Aren't single quotes supposed to be "hard" quotes in POSIX shells?
Why does dash seem to process escapes inside them, unlike Bash?
$ bash --posix -c "echo ['[\\n]']"
[[\n]]
$ dash -c "echo ['[\\n]']"
[[
]]
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user541686
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1 Answers
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The single quotes do behave identically, it's the echo implementation that's different.
When you run sh -c "echo ['[\\n]']" (for any sh), the first thing that happens is that your interactive shell processes the double-quoted string, turning the \\ into a single \. The string that gets passed to the shell for execution is then echo ['[\n]'].
The echo command (probably builtin) launched within that inner shell sees the single argument [[\n]], and some implementations of echo process C-style backslash-escapes like \n for newline, while some others don't.
Try sh -c "printf '%s\\n' ['[\\n]']" instead, and see Why is printf better than echo?
ilkkachu
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bash --posix -O xpg_echo -c "echo '[\\n]'"for theechobuiltin ofbashto be fully POSIX compliant. On some systemsxpg_echois enabled by default (at least for builds of bash intended for /bin/sh). See Why is printf better than echo? for details. – Stéphane Chazelas Dec 23 '22 at 08:44