Get the exitcode of the shell script in a “trap EX

问题:

I want to have a cleanup action in my Bash scripts, like this:

#! /bin/bash
set -eu
trap 'echo "E: failed with exitcode $?" 1>&2' ERR

true
false

Using $? came to mind as a natural choice, but this isn't the case. It always contains 0. Is there any way that I can "spy" on the exitcode in the ERR trap?

[Update:] I have no idea what I had tested before. This code works like a charm, so I'm leaving it here as a small and good example.

回答1:

Your (probably simplified) example doesn't exhibit the problem you've mentioned:

+ set -eu
+ trap 'echo "E: failed with exitcode $?" 1>&2' ERR
+ true
+ false
++ echo 'E: failed with exitcode 1'
E: failed with exitcode 1

Chances are that the command returning ERR is executed in a && or ||, or subject to other conditions mentioned in the snippet below. Quoting from the manual:

If a sigspec is ERR, the command arg is executed whenever a simple command has a non-zero exit status, subject to the following conditions. The ERR trap is not executed if the failed command is part of the command list immediately following an until or while keyword, part of the test following the if or elif reserved words, part of a command executed in a && or || list, or if the command’s return status is being inverted using !. These are the same conditions obeyed by the errexit option.

So if you have, for example, the following:

#! /bin/bash
set -eu
trap 'echo "E: failed with exitcode $?" 1>&2' ERR

false && true

Executed it wouldn't cause the failure to be trapped:

+ set -eu
+ trap 'echo "E: failed with exitcode $?" 1>&2' ERR
+ false