Where is the setting that disallows duplicate command history?

Question

Let's say I type the following sequence of commands:

$ cd
$ ls
$ ls

When I go back in history by pressing up twice, I end up with cd, instead of ls.

I've noticed this isn't always the case, so it likely is some bash setting somewhere. Which is it?

Answer 1

The is controlled by the HISTCONTROL variable. If is contains ignoredups, then duplicate commands will not be saved in the history. Without this string, they will be saved. This would be normally set in .profile or .bashrc, using something like

export HISTCONTROL=ignoredups

Other values that can be used in HISTCONTROL (multiple can be used, separated by :)

• ignorespace any commands starting with a space are not added to the history
• erasedups duplicate commands are erased from the history, so the history only contains the latest command
• ignoreboth identical to ignoredups:ignorespace

Answer 2

HISCONTROL=ignoredups

http://www.gnu.org/software/bash/manual/html_node/Bash-Variables.html#Bash-Variables