Re: history -f filename

[Chet Ramey]

On 11/30/24 9:36 PM, Dan Jacobson wrote:
> $ history |wc - $HISTFILE|sed \$d
>     7622   75741  532254 -
>    14973   29625  320996 /home/jidanni/.bash_history_jidanni
> Works as expected.

Sure. You have fewer in-memory history entries than in your history file,
which is kind of unusual, but not unheard of.

> $ (a=~/.bash_history_jidanni-emacs; HISTFILE=$a history |wc - $a|sed \$d)
>     7625   75780  532531 - ### I expected to see about 1399/2 here.
>     1399    3535   35312 /home/jidanni/.bash_history_jidanni-emacs
> We see for simple
> $ history
> output, there's no way to change the file it reads, 

It doesn't read a file. After reading HISTFILE when the shell starts up,
the only thing changing HISTFILE does is to change where the shell writes
the history when it exits. You have to use the history options that read
and write files to import/export this in-memory list.

at least with the
> current (same PID) shell.

This is just wrong. You can write a shell function and temporarily swap
history files to populate the in-memory list as much as you want without
forking a new shell.

> > > > > > "CR" == Chet Ramey <chet.ramey@case.edu> writes:
> CR> Use `history -r'?
> That would change files (by more than just one line).

I think there's a fundamental misunderstanding here about the in-memory
nature of the history list.

You can always generate a temp file name, write the current history to it,
read a new history file into memory, do what you want, optionally write it
back out, and then reload the history from the temporary file.

> All I want is a read-only operation.
>
> CR> Because those are the options that act on the history file. The rest act
> CR> on the in-memory history list.
>
> > > So maybe a new -f is needed:
> > > $ history -f filename
> > > to simply read from filename.
>
> CR> What exactly would this do?
>
> It would do just like plain
> $ history
> but instead of reading from the in memory list,
> (which is about the same as $HISTFILE + or - one line)

It is not, unless you're doing something like syncing the history list to
the history file from $PROMPT_COMMAND. Look at your first example.

> it would read from the given file.

Use a shell function as described above. But there's little difference
between that and `cat', unless you want to restrict the output to a
subset of the history file entries or take advantage of timestamps.


--
``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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