How to consistently record command output in a variable in UNIX

Question

In some cases what I see in a console output is different from what I get recorded after redirection. I see this on Linux/bash but this example is ksh/OpenBSD. Is there a way around this?

For example:

# pfctl -ttable -Ttest 123.123.123.123 > result.txt
0/1 addresses match.
# more result.txt
result.txt (END)

In other words the "0/1 addresses match." is printed on the console, but I cannot for the life of me get it into a file, variable or anything. I've used $() and > which work for most commands, but every now and then there is a command that spits out stuff on the screen but I get nothing via the redirect/pipe. I hope someone can shed light on this peculiarity.

So again contrast this:

# OUTP=$(pfctl -tscanners -Ttest 123.123.123.123)
0/1 addresses match.
# echo $OUTP

#

(nothing echoing, the variable does not hold the console output) with this:

# OUTP=$(date)
# echo $OUTP
Sun Aug 21 08:33:37 PDT 2016
#

(the variable contains the entire console output)

Thanks again for any help.

Possible duplicate of Bash scripting assign ALL output from command to variable — tripleee
– tripleee, Commented Aug 21, 2016 at 16:17

Walter A · Accepted Answer · 2016-08-21 16:14:33Z

1

Your command has 2 different output streams.
You need to rederict the second (stderr) to the first.

pfctl -ttable -Ttest 123.123.123.123 > result.txt 2>&1

answered Aug 21, 2016 at 16:14

Walter A

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4 Comments

Laine Mikael Over a year ago

Omg I had tried 2>&1 but had put that right after the command. Putting it at the very end works! For both cases. Thanks!

Walter A Over a year ago

I get confused every time I use it. I try to remember it with : connect 2 with the target of 1. When you first connect stderr to the console, redirecting stdout is out of the picture. When you first connect stdout to a file, connecting 2 to the target is the same file.

Laine Mikael Over a year ago

Just for completeness sake, here is how to get the output using command substitution: OUTP=$(pfctl -tscanners -Ttest 1.1.1.1 2>&1)

Walter A Over a year ago

@Laine: When you want to use $OUTP (or better ${outp} lowercase and braces), do not forget quotes: echo "${OUTP}" and you get newlines when they were in the original output.

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