Re: `history -r` can not read from /dev/stdin ?

[Techlive Zheng]

2012/8/17 Chet Ramey <chet.ramey@case.edu>:
> On 8/16/12 10:11 PM, 郑文辉(Techlive Zheng) wrote:
>> 2012/8/17 Chet Ramey <chet.ramey@case.edu>:
>>> On 8/16/12 9:17 AM, 郑文辉(Techlive Zheng) wrote:
>>>> I was trying to reload the bash history file which changed by another
>>>> bash session with the following commands, but it wouldn't work, please
>>>> help me, why?
>>>>
>>>> ```
>>>> new_history=$(history -a /dev/stdout)
>>>> history -c
>>>> history -r
>>>> echo "$new_history" | history -r /dev/stdin
>>>> ```
>>>
>>> One possible cause that springs to mind is the fact that the `history -r'
>>> at the end of the pipeline is run in a subshell and cannot affect its
>>> parent's history list.
>>
>> So, How could I accomplish this kind of thing?
>
> Why not just use a regular file?
>
> --
> ``The lyf so short, the craft so long to lerne.'' - Chaucer
>                  ``Ars longa, vita brevis'' - Hippocrates
> Chet Ramey, ITS, CWRU    chet@case.edu    http://cnswww.cns.cwru.edu/~chet/

Actually, I was tring to erase duplicate entries and share history
across bash sessions.'erasedups' in `HISTCONTROL` only have effect for
history list in the memory, so my solution is to load entire history
file into memory and save it after every command finished.

Here is what I am currently have in .bashrc, and it works as expected.

    reload_history() {
        local HISTHASH_NEW=`md5sum $HOME/.bash_history | cut -d' ' -f1`
        if [ "$HISTHASH" = "$HISTHASH_NEW" ]; then
            history -w
            # This is necessay because we need
            # to clear the last append signture
            history -c
            history -r
        else
            HISTTEMP=`mktemp`
            history -a $HISTTEMP
            history -c
            history -r
            history -r $HISTTEMP
            history -w
            rm $HISTTEMP
        fi
        HISTHASH=`md5sum $HOME/.bash_history | cut -d' ' -f1`
    }

    export PROMPT_COMMAND="reload_history;$PROMPT_COMMAND"

Considering `mkemp` then remove the temp file on every prompt command
is a little bit expensive, I want to directly pipe the output of the
`history -a` to `hisotory -r` like below, unfortunately, this wouldn't
work, because `history -r` could not handle /dev/stdin.

reload_history() {
        local HISTHASH_NEW=`md5sum $HOME/.bash_history | cut -d' ' -f1`
        if [ "$HISTHASH" = "$HISTHASH_NEW" ]; then
            history -w
            # This is necessay because we need
            # to clear the last append signture
            history -c
            history -r
        else
            new_history=$(history -a /dev/stdout)
            history -c
            history -r
            echo "$new_history" | history -r /dev/stdin
            history -w
        fi
        HISTHASH=`md5sum $HOME/.bash_history | cut -d' ' -f1`
    }

    export PROMPT_COMMAND="reload_history;$PROMPT_COMMAND"