Strange bash history behaviour when running multiple sessions

To understand the behaviour of the bash history first you have to know the following:

1. There is the history in the history file.
2. There is the history in the memory of a bash process.
3. The history in the memory of one bash process is not synced with the history in the memory of any other bash process.
4. The history in the memory of a bash process is not synced with the history in the file, unless explicitly asked to or during some specific event (see below).

Using the default settings, the lifecycle of a bash session with regard to the history is as follows:

1. During startup bash will read the history file. The content of the history file is now in the memory of the bash process.
2. During normal use only the history in memory is manipulated.
3. During shutdown the history in memory is written to the history file, overwriting any previous content of the history file.

The seemingly nondeterministic behaviour you have observed is mostly because the content of the history file is always the history of the last closed bash session, and bash only reads the history file during startup.

Read the bash manual for a more detailed explanation of the startup and shutdown process.

Note that with default settings I mean the default settings from bash. Your distribution might have provided a .bashrc (or /etc/bash.bashrc) which change this behaviour.

By enabling the shell option histappend you can tell bash to append instead of overwriting the history file. You can enable histappend using the command shopt -s histappend. To have this option always enabled you have to put the command in your .bashrc (or other initialization file). Read more about the shopt command in the bash manual

Note that enabling histappend will not reduce the seemingly nondeterministic behaviour by much. This is because every bash session still has it's own history in memory. It is possible to have a mostly synchronized bash history. There is a guide how to get every bash process to have a mostly synced history in a thread on stack overflow.

using the builtin command history you can explicitly tell bash to read the history from file to memory, or the write from memory to file. For example: history -r will read the content of the file and append it to the history in memory. history -w will write the current history from memory to file, overwriting the previous content. This is basically what happens during shutdown. Read more about the history command in the bash manual

For completeness here is a list of the internal variables which modify the history behaviour:

• HISTFILE: the file to read from and write the history to.
• HISTFILESIZE: the maximum line count for the history file.
• HISTSIZE: the maximum line count for the history in memory.
• HISTCONTROL, HISTIGNORE, HISTTIMEFORMAT: not relevant for this discussion. Read the bash manual for details.

http://www.gnu.org/software/bash/manual/bashref.html#Using-History-Interactively

You might be able to manipulate how the history file is written to with one of the terminals, i.e., execute "history -a" or "history -w" in the terminal you want to save history, and then "history -r" in the other terminals. Depends on what you want to do.

AFAIK, the bash commands are saved after the SSH session is terminated. So, the commands are not saved when a session terminates abnormally (for example, due to network failure). I am talking here about SSH sessions. The local terminals may use similar approach.

When opening multiple sessions at the same time, the commands typed on one session are not seen on the other while they are both active. However, you will see these commands when you terminate your session re-open it.