redirect error output to function in bash?

Question

How do I redirect the error output from a command to a function?

E.g. I have:

function logit(){
    read output
    echo "error occured -- $output" >> $file
}

mkdir /tmp/pop.txt | logit

But the above doesn't work, the variable "$output" doesn't contain anything.

You have to redirect stderr(2) to stdout(1) to make it work like this: mkdir /tmp/pop.txt 2>&1 | logit — Ivan
– Ivan, Commented Oct 3, 2022 at 15:29
It works, but now I have another issue, I have other commands following mkdir, like chown, and even though they don't trigger any errors, they still trigger the "logit" function. Is there a way to trigger the command upon receiving stderr? — Dmytro Lysak
– Dmytro Lysak, Commented Oct 3, 2022 at 15:42

William Pursell · Accepted Answer · 2022-10-03 16:20:48Z

1

It's not entirely clear what you mean (in a comment), but perhaps you are looking for something like:

logit(){
    printf "error occured -- ";
    cat
} >> "$file"

exec 3>&1
{
    mkdir /tmp/pop.txt 
    chown ...
    chmod ... 
} 2>&1 1>&3  | logit

This routes the stdout of all the commands in the block to the original stdout of the script while directing the errors streams of all to the logit function. Rather than simply dumping error occurred -- as a header and ignoring newlines in the input, it might be better to implement logit as:

logit(){ sed 's/^/ERROR: /' >> "$file"; }

or maybe even add a timestamp:

logit(){ perl -ne 'printf "%s: ERROR: %s", scalar gmtime, $_'; } >> "$file"

edited Oct 3, 2022 at 16:20

answered Oct 3, 2022 at 15:57

William Pursell

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1 Comment

Dmytro Lysak Over a year ago

ah, interesting, thanks.

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