Why do 2>&1 (redirect stderr to stdout) with <&- >&- 2>&- (close stdin, stdout, stderr)

io-redirection

Why would someone do this

(exec <&- >&- 2>&-; gzip somefile.txt) 2>&1 &

<&- >&- 2>&- "…means close stdin, stdout and stderr, respectively.."

but why do "2>&1" if it's closed?

Best Answer

The statement mentioned is incorrect because the standard file descriptors must be open.

POSIX specification: C.2.7 Redirection

Applications should not use the [n]<&- or [n]>&- operators to execute a utility or application with file descriptor 0 not open for reading or with file descriptor 1 or 2 not open for writing, as this might cause the executed program (or shell built-in) to misbehave. In order not to pass on these file descriptors to an executed utility or application, applications should not just close them but should reopen them on, for example, /dev/null. Some implementations may reopen them automatically, but applications should not rely on this being done.

The instruction would rather be

gzip somefile.txt >/dev/null 2>&1 &

Example

$ echo "hi" 2>&1 | logger &
[1] 26818
[1]+  Done                    echo "hi" 2>&1 | logger

$ sudo tail /var/log/messages
Apr 12 17:53:57 greeneggs saml: hi

You can use the abbreviated notation here as well, if used cautiously in a actual Bash shell (not to be confused with Dash):

$ echo "hi" |& logger &

NOTE: This is equivalent to <cmd1> 2>&1 | <cmd2>. Again only use the above when making use of an actual Bash shell interactively, would be a good way to approach it.

excerpt from ABSG

# |& was added to Bash 4 as an abbreviation for 2>&1 |.

References

Advanced Bash Scripting Guide - Chapter 20. I/O Redirection

IO Redirection – How to Tee stdout to stderr

tee can duplicate to the current console by using tee /dev/tty

git status --short | cut -b4- | tee /dev/tty | xargs gvim --remote

Alteratively, you can use /dev/stdoutor /dev/stderr but they could be redirected if your command is within a script. Note that /dev/tty will always be the console (and may not exist in a non-interactive shell). This is wrong, read the comments.

Best Answer

Related Solutions

Linux Shell – How to Redirect stdout & stderr to Logger

Example

References

IO Redirection – How to Tee stdout to stderr

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