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In POSIX sh, FDs outside of 0-9 are undefined.

(or "In dash, ... are not supported." when using dash)

Problematic code:

# Open FD #10 for writing
exec 10> file

Correct code:

Either rewrite the script to only use FDs in the valid range, or switch to a shell that allows a larger range like bash or ksh.

Rationale:

The shell grammar is only required to recognize a single digit before redirections to set the file descriptor, which limits the portable range to 0-9.

Exceptions:

None.

Related resources:

• Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!

ShellCheck is a static analysis tool for shell scripts. This page is part of its documentation.