`history -r` can not read from /dev/stdin ?
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 ```
Re: `history -r` can not read from /dev/stdin ?
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. -- ``The lyf so short, the craft so long to lerne.'' - Chaucer ``Ars longa, vita brevis'' - Hippocrates Chet Ramey, ITS, CWRUc...@case.eduhttp://cnswww.cns.cwru.edu/~chet/
Re: `history -r` can not read from /dev/stdin ?
2012/8/17 Chet Ramey : > 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. > > > > -- > ``The lyf so short, the craft so long to lerne.'' - Chaucer > ``Ars longa, vita brevis'' - Hippocrates > Chet Ramey, ITS, CWRUc...@case.eduhttp://cnswww.cns.cwru.edu/~chet/ So, How could I accomplish this kind of thing?
Re: `history -r` can not read from /dev/stdin ?
On 8/16/12 10:11 PM, 郑文辉(Techlive Zheng) wrote: > 2012/8/17 Chet Ramey : >> 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, CWRUc...@case.eduhttp://cnswww.cns.cwru.edu/~chet/
Shell comment ignored
Hi, I first noticed this in a shell script under mingw but tested it in bash under Linux and it has the same behaviour. some_var=# What I thought was a comment. The '#' is eaten by the variable assignment so that some_var gets a null value and the rest of the line is not treated like a command. If there is no space after the '#': some_var=#What Then some_var is set to '#What'. Regards, Keith Clifford.
Re: Shell comment ignored
On Thursday 16 August 2012 20:36:45 Keith Clifford wrote: > some_var=# What I thought was a comment. > > The '#' is eaten by the variable assignment so that some_var gets a null > value and the rest of the line is not treated like a command. this is correct (if sometimes confusing to people) behavior. you need to have some byte in there that breaks up the tokens -- be it a tab, space, or newline. -mike signature.asc Description: This is a digitally signed message part.
Re: Shell comment ignored
On 08/16/2012 06:36 PM, Keith Clifford wrote: > > some_var=# What I thought was a comment. ^ Not the first byte of a word. # starts a comment only when it is the first byte of a word. You can write: some_var= #Now this is a comment, since there is space between = and # if you meant for some_var to be assigned the empty string, or write: some_var='# Embedded as part of the assignment' if you meant to include more than just the # in the value assigned to the var. > > some_var=#What > > > > Then some_var is set to '#What'. Yep, and that is correct behavior as mandated by POSIX. -- Eric Blake ebl...@redhat.com+1-919-301-3266 Libvirt virtualization library http://libvirt.org signature.asc Description: OpenPGP digital signature
Re: `history -r` can not read from /dev/stdin ?
2012/8/17 Chet Ramey :
> On 8/16/12 10:11 PM, 郑文辉(Techlive Zheng) wrote:
>> 2012/8/17 Chet Ramey :
>>> 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, CWRUc...@case.eduhttp://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"
