Also called "ANSI-C Quoting" in info:(bash) ANSI-C Quoting.
The escaping rules are a POSIX proposal, and are described in
http://austingroupbugs.net/view.php?id=249. There's a lot of back-and-forth on
the details of escaping of control characters, but we'll be only using a small
subset of the syntax that is common to all proposals and is widely supported.
Unfortunately dash and fish and maybe some other shells do not support it (see
the man page patch for a list).
This allows environment variables to be safely exported using show-environment
and imported into the shell. Shells which do not support this syntax will have
to do something like
export $(systemctl show-environment|grep -v '=\$')
or whatever is appropriate in their case. I think csh and fish do not support
the A=B syntax anyway, so the change is moot for them.
Fixes#5536.
v2:
- also escape newlines (which currently disallowed in shell values, so this
doesn't really matter), and tabs (as $'\t'), and ! (as $'!'). This way quoted
output can be included directly in both interactive and noninteractive bash.
rework C11 utf8.[ch] to use char32_t instead of uint32_t when referring
to unicode chars, to make things more expressive.
[
@zonque:
* rebased to current master
* use AC_CHECK_DECLS to detect availibility of char{16,32}_t
* make utf8_encoded_to_unichar() return int
]
Not every byte sequence is valid utf8. We allow escaping of non-utf8
sequences in strings by using octal and hexadecimal escape sequences
(\123 and \0xAB) for bytes at or above 128. Users of cunescape_one
could infer whether such use occured when they received an answer
between 128 and 256 in *ret (a non-ascii one byte character). But this
is subtle and misleading: the comments were wrong, because ascii is a
subset of unicode, so c != 0 did not mean non-unicode, but rather
ascii-subset-of-unicode-or-raw-byte. This was all rather confusing, so
make the "single byte" condition explicit.
I'm not convinced that allowing non-utf8 sequences to be produced is
useful in all cases where we allow it (e.g. in config files), but that
behaviour is unchanged, just made more explicit.
This also fixes an (invalid) gcc warning about unitialized variable
(*ret_unicode) in callers of cunescape_one.
