How can we disconnect a file descriptor from any file?

Question

In bash we can create a connection between a file descriptor and a file, e.g. by

exec > /path/to/myfile

which connects fd 1 to the file.

I was wondering how we can disconnect a fd from any file connected to it, so that the fd isn't connected to any file? Note that /dev/null and /dev/tty are files.

Thanks.

My purpose is to figure out Does a shell automatically connect file descriptors 0, 1 and 2 to its controlling terminal?, by disconnect fd 0, 1 and 2 in bash, before running bash in the same bash shell.

Answer 1

What you want is close the file descriptor which in Bourne-like shell is done with the >&- (defaults to fd 1) or <&- (defaults to fd 0) operator:

exec >&-
exec 1>&-
exec 1<&-

are all equivalent and result in a close(1) system call being made.

Note that it's generally a bad idea to close file descriptors 0, 1 and 2, as most commands expect those to be open (it's their stdin, stdout and stderr), and them opening a file could cause that file to inadvertently become their stdin/stdout/stderr and what they print on stdout or stderr could end up being written in those files.