2016-11-01 17:19:30 +0000, Martijn Dekker: [...] > | -C (Uppercase C.) Prevent existing files from being overwritten by > | the shell's '>' redirection operator (see Redirecting Output); > > This says nothing about how long the files are supposed to have existed > or how stable they are supposed to be before -C prevents them from being > overwritten. As written, the spec inherently requires atomicity. > > AFAIK, there also is no other POSIX shell mechanism that could create > files both atomically and non-destructively. > > If 'mktemp' were a POSIX standard utility, this wouldn't be an issue, > but it's not, so it is. [...]
And as already discussed that makes for security vulnerabilities in scripts that assume "set -C" will give you the guarantee that redirections will not clobber file or follow symlinks (like O_EXCL) does. See also https://stackoverflow.com/questions/12187859/create-new-file-but-add-number-if-filename-already-exists-in-bash/12194427#12194427 zsh has sysopen -o excl to open a file with O_EXCL (also has nofollow). Generally, you use "-C" for its O_EXCL, the only reason that exclusion for device/fifos is there is because of redirections to /dev/null. Ideally, we want to decide of the flags for each redirection. In ls > file 2> /dev/null we (may) want the O_EXCL for "file", but not for "/dev/null". In zsh, that can be addressed with: (sysopen -wu 1 -o excl file && ls 2> /dev/null) Which is a bit unwieldy and doesn't exit the script upon failure of that sysopen Ideally, we'd want a different redirection operator like ls >? file 2> /dev/null Related: http://www.zsh.org/mla/workers/2015/msg01575.html -- Stephane
