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Forcing builtins (interactive, non-interactive shells) to execute in for


From: Matthew Giassa
Subject: Forcing builtins (interactive, non-interactive shells) to execute in fork()'ed process.
Date: Wed, 26 Oct 2016 15:00:20 -0700
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I'm doing some testing with an on-screen colorizer library I wrote,
which does
some PTY manipulation, captures screen output, and renders certain regex
patterns in certain colors. It's designed to help testers notice certain
keywords in a test environment that is not easily automated.

On bash 4.3.36 on Ubuntu 16.04 LTS, I've noticed that all commands seem
to
fork() a separate process, even shell builtins. This makes it easy to
detect
the start of any command by hooking fork() in libc. However, on bash
4.3.11 on
Ubuntu 14.04 LTS, this behavior is not observed (ie: only "disk
commands" cause
a fork(), while builtins do not trigger a fork). Additionally, when
running in a
non-interactive bash instance, shell builtins never cause a fork,
regardless of
whether I'm running a script (ie: ./test.sh), or using bash via the "-c"
option
(ie: bash -c "ls; ls -la").

Is there any way, either via environment variables, BASHRC settings, or
bash
startup flags, or some other means (ie: not recompiling bash) where I
can:

1) Have bash 4.3.11 fork() for every interactive command, like in bash
4.3.36.
2) Have both bash 4.3.11 and 4.3.36 trigger a fork() for builtins being
executed
in a non-interactive shell?

I could potentially use PROMPT_COMMAND to detect the most recent
command, "type"
it, and force running in a subshell, but I want to avoid breaking other
scripts
by changing how commands are actually executed, save for forking.

On a side note, its interesting how bash 4.3.36 does this, even for the
"cd"
command, since launching it in a subshell can't change the CWD for the
parent
process.

Thank you.



============================================================
Matthew Giassa, MASc, BASc, EIT
Security and Embedded Systems Specialist
e-mail:   address@hidden
website:  www.giassa.net




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