Re: history file missing timestamp when HISTFILESIZE reached

[Rob Foehl]

On Sun, 19 Mar 2023, Grisha Levit wrote:

> When HISTTIMEFORMAT is set and history file truncation is performed,
> the first line of the history file (i.e. the timestamp of the first
> entry) seems to always be missing
>
> > /tmp/hist
> HISTTIMEFORMAT= HISTFILESIZE=3 HISTFILE=/tmp/hist bash --norc -in <<<$'1\n2\n3'
>
> $ cat /tmp/hist
> 1
> #1679274410
> 2
> #1679274410
> 3


For whatever it's worth, I'd noticed history truncation to roughly 
HISTSIZE/2 not long after first enabling HISTTIMEFORMAT, and have used 
this mnemonic since:

HISTTIMEFORMAT='%Y-%m-%d %H:%M:%S  '
HISTSIZE=$(( 10000 * 2 ))
HISTFILESIZE=$HISTSIZE

Losing the first timestamp line was a frequent occurrence with unintended 
truncation due to a long-standing Fedora default [1], and my guess as to 
why was something like:

- load history without HISTTIMEFORMAT set
- append a single command line, with no timestamp
- write last (default) 500 lines of "history" back to disk

Though I never tried to confirm this, being primarily concerned with the 
data loss.  Also never tried with an odd number for HISTSIZE.

-Rob

[1] https://bugzilla.redhat.com/show_bug.cgi?id=1871744