Iterate over the output of a command in bash without a subshell

Question

I want to loop over the output of a command without creating a sub-shell or using a temporary file.

The initial version of of my script looked like this, but this doesn't work since it creates a subshell, and the exit command terminates the subshell instead of the main script which is required. It is part of a much larger script to configure policy routing, and it is halt the execution if it detects a condition that will cause routing to fail.

sysctl -a 2>/dev/null | grep '\.rp_filter' | while read -r -a RPSTAT ; do

  if [[ "0" != "${RPSTAT[2]}" ]] ; then
    echo >&2 "RP Filter must be disabled on all interfaces!"
    echo >&2 "The RP filter feature is incompatible with policy routing"
    exit 1
  fi
done

So one of the suggested alternatives is to use a command like this to avoid the subshell.

while read BLAH ; do echo $BLAH; done </root/regularfile

So it seems to me that I should also be able use a command like this to avoid the subshell and still get the output from the program I want.

while read BLAH ; do echo $BLAH; done <(sysctl -a 2>/dev/null | grep '\.rp_filter')

Unfortunately, using that command results in this error.

-bash: syntax error near unexpected token `<(sysct ...

I get really confused since this does work.

cat <(sysctl -a 2>/dev/null | grep '\.rp_filter')

I could save the output of that command to a temporary file, and use redirect the on the temporary file, but I wanted to avoid doing that.

So why is the redirection giving me an error, and do I have any options other then creating a temporary file?

Solution

You missed a <. Should be:

while read BLAH ; do echo $BLAH; done < <(sysctl -a 2>/dev/null | grep '\.rp_filter')

Think of <(sysctl -a 2>/dev/null | grep '\.rp_filter') being a file.

Comments

[Admin]

fvue.nl/wiki/… tldp.org/LDP/abs/html/subshells.html

[dribler]

Thanks a lot. Just for my edification, can point me at a man page, help, or web page, that documents this? I couldn't get Google to point me at anything useful while trying to search for this.

[CurtainDog]

Google "process substitution". e.g. tldp.org/LDP/abs/html/abs-guide.html#PROCESS-SUB

[ewindisch]

It should be noted that the <() substitution uses file descriptors and on some operating systems will transparently create & use temporary files.

[Gilles 'SO- stop being evil']

@Zoredache: Simply search for <( in the bash manual (it's under “process substitution”).

[dribler]

@ewindisch, if a tool transparently creates a tempfile that is fine, I just didn't want to manually create one, when I was pretty sure I didn't need to.