Re: history bug: new shell separates lines from the history file

[Chet Ramey]

On 11/21/24 9:59 AM, Michael Tosch wrote:

> Bash Version: 5.1
> Patch Level: 16
> Release Status: release
>
> Description:
>
> In bash I do
>
> bash-5.1$
> bash-5.1$ shopt| grep hist
> cmdhist         on
> histappend     off
> histreedit     off
> histverify     off
> lithist         on
> bash-5.1$ echo $HISTTIMEFORMAT
> %F %T
> bash-5.1$ for i in a b c
> > do
> >    echo $i
> > done
> a
> b
> c
> bash-5.1$ history 5
>    503  2024-11-21 13:44:22 ls
>    504  2024-11-21 13:44:31 shopt| grep hist
>    505  2024-11-21 13:45:11 echo $HISTTIMEFORMAT
>    506  2024-11-21 13:45:26 for i in a b c
> do
>    echo $i
> done
>    507  2024-11-21 13:45:51 history 5
> bash-5.1$ exit
>
>
> In a new shell I get
>
> bash-5.1$ history 10
>    491  2024-11-21 13:44:22 ls
>    492  2024-11-21 13:44:31 shopt| grep hist
>    493  2024-11-21 13:45:11 echo $HISTTIMEFORMAT
>    494  2024-11-21 13:45:26 for i in a b c
>    495  2024-11-21 13:48:30 do
>    496  2024-11-21 13:48:30   echo $i
>    497  2024-11-21 13:48:30 done
>    498  2024-11-21 13:45:51 history 5
>    499  2024-11-21 13:48:27 exit
>    500  2024-11-21 13:48:46 history 10
> bash-5.1$

This is what happens when the calling application (bash, in this case)
doesn't set the flag that tells the history library to treat lines as
potential timestamps before it reads the history file.

Bash does this by setting it when you set HISTTIMEFORMAT to a value. If you
don't set HISTTIMEFORMAT, the history library uses the historical behavior
of line-oriented history, where each line gets a timestamp that bash can
manage. If you do, the history library assumes timestamps are command
delimiters.

The idea is that history files are usually unique per application. If a
previous session writes timestamps to the history file, and the same
application reads it (with the same settings managed in an application-
specific way) in a subsequent session, you'll get the same set of entries.
Bash uses HISTTIMEFORMAT to control writing timestamps, and the same
thing to determine whether or not history entries should be timestamp-
delimited.

This came in in bash-4.4. Here's a message from 2015 describing it:

https://lists.gnu.org/archive/html/bug-bash/2015-12/msg00206.html

--
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    chet@case.edu    http://tiswww.cwru.edu/~chet/

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