Using trap with ERR: How do I send $?

Question

In bash 4.x If I have:

err_handler() {
 echo You received error 1ドル|mail -s "script error" MAILTO
}
trap err_handler ERR

how do I send the actual stderr output to the function when the trap is called? I want to avoid having to put logic on every line to redirect stderr, thats the point of a trap I think.

• celtschk: What about redirecting stderr to a file from the beginning (for the complete script), and then just sending the contents of that file on error? As added bonus, if mailing fails for whatever reason, you still have the file locally to look what happened.
• Felix Frank: Uhm, no. The purpose of the trap is to invoke specific behavior when a signal has been received. The signal carries no further data, not from stdout nor stderr of any subprocesses.
• celtschk: @FelixFrank: From bash's man page: "A SIGNAL_SPEC of ERR means to execute ARG each time a command's failure would cause the shell to exit when the -e option is enabled." So no signal here. And yes, that event itself is also not associated with standard error, but usually failing commands write something to standard error before they exit with non-zero exit status (which is what Gregg Leventhal wants to send). And if you store everything written to standard error by your script into a file, then that file will especially contain the error output of the failing command.
• Felix Frank: I see. My bad. I'm basically with celtschk then. You could just globally redirect all stderr to a file, then include the contents of that in the mail.
• celtschk: exec 2>somefile at the beginning of the script should suffice.

Answer 1

There is no way to retroactively capture the error output, because at the time the command returns with a non-zero exit status, the output already has been done. However you can redirect all stderr output of your script from the beginning to a file by issuing the command

exec 2>/some/file

at the beginning of your script. This command-less form of exec applies the redirections to the current shell process. You then can send that file in case of an error.

An additional advantage of this method is that if sending the mail fails for any reason, you still have the file around to look at locally (well, unless the reason is a failure of that disk, of course).