Re: [Help-bash] Why 'exec' has to be used in with fd<>filename? And the difference between <> and >>?

[Greg Wooledge]

On Tue, Feb 11, 2014 at 05:28:59PM -0600, Peng Yu wrote:
> exec fd<>fileName

> In the above document, I don't see 'exec' should be used. Where is
> this documented? Thanks.

As Chet said, exec is used when you want the shell to open or close
a file descriptor.  Think of it as bash's builtin version of open()
and close() with a silly name inherited from the Bourne shell.

> Also, what is the benefit of using exec fd<>filename.

Pretty much the only time I would ever use <> in a script is with a
socket connection.  For example:

exec 3<>/dev/tcp/"$hostname"/80     # open
printf >&3 '%s\n' \
  'HEAD / HTTP/1.1' \
  "Host: $hostname" \
  'Connection: close' \
  ''
cat <&3                             # without 'Connection: close' this hangs
exec 3>&-                           # close

For regular files, bidirectional I/O is very, very rare in shell scripts.