Re: A snapshot

[Maroloccio]

On 16/05/2020 23:27, Greg Wooledge wrote:
> On Sat, May 16, 2020 at 09:10:07AM -0700, address@hidden wrote:
> > I would like to create a file -- call it "snapshot" -- that will record things
> > like the current command history and the value of all shell and env variables.
> > And, then, at a later time, possibly from a different terminal and invocation
> > of bash, access the file, so that to the extent practicable, it will be as
> > though I continued the bash session at the time of snapshot's creation.
>
> Well... there are several things you'll need to save in a machine-readable
> format.  The challenge is thinking of everything.
>
> The 'declare -p' command gets you machine-readable copies of all shell
> and environment variables, but not functions.  The 'declare' commands
> gets you variables and functions, but is missing the distinction between
> shell and environment variables.  I'd probably go with 'declare -p' for
> the variables, and then pick up the functions separately.
>
> So, then 'declare -pf' for functions.
>
> 'alias' for aliases.
>
> 'shopt -p' for shell options.
>
> I'll let you figure out how to get shell history.  Maybe setting HISTFILE
> to a temporary file, and keeping that separate from the other settings?
> History is just a mess, and is not one of the things I've spent a lot of
> time trying to wrap my head around.
>
> Maybe 'stty -a' to save the terminal state?  But I doubt you'd want to
> restore the terminal in a nonstandard state anyway.
>
> Of course, you will lose all running child processes, but I'm assuming
> you've already come to terms with that.

The cases Greg mentioned seem like the only practical and common cases.

On Linux, if you had a file descriptor redirection you wanted to
recreate upon returning, such as:

exec 3>/tmp/foo

you could look at /dev/fd/ for file descriptor numbers and then, using
sudo, find your shell in /proc via $$ and readlink in the fd directory
to find out where those descriptors pointed to, e.g.:

$ sudo readlink /proc/$$/fd/3
[sudo] password for maroloccio:
/tmp/foo

If you needed open modes to be recreated too, you'd have to do more
work.

As Greg said, you would not have child processes, so file descriptor
redirection to processes, rather than files, would not work unless you
restarted the children, e.g.:

exec 4> >(grep bar)

would only report a "pipe:[n]" kernel object via readlink.

You could lsof to see which process is attached to that pipe and from
there `ps -o args $pid' to find out more, but recreation would be too
brittle due to lost state.

I am assuming you don't need anything core dumped and resurrected.