I've been implementing something in my shell where I'm
using multiple history files, and I switch between
them. This changes the shell's "identity," to
basically swap its history file with another. This is being
used to have per-project histories.
The procedure I am using to "switch" is something like
this:
history -a # flush anything left
history -c # start anew
HISTFILE=newhist # prepare to load this one
history -r # and do it (identities changed)
however I have also tried this:
history -a
history -cr newhist
HISTFILE=newhist
At this point we should be able to enter new commands,
and when we decide to, "history -a" (append them to the
`newhist' file).
However, bash fails to do this in the case that the new
history file is either very small, or "new," i.e. zero
bytes.
I believe I have tracked down the reason this is
happening:
`bash' keeps the notion of how many lines it has
inserted into the history since the history file was
read (see `bashhist.c:history_lines_this_session').
This amount is zeroed after a successful append.
If this amount ever becomes greater than or equal to
the size of the new history that has been read (which
is what is returned from the global `history_offset' in
`history.c:where_history()'), bash will never allow the
append to succeed again (see the check on whether
history should be appended or not, which can be found
in `bashhist.c:maybe_append_history()').
BUG: This situation arises if there are a few commands
in between the successful append to the old history,
and the import of the new history, IFF the new history
is smaller than said number.
BUG: This happens also for NEW histories. Even with a
"history -cr"
(resulting in `history_lines_this_session' value of `1')
instead of
"history -c; history -r"
(`history_lines_this_session' value of `2'), the value
is still greater than zero, which is the size of a new
history file just read, so it will NEVER work again and
all subsequent history will NOT be written with a
"history -a", which silently fails to append data when
it should.
Note that on the append which occurs at shell exit,
the data WILL be appended, for two reasons:
(1) the test in `maybe_append_history()':
history_lines_this_session < where_history()
differs from that in `maybe_save_shell_history()':
history_lines_this_session <= where_history()
^^
(2) the history is appended unconditionally anyways if
`force_append_history' is set (which corresponds
to `shopt -s appendhist'), whereas no such
condition is checked for in `maybe_append_history()'.
I am wondering, why not zero `history_lines_this_session'
upon a successful "history -c" ? As it is, clearing the
history does not change the number at all and I can
keep entering commands and clearing them all day;
`history_lines_this_session' continues to rise. What
reason is there to keep this variable with a positive
value after all history settings are cleared? There's
nothing to write.
The following demonstrates the problem:
$ id -un # starting as self
smcdermott
$ sudo -u test rm -f ~test/.bash_history # give test account
$ sudo -u test touch ~test/.bash_history # empty history
$ sudo su - test # become test user
$ wc -l $HISTFILE # was indeed empty
0 /home/test/.bash_history
$ shopt -s histappend # force append on exit
$ logout
$
$ sudo su - test # now go back and load
$ wc -l $HISTFILE # 2 commands appended
2 /home/test/.bash_history
$ cp $HISTFILE $HISTFILE.new # which we stow
$ history -a # note append works
$ wc -l $HISTFILE
5 /home/test/.bash_history
$ history -cr $HISTFILE.new # clear, load stowed 2
$ HISTFILE=$HISTFILE.new # switch to stow file
$ test 1 # add a few commands
$ test 2 # that should be
$ test 3 # appended
$ history -a # like so
$ wc -l $HISTFILE # but they're not
2 /home/test/.bash_history.new
Now note when I add ONE extra command, otherwise unchanged:
-----------------------------------------------------------
$ id -un
smcdermott
$ sudo -u test rm -f ~test/.bash_history
$ sudo -u test touch ~test/.bash_history
$ sudo su - test
$ wc -l $HISTFILE
0 /home/test/.bash_history
$ shopt -s histappend
$ test 1 # <--- here makes 3 to save
$ logout
$ sudo su - test
$ wc -l $HISTFILE
3 /home/test/.bash_history
$ cp $HISTFILE $HISTFILE.new
$ history -a
$ wc -l $HISTFILE
6 /home/test/.bash_history
$ history -cr $HISTFILE.new
$ HISTFILE=$HISTFILE.new
$ test 1
$ test 2
$ test 3
$ history -a
$ wc -l $HISTFILE
10 /home/test/.bash_history.new # this time it appended!!
(Note that I used control-d to logout, that's why it was not
saving the `logout' in history.)
...
I believe this shows the bug. I suggest zeroing
`history_lines_after_session' upon "history -c" OR having
`force_append_history' cause it to do just that in
`maybe_append_history()' in addition to `maybe_save_history()'
as it does already.
Please keep me copied as I am not on the list. Thanks.
--
Scott
