Ubuntu – centralized history file for all the commands run from various terminals

The bash session that is saved is the one for the terminal that is closed the latest. If you want to save the commands for every session, you could use the trick explained here.

export PROMPT_COMMAND='history -a'

To quote the manpage: “If set, the value is executed as a command prior to issuing each primary prompt.”

So every time my command has finished, it appends the unwritten history item to ~/.bash_history before displaying the prompt (only $PS1) again.

So after putting that line in /etc/bash.bashrc I don’t have to find myself reinventing wheels or lose valuable seconds re-typing stuff just because I was lazy with my terminals.

Anyway, you'll need to take into account that commands from different sessions will be mixed in your history file so it won't be so straightforward to read it later.

See also:

• https://unix.stackexchange.com/questions/1288/preserve-bash-history-in-multiple-terminal-windows

It looks like you're trying to access the other terminal's history before the sync takes place. PROMPT_COMMAND is executed right before a new prompt is printed, i.e., after you run a command and before you type the next command. So it won't happen right away in T1; you have to cause a new prompt to be displayed.

To test this, try this variant on your steps (I added an extra <enter> in T1):

T1:
ls -lah <enter>
# the list of files and directory is shown

T2:
cd Documents <enter>

T1:
<enter>
<up> (i.e. I press the up arrow)

With this extra press of enter, you get a new prompt, which runs PROMPT_COMMAND and syncs your history, and so I would expect this up arrow to retrieve the cd instead of the ls, as you wanted. Unfortunately, I don't think there is a way to make the sync happen instantaneously in all terminals without running any commands as you seem to want; effectively this would require all of your login sessions to be synchronizing their history lists continuously all the time, which would be a huge waste of CPU and disk throughput.