CODEWITHSUNDEEP
bash
Roguebantha
Roguebantha

Reputation: 824

Pipe stdout to another process's file descriptor

This one is foiling me so far. I know how to redirect my stdout to another file descriptor in the same process. I know how to pipe stdout to another process's stdin. But what if I want to pipe a process's stdout to a file descriptor in another process?? Specifically, for the case of while read...

  cat file | while read -u 9 line; do //some stuff; done

How do I get cat's output onto file descriptor 9 of the while loop?

Upvotes: 3

Views: 2095

Answers (3)

John Kugelman
John Kugelman

Reputation: 361565

Skip the useless use of cat and directly open fd 9:

while read -u 9 line; do //some stuff; done 9< file

If you do want to use cat or some other arbitrary command, you wouldn't use a pipe at all. A pipe hooks stdout to stdin, but fd 9 is neither of those. Instead you'd want to open up the file descriptor first and then directly run the command that reads from it.

exec 9< <(cat file)
while read -u 9 line; do //some stuff; done

You could also combine it with the first answer if you want to do it all in one line.

while read -u 9 line; do //some stuff; done 9< <(cat file)

Upvotes: 1

chepner
chepner

Reputation: 530872

Pipes work specifically with standard input and output (file descriptors 0 and 1); they don't generalize to other descriptors. Use process substitution and input redirection instead.

while read -u 9 line; do
  ...
done 9< <(cat file)

Of course, you shouldn't use cat like this; just use regular input redirection

while read -u 9 line; do
  ...
done 9< file

Bonus, POSIX-compliant answer: use a named pipe.

mkfifo p
cat file > p &
while read line <&9; do
  ...
done 9< p

Upvotes: 6

Lewis M
Lewis M

Reputation: 548

Not sure why you are reading from file descriptor 9 in a pipeline like this, but can you try something like this:

cat test.in 9>&1 | while read -u 9 line; do
    echo ${line}
done 9<&0

Hope this helps

Upvotes: 0

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