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Zucca
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Joined: 14 Jun 2007
Posts: 3965
Location: Rasi, Finland
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 Posted: Wed Mar 05, 2025 5:30 pm Post subject: Complex shell redirections - help |
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I've been developing few scripts to simplify a lot of things on all of my systems.
To avoid bashisms (which I most of the times do) for little more portability, I've tried to find a solution to use exec to redirect stdout and stderr to log. That's pretty easy just by | Code: | | exec > stdout.log 2> stderr.log |
... for example.
But when it comes to passing these streams to logger, things get orders of magnitude more complex if I want to avoid bashisms.
With bash one can | Code: | | exec >(logger [arguments]) |
, but ~POSIX shells (I tend to target busybox sh and alternatively mksh and dash too) don't support such syntax.
After quite some time (frankly) of digging around the net, I found this: | SE - mikeserv: | _log()( p=
mkfifo "${p:=$(mktemp -u)}" &&
printf %s "$p" &&
exec <&- >&- <>/dev/null >&0 &&
{ rm "$p"
logger --priority user."$1" --tag "${0##*/}"
} <"$p" &
) |
... that code is meant to be used as: | Code: | exec >"$(_log notice)" 2>"$(_log error)"
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... for example. Neat really.
I can follow this code at every line except the line with exec.
I cannot decipher what it does.
(I really do understand why novices find it sometimes hard to understand bourne -like shell scripting. :D)
Anyway what I think the exec line does:- <&- >&- closes stdin and stdout
- <>/dev/null connects stdin and stdout to NULL
- >&0 redirects stdout to stdin
... yeaaah... That doesn't make any sens whatsoever to me.
Living something like 20 years on sh and bash and I still stumble upon some "magic" like this which I don't understand.
Anyone to decipher this to me?
And if you have better/alternative, sh prefereably, solutions, speak up?
_________________
..: Zucca :..
| My gentoo installs: | init=/sbin/openrc-init
-systemd -logind -elogind seatd |
| Quote: | | I am NaN! I am a man! |
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Hu
Administrator
Joined: 06 Mar 2007
Posts: 23153
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| Posted: Wed Mar 05, 2025 6:00 pm Post subject: Re: Complex shell redirections - help |
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When in doubt, use strace to see what is done.
| Zucca wrote: | Anyway what I think the exec line does:- <&- >&- closes stdin and stdout
- <>/dev/null connects stdin and stdout to NULL
- >&0 redirects stdout to stdin
... yeaaah... That doesn't make any sens whatsoever to me. |
Per info bash: | info '(bash)Redirections': | The redirection operator
[N]<>WORD
causes the file whose name is the expansion of WORD to be opened for
both reading and writing on file descriptor N |
Presumably, this is not a bashism, so we can rely on the documentation for bash to address your question. If so, then your interpretation is slightly incorrect. <>/dev/null opened null onto stdin only, but did so as a read-write descriptor, so that the subsequent >&0 (which sets stdout=stdin) does not lead to weird errors with a stdout that is not writable. To show why this is needed, consider: | Code: | $ (exec </dev/null >&0; echo a)
-bash: echo: write error: Bad file descriptor
$ (exec <>/dev/null >&0; echo a)
$ |
Regarding strace: | Code: | $ strace -e trace=file,close,dup,dup2,dup3 /bin/busybox sh -c 'exec <&- >&- <>/dev/null >&0'
execve("/bin/busybox", ["/bin/busybox", "sh", "-c", "exec <&- >&- <>/dev/null >&0"], 0x7ffd44f0e498 /* 0 vars */) = 0
readlinkat(AT_FDCWD, "/proc/self/exe", "/bin/busybox", 4096) = 12
newfstatat(AT_FDCWD, "/", {st_mode=S_IFDIR|0755, st_size=4096, ...}, 0) = 0
newfstatat(AT_FDCWD, ".", {st_mode=S_IFDIR|0755, st_size=4096, ...}, 0) = 0
close(0) = 0
close(1) = 0
openat(AT_FDCWD, "/dev/null", O_RDWR|O_CREAT, 0666) = 0
dup2(0, 1) = 1
close(10) = 0
close(11) = 0
+++ exited with 0 +++
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In particular, this shows directly that the <> is a read-write descriptor for /dev/null. |
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Zucca
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Joined: 14 Jun 2007
Posts: 3965
Location: Rasi, Finland
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| Posted: Thu Mar 06, 2025 8:49 am Post subject: |
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Hu,
Thanks again for your input. :)
I really should use strace more often.
Now that the function makes more sense, I'll start crafting and testing my own.
Thanks again!
_________________
..: Zucca :..
| My gentoo installs: | init=/sbin/openrc-init
-systemd -logind -elogind seatd |
| Quote: | | I am NaN! I am a man! |
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