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Old 04-29-2017, 01:56 PM   #1
Weapon S
Registered: May 2011
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[Ubuntu] Process substitution prints out file descriptor?

Every time I use process substitution, it prints out the file descriptor of the temporary pipe afterwards. Simplest example (I wouldn't expect any output):
$ echo <( false )
> /dev/fd/63
I don't remember I've ever had this issue with any other Linux while using process substitution. Is there any option, command, or syntax I can use to suppress this output? Or did I just remember incorrectly?
Old 04-29-2017, 05:31 PM   #2
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Try cat instead of echo. Do you literally=actually get that > character displayed on the output=result line? (I don't)
Now that I've knocked this thread off of pristene ZRT status, hopefully LQgurus will come along with a detailed explanation
A web-search of "/dev/fd/63" returns somethings I remember searching for in the past, like:

Edit: Found MUCH BETTER! link:
And, adding ThreadTitle,
Wiki has echo<(true)

But I don't have a precise&full explanation. (Interesting study tho!)
p.s. Minor 'nit': suggest adding "bash" to ThreadTitle (so viewers don't think 'krnl' e.g. )

Last edited by !!!; 04-29-2017 at 06:17 PM.
Old 04-29-2017, 07:21 PM   #3
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Originally Posted by Weapon S View Post
$ echo <( false )
> /dev/fd/63
Did you think you had redirected the output from false to the stdin of the echo command? That is not what you did. That form of process redirection sets up a file descriptor and passes an argument that is a pathname that can be opened to access that file descriptor. The echo command does not open files. It just echoes its arguments.

If you want to redirect a command's stdin as with a pipe, you can do it this way:
command1 < <( command2 )
Note the space between the two "<" characters.

I'm sure some will wonder why on Earth you would do that rather than simply writing "command2 | command1", but that construct is useful for streams other than stdin and for redirecting the stdin of loops in shell scripts:
while read Var; do
    $((X += Var))
done < <(command)
echo "Sum=$X"
If you tried to do that with "command | while read Var; do ... done", the while loop would be run in a sub-shell, and the incremented value of X would not be available outside the loop.
2 members found this post helpful.
Old 04-30-2017, 12:12 AM   #4
Weapon S
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Yep, I'm stupid. The actual program I had trouble with was wc (to be exact: "wc -l <( find /home/me -size -2k )" ). Then to make a minimum replication of the problem, I used echo. Echo will just print out the argument it gets, and wc will print out the file(s) it counted in at the end.


bash, file descriptor, process substitution, ubuntu

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