How to store standard error in a variable

Question

Let\'s say I have a script like the following:

useless.sh

echo \"This Is Error\" 1>&2
echo \"This Is Output\" 

And I have an

Answer 1

This post helped me come up with a similar solution for my own purposes:

MESSAGE=`{ echo $ERROR_MESSAGE | format_logs.py --level=ERROR; } 2>&1`

Then as long as our MESSAGE is not an empty string, we pass it on to other stuff. This will let us know if our format_logs.py failed with some kind of python exception.

Answer 2

It would be neater to capture the error file thus:

ERROR=$(</tmp/Error)

The shell recognizes this and doesn't have to run 'cat' to get the data.

The bigger question is hard. I don't think there's an easy way to do it. You'd have to build the entire pipeline into the sub-shell, eventually sending its final standard output to a file, so that you can redirect the errors to standard output.

ERROR=$( { ./useless.sh | sed s/Output/Useless/ > outfile; } 2>&1 )

Note that the semi-colon is needed (in classic shells - Bourne, Korn - for sure; probably in Bash too). The '{}' does I/O redirection over the enclosed commands. As written, it would capture errors from sed too.

WARNING: Formally untested code - use at own risk.

Answer 3

Here's how I did it :

#
# $1 - name of the (global) variable where the contents of stderr will be stored
# $2 - command to be executed
#
captureStderr()
{
    local tmpFile=$(mktemp)

    $2 2> $tmpFile

    eval "$1=$(< $tmpFile)"

    rm $tmpFile
}

Usage example :

captureStderr err "./useless.sh"

echo -$err-

It does use a temporary file. But at least the ugly stuff is wrapped in a function.

Answer 4

alsoUseless.sh

This will allow you to pipe the output of your useless.sh script through a command such as sed and save the stderr in a variable named error. The result of the pipe is sent to stdout for display or to be piped into another command.

It sets up a couple of extra file descriptors to manage the redirections needed in order to do this.

#!/bin/bash

exec 3>&1 4>&2 #set up extra file descriptors

error=$( { ./useless.sh | sed 's/Output/Useless/' 2>&4 1>&3; } 2>&1 )

echo "The message is \"${error}.\""

exec 3>&- 4>&- # release the extra file descriptors

Answer 5

$ b=$( ( a=$( (echo stdout;echo stderr >&2) ) ) 2>&1 )
$ echo "a=>$a b=>$b"
a=>stdout b=>stderr

Answer 6

Capture AND Print stderr

ERROR=$( ./useless.sh 3>&1 1>&2 2>&3 | tee /dev/fd/2 )

Breakdown

You can use $() to capture stdout, but you want to capture stderr instead. So you swap stdout and stderr. Using fd 3 as the temporary storage in the standard swap algorithm.

If you want to capture AND print use tee to make a duplicate. In this case the output of tee will be captured by $() rather than go to the console, but stderr(of tee) will still go to the console so we use that as the second output for tee via the special file /dev/fd/2 since tee expects a file path rather than a fd number.

NOTE: That is an awful lot of redirections in a single line and the order matters. $() is grabbing the stdout of tee at the end of the pipeline and the pipeline itself routes stdout of ./useless.sh to the stdin of tee AFTER we swapped stdin and stdout for ./useless.sh.

Using stdout of ./useless.sh

The OP said he still wanted to use (not just print) stdout, like ./useless.sh | sed 's/Output/Useless/'.

No problem just do it BEFORE swapping stdout and stderr. I recommend moving it into a function or file (also-useless.sh) and calling that in place of ./useless.sh in the line above.

However, if you want to CAPTURE stdout AND stderr, then I think you have to fall back on temporary files because $() will only do one at a time and it makes a subshell from which you cannot return variables.