Re: [PLUG] Bash History question

[JP Vossen <jp@jpsdomain.org>]

> Date: Thu, 13 Sep 2007 09:22:28 -0400
> From: Dan Widyono <dan@widyono.net>
> Subject: Re: [PLUG] Bash History question
>
> On Thu, Sep 13, 2007 at 02:00:14PM +0100, Stephen Gran wrote:
> > On Thu, Sep 13, 2007 at 08:00:07AM -0400, Mag Gam said:
> > > Just started to use Linux exclusively, and I have a question pertaining Bash
> > > History. I know the history is logged in .bash_history or ($HISTFILE var).
> > > My question is, when I fork a new shell ie.
> > > $ echo $$
> > > 2232
> > > $ bash
> > > $ echo $$
> > > 4434
> > >
> > > Where is the history kept for bash.4434? and what happens to the history of
> > > bash.2232? How does this work, very confusing to me....
>
> In addition to Stephen's answer regarding the writing of the history, each
> shell process holds its own idea of its command history in memory.  As
> Stephen mentioned, it isn't written to disk until logout.
>
> Since each process has its own idea of memory (controlled by the virtual
> memory management of the kernel), PID2232 will have one idea of the history
> "array" while PID4434 will have a separately maintained idea.

I'll add a bit to the two great answers already given.

If you are interested in synchronizing the history across different 
sessions/shells/pids or however you want to think about it, you can.  At 
a high level, there are two ways to do it: manually or "automatically." 
 <Shameless plug: this is basically recipe 16.11 of _bash Cookbook_>

For the manual method, you simply use the built-in bash tools to sync 
into the current shell:
	history -a
	history -n
	--or-- "history sync"
	alias hs='history -a ; history -n'
	
'-a' means to append history lines from this session to the history file.
'-n' means to read all history lines not already read from the history 
file and append them to the history list.

For the automatic method, you can use:
	PROMPT_COMMAND='history -a ; history -n'

The problem here is that $PROMPT_COMMAND contains a command to run just 
before the prompt is displayed, *every time*.  So anything you put there 
gets run a *lot* and may slow down an older or heavily used system.  I 
tend to use a history with 5000 lines, so I personally would not use 
$PROMPT_COMMAND.

Also, the manual method has a bit of a chicken and egg problem in that 
if you sync shell_1, then shell_2, then shell_3, 1&2 won't have 3's 
history.  You basically have to loop through them all twice at one time 
to be sure.


See also 'help history' from the bash command line.

Hope this is useful,
JP
----------------------------|:::======|-------------------------------
JP Vossen, CISSP            |:::======|        jp{at}jpsdomain{dot}org
My Account, My Opinions     |=========|      http://www.jpsdomain.org/
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