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I/O redirection

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# 1  
Old 03-08-2011
I/O redirection
Hello everyone,I'm reading a book and there's code fragment:
Code:
exec 3>&1
ls -l 2>&1 >&3 3>&- | grep bad 3>&-
exec 3>&-

It says that the red part of that code does not close fd 3 but the green does close the fd 3.I can't understand that.....Why?Any predicate will be appreciated.Smilie
# 2  
Old 03-08-2011
Maybe the right side runs first, locally and the left side is a sub-process?
# 3  
Old 03-08-2011
It looks to me that both red and green are closing fd 3. What book are you reading?
# 4  
Old 03-08-2011
Quote:
Originally Posted by homeboy
It says that the red part of that code does not close fd 3 but the green does close the fd 3.I can't understand that.....Why?Any predicate will be appreciated.Smilie
It'd be more accurate to say that grep and ls get separate sets of file descriptors, so they close them individually. The shell forks twice, once to create each of those processes. FD 3 is closed in the forks but not in the original shell itself. Once they return, the exec closes FD 3 in the shell itself. That's what the exec does, changes the original shell's own file descriptors.
# 5  
Old 03-08-2011
Quote:
Originally Posted by Perderabo
It looks to me that both red and green are closing fd 3. What book are you reading?
I do agree with you,the book named "Advanced Bash-scripting Guide"
Quote:
Originally Posted by Corona688
It'd be more accurate to say that grep and ls get separate sets of file descriptors, so they close them individually. The shell forks twice, once to create each of those processes. FD 3 is closed in the forks but not in the original shell itself. Once they return, the exec closes FD 3 in the shell itself. That's what the exec does, changes the original shell's own file descriptors.
Thanks for your reply.What your said is the same as what i thought.We all think that FD 3 is closed in each process while the book said the process which executed the "ls" did NOT closed that.Confused...Smilie
# 6  
Old 03-08-2011
http://www.tldp.org/LDP/abs/html/io-redirection.html
Quote:
Originally Posted by Advanced Bash Scripting Guide
Code:
# Redirecting only stderr to a pipe.

exec 3>&1                              # Save current "value" of stdout.
ls -l 2>&1 >&3 3>&- | grep bad 3>&-    # Close fd 3 for 'grep' (but not 'ls').
#              ^^^^   ^^^^
exec 3>&-                              # Now close it for the remainder of the script.

In my opinion, that's simply a poorly-worded comment on their part. fd 3 is closed for both. Perhaps what the author was trying to convey is that ls still has access to fd 3's destination because it was dup'd before closing (the >&3 before 3>&-). But, then, so does grep, since its inherited stdout leads to the same place as fd 3.

My advice would be to not get hung up on this example.

Regards,
Alister
# 7  
Old 03-09-2011
Quote:
Originally Posted by alister
In my opinion, that's simply a poorly-worded comment on their part. fd 3 is closed for both. Perhaps what the author was trying to convey is that ls still has access to fd 3's destination because it was dup'd before closing (the >&3 before 3>&-). But, then, so does grep, since its inherited stdout leads to the same place as fd 3.

My advice would be to not get hung up on this example.

Regards,
Alister
Thank you so much.Smilie
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