string and enter the literal single quote using another quoting mechanism (like \ or "):

alias shopt='echo "You'\''re looking for setopt. This is Z shell, man, not Bash."'

Or:

alias shopt='echo "You'"'"'re looking for setopt. This is Z shell, man, not Bash."'

Though you could also do:

alias shopt="echo \"You're looking for setopt. This is Z shell, man, not Bash.\""

("..." are weaker quotes inside which several characters, including \ (here used to escape the embedded ") are still special).

Or:

alias shopt=$'echo "You\'re looking for setopt. This is Z shell, man, not Bash."'

($'...' is yet another kind of quotes from ksh93, where the ' can be escaped with \').

(and BTW, you can also use the standard set -o in place of setopt in zsh. bash, for historical reasons, has two sets of options, one that you set with set -o one with shopt; zsh like most other shells has only one set of options).

---

¹ In `rc`, the shell of Plan9, with a version for unix-likes also available, [single quotes are the only quoting mechanism](/a/296147) (backslash and double quotes are ordinary characters there), the only way to enter a literal single-quote there is with `''` inside single quotes, so with `zsh -o rcquotes`, you could do:

alias shopt='echo "You''re looking for setopt. This is Z shell, man, not Bash."'

Answer 2

shopt='echo "You\'

This is not voodoo. This is normal POSIX shell quoting in action. There is no escaping mechanism within single-quoted strings. They always terminate at the next single quote. There is a Z shell extension that makes two successive single-quoted strings get a single quote placed between them, which you could employ. Or you could just terminate the single-quoted string, use an escaped (or indeed non-single-quote quoted) single quote, and then start a second single-quoted string.

Or you could not use contractions in your messages. ☺

Further reading

• "Single quotes". Shell command language. Base Specifications. IEEE 1003.1:2017. The Open Group
• "Quoting". Shell Grammar. The Z Shell manual.

Answer 3

The other answers do a good job of explaining why you're seeing this behavior. But if I may make a suggestion for how to actually solve this problem:

Don't use aliases for anything even remotely complicated.

Sure, you can tie your brain up in knots trying to figure out how to nest N layers of quotes, but it's rarely worth it for an alias. When an alias is complicated enough that its quoting becomes nontrivial, just switch to a shell function:

shopt(){
    echo "You're looking for setopt. This is Z shell, man, not Bash."
}

This removes an entire layer of quotes, allows you to add more code to the shell function later, and is generally much easier to read. It also allows much finer control over how and where arguments are inserted, instead of the alias approach of "just replace the beginning of the line and let the remaining words fall where they may." For example, with your (corrected) alias, if you type this:

shopt -s some_opt

...then you will get this output:

You're looking for setopt. This is Z shell, man, not Bash. -s some_opt

That's probably not what you wanted. The shell function will consume whatever arguments you pass to it, and silently discard them.

Answer 4

For days when you don't feel like fighting the quotes, use another character like Unicode U+2019 instead of ' for apostrophes.

Press and hold Ctrl+Shift and type u2019 and this character will appear ’ (er... depending on your locale?).

% alias shopt='echo You’re looking for setopt. This is a Z shell, woman, not Bash.'
% shopt -s dotglob
You’re looking for setopt. This is a Z shell, woman, not Bash. -s dotglob

Such usage of ’ (U+2019) is correct because this character is officially intended to be used as an apostrophe. The Unicode Standard, Version 10.0 explicitly recommends such usage. The standard acknowledges that ' (U+0027) is commonly used as an apostrophe due to its presence in ASCII and on keyboards, and appears to permit that. But it then states that ’ (U+2019) is preferred for apostrophes used as punctuation (and gives a contraction as an example). The only apostrophes for which ’ (U+2019) is not preferred are those used as diacritic marks rather than punctuation marks; those are best written ʼ (U+02BC). From Apostrophes (p. 276) in section 6.2 of the standard:

Apostrophes

U+0027 _^APOSTROPHE is the most commonly used character for apostrophe. For historical reasons, U+0027 is a particularly overloaded character. In ASCII, it is used to represent a punctuation mark (such as right single quotation mark, left single quotation mark, apostrophe punctuation, vertical line, or prime) or a modifier letter (such as apostrophe modifier or acute accent). Punctuation marks generally break words; modifier letters generally are considered part of a word.

When text is set, U+2019 _{^{RIGHT SINGLE QUOTATION MARK}} is preferred as apostrophe, but only U+0027 is present on most keyboards. Software commonly offers a facility for automatically converting the U+0027 _^APOSTROPHE to a contextually selected curly quotation glyph. In these systems, a U+0027 in the data stream is always represented as a straight vertical line and can never represent a curly apostrophe or a right quotation mark.

Letter Apostrophe. U+02BC _{^{MODIFIER LETTER APOSTROPHE}} is preferred where the apostrophe is to represent a modifier letter (for example, in transliterations to indicate a glottal stop). In the latter case, it is also referred to as a letter apostrophe.

Punctuation Apostrophe. U+2019 _{^{RIGHT SINGLE QUOTATION MARK}} is preferred where the character is to represent a punctuation mark, as for contractions: “We’ve been here before.” In this latter case, U+2019 is also referred to as a punctuation apostrophe.

An implementation cannot assume that users’ text always adheres to the distinction between these characters. The text may come from different sources, including mapping from other character sets that do not make this distinction between the letter apostrophe and the punctuation apostrophe/right single quotation mark. In that case, all of them will generally be represented by U+2019.

The semantics of U+2019 are therefore context dependent. For example, if surrounded by letters or digits on both sides, it behaves as an in-text punctuation character and does not separate words or lines.

Answer 5

A now-deleted comment tipped me off and got me half-way to the answer.

It's impossible to escape a single quote inside a single-quoted string.

This limitation is not present in double quoted strings, as I'm certainly escaping a single quotation mark inside a double-quoted string in my final solution:

alias shopt="echo You\'re looking for setopt. This is Z shell, man, not Bash."