When and how is bash_history appended to?

As far as I can tell, the history of an individual shell is only appended to the .bash_history file when the shell is closed. This means that I frequently lose individual shells' histories on crashes / kernel panics.

Why does the history only get written on closing a session? Is there a way to have each command written at the time of execution? Is there any reason I should not want to have each command written at the time of execution (e.g., performance issues when writing to an enormous bash_history file)?

(I'm using Mac OS X 10.6 and running many tabbed terminals simultaneously with iTerm. I have histappend set along with some other options.

export HISTSIZE=1000000
export HISTTIMEFORMAT="%d/%m/%y %T "
shopt -s histappend

)

You're missing one line in your .bashrc. Add:

PROMPT_COMMAND="$PROMPT_COMMAND;history -a"

after

shopt -s histappend

PROMPT_COMMAND is executed just before the next prompt is printed (IIRC). You're right about when the history is written, by the way. The modified prompt command will flush the history.