How to store standard error in a variable in a Bas

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 another shell script:

alsoUseless.sh

./useless.sh | sed 's/Output/Useless/'

I want to capture "This Is Error", or any other stderr from useless.sh, into a variable. Let's call it ERROR.

Notice that I am using stdout for something. I want to continue using stdout, so redirecting stderr into stdout is not helpful, in this case.

So, basically, I want to do

./useless.sh 2> $ERROR | ...

but that obviously doesn't work.

I also know that I could do

./useless.sh 2> /tmp/Error
ERROR=`cat /tmp/Error`

but that's ugly and unnecessary.

Unfortunately, if no answers turn up here that's what I'm going to have to do.

I'm hoping there's another way.

Anyone have any better ideas?

Answer 1

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

Answer 2

If you want to bypass the use of a temporary file you may be able to use process substitution. I haven't quite gotten it to work yet. This was my first attempt:

$ .useless.sh 2> >( ERROR=$(<) )
-bash: command substitution: line 42: syntax error near unexpected token `)'
-bash: command substitution: line 42: `<)'

Then I tried

$ ./useless.sh 2> >( ERROR=$( cat <() )  )
This Is Output
$ echo $ERROR   # $ERROR is empty

However

$ ./useless.sh 2> >( cat <() > asdf.txt )
This Is Output
$ cat asdf.txt
This Is Error

So the process substitution is doing generally the right thing... unfortunately, whenever I wrap STDIN inside >( ) with something in $() in an attempt to capture that to a variable, I lose the contents of $(). I think that this is because $() launches a sub process which no longer has access to the file descriptor in /dev/fd which is owned by the parent process.

Process substitution has bought me the ability to work with a data stream which is no longer in STDERR, unfortunately I don't seem to be able to manipulate it the way that I want.

Answer 3

POSIX

STDERR can be captured with some redirection magic:

$ { error=$( { { ls -ld /XXXX /bin | tr o Z ; } 1>&3 ; } 2>&1); } 3>&1
lrwxrwxrwx 1 rZZt rZZt 7 Aug 22 15:44 /bin -> usr/bin/

$ echo $error
ls: cannot access '/XXXX': No such file or directory

Note that piping of STDOUT of the command (here ls) is done inside the innermost { }. If you're executing a simple command (eg, not a pipe), you could remove these inner braces.

You can't pipe outside the command as piping makes a subshell in bash and zsh, and the assignment to the variable in the subshell wouldn't be available to the current shell.

bash

In bash, it would be better not to assume that file descriptor 3 is unused:

{ error=$( { { ls -ld /XXXX /bin | tr o Z ; } 1>&$tmp ; } 2>&1); } {tmp}>&1; 
exec {tmp}>&-  # With this syntax the FD stays open

Note that this doesn't work in zsh.

---

Thanks to this answer for the general idea.

Answer 4

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 5

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 6

For error proofing your commands:

execute [INVOKING-FUNCTION] [COMMAND]

---

execute () {
    error=$($2 2>&1 >/dev/null)

    if [ $? -ne 0 ]; then
        echo "$1: $error"
        exit 1
    fi
}

---

Inspired in Lean manufacturing:

• Make errors impossible by design
• Make steps the smallest
• Finish items one by one
• Make it obvious to anyone