Linux Section

Bash HISTCONTROL

I happened to come across Linux Tips: take control of your bash_history. This explains that setting "export HISTCONTROL=ignoreboth" causes Bash not to store duplicate commands in history. For example, if you are checking "ps" waiting for some process to end:

ps -e | grep foo

and keep pressing up arrow every few seconds to repeat it, you'll end up with a long list of those in your history if this isn't set. If it is set, you get only the last.

Actually there's much more to this than that little post exlained.

First, there's actually several values HISTCONTROL can take. On newer bash, it can be set to "ignorespace", "ignoredups", "ignoreboth" or "erasedups". Bash 2.05 doesn't have "erasedups".

If you want manual control over history, set "ignorespace". Then remember to use a space to precede any command you don't want to remember in history. Bash ignores leading spaces, so "  ls" is as good as "ls", but with "ignorespace" set, "  ls" won't be saved.

The "ignoredups" setting ignores repeated lines. Typing "ls" twenty times in a row only ends up with one of them in history. However, typing "ls" and then "ps" and then "ls" again will store "ls" and "ps" every time - unless you have bash 3 and set "erasedups". If that's set, no duplicates get entered in bash history at all.

You can also control bash history with HISTIGNORE. For example, if I
"declare -x HISTIGNORE=ls:ps"
, neither "ls" or "ps" will appear in my history. To ignore all two character commands, use "declare -x HISTIGNORE='??'".

I don't like extra commands cluttering up my history file, so I use "ignoreboth". When available, I use "erasedups", but if you are using history as a record of commands rather than just for the convenience of repetition, you don't want that.

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Sun Jun 8 02:22:30 2008:   anonymous


I am hoping that the following is just for example's sake, but if not, type "man watch" to learn about your new best friend- in this case the simplest version is: watch "ps -e | grep foo"

"if you are checking "ps" waiting for some process to end:
ps -e | grep foo
and keep pressing up arrow every few seconds to repeat it,"




Sun Jun 8 10:39:41 2008:   TonyLawrence


Yes, it was just for example, but keep in mind that bash runs on lots of systems that do NOT have "watch". or anything like it.

Old Unix folk like me would be unlikely to use it anyway. Out of long habit, we'd write

while :
ps -e | grep foo
sleep 2
done

or whatever. So "watch" will never be our new best friend :-)





Mon Jun 9 01:18:00 2008:   BigDumbDinosaur
http://bcstechnology.net

Hey! Watch it with the "old UNIX folks" stuff. I resemble that remark!



Thu Apr 7 15:33:29 2011:   Brade
http://www.bradezone.com/

Thanks for the tips--I've seen similar advice from other pages on the web, including the one you linked. The problem is that, on my system, once the bash session ends, the duplicates reappear the next time a session is opened.

The problem was described by a stackoverflow member here: http://stackoverflow.com/questions/338285/prevent-duplicates-from-being-saved-in-bash-history

This is the exact problem I'm having. No matter if I use "ignoredups" or "erasedups" or both, The history updates correctly while my session is open, but once I close Terminal and reopen later, those duplicates are back. Extremely frustrating, to say the least. You wouldn't happen to know the reason for this? (Or can you duplicate the problem on a OS 10.6.7 machine?)



Thu Apr 7 16:44:12 2011:   TonyLawrence


Running OS X 10.6.7 with
HISTCONTROL=ignoreboth

I get no dupes.

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