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I am trying to build a pre-exec checker in bash (using bash-preexec.sh)

In order to fail the command when the pre-exec hook fails, I need to run shopt -s extdebug (which fails the command when trap DEBUG command returns non-zero RC).

If I run shopt -s extdebug in my shell as a command, this works perfectly fine (the preexec checker just grepped for "OK" in a file containing word "FAIL", so command shouldn't execute):

> grep shopt .bashrc
> bash
bash-4.1$ echo TEST2
TEST2   <=== Expected, since I didn't turn on extdebug yet
bash-4.1$ shopt -s extdebug > /dev/null 2>&1
bash-4.1$ echo TEST2
bash-4.1$ # WHAT WAS EXPECTED. Command failed to execute.

However, when I add the command to my .bashrc, it has no effect:

> grep shopt .bashrc
shopt -s extdebug > /dev/null 2>&1
> bash
bash: /usr/share/bashdb/bashdb-main.inc: No such file or directory
bash: warning: cannot start debugger; debugging mode disabled
bash-4.1$ echo TEST2
TEST2

What am I doing wrong, and how can I execute this in .bashrc (I need to execute it anytime the user logs in)?

  • Not sure if this belongs here or unix.SE. Please migrate if the latter. – DVK Jan 8 '18 at 23:31
  • Most likely your .bashrc is not used in your login shell. It is usually the case that you call it from .profile. – eckes Jan 9 '18 at 8:36
  • @eckes - sorry, that does not seem to be the case: (1) .bashrc is executed because the pre-exec hooks ARE getting installed successfullly (and they are in .bashrc). (2) I just moved the code from .bashrc to .profile and the behavior was 100% same as with .bashrc (the hooks work but the extdebug doesn't have any effect till I explicitly run it in the shell as a command) – DVK Jan 9 '18 at 14:40
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OK, figured out the answer.

    set -o functrace > /dev/null 2>&1

is required (in addition to shopt -s extdebug) to have the functionality work in sub-shells. And somehow, .bashrc is treated as supershell of actual interactive shell, in a sense, if you don't add this, it won't take effect on interactive command prompt.

So, you need to add both command to .bashrc

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