You keep two bash terminals open all day, and commands typed in the first are missing from history after the second one exits. How does bash write its history file, and which settings stop concurrent sessions from clobbering each other?

## The model: memory now, file later Bash's history is a per-process, in-memory list. At startup an interactive shell reads `$HISTFILE` (defaulting to `~/.bash_history`) into that list. Everything you type is added to the list, and everything `Ctrl-R` or the up-arrow searches comes from the list — not from the file. The file is touched again only when the shell exits, or when you explicitly ask via the `history` builtin. That single design fact explains the symptom. Two shells opened at 09:00 both loaded the same snapshot. Each has been appending to its own private list. When shell A exits it writes *its* list to the file. When shell B exits it writes *its* list — and with the default `histappend` off, that write truncates the file first. B's lines survive; A's are gone. ## The settings that matter ```bash shopt -s histappend # append on exit instead of overwriting HISTSIZE=100000 # lines kept in memory for this session HISTFILESIZE=200000 # lines kept in the file after truncation HISTCONTROL=ignoreboth # ignorespace + ignoredups HISTIGNORE='ls:bg:fg:history' # patterns never recorded at all HISTTIMEFORMAT='%F %T ' # store and display timestamps ``` - **`shopt -s histappend`** is the fix for the clobbering itself. It belongs in `~/.bashrc`, because it is per-shell state. - **`HISTSIZE` vs `HISTFILESIZE`** is the classic confusion: the first bounds the in-memory list, the second truncates the file after it is written. Appending faithfully into a file capped at 500 lines still leaves you with 500 lines. In bash 4.3 and later, a value of `-1` or an empty value means unlimited. - **`HISTCONTROL`** takes `ignorespace` (a command starting with a space is not recorded), `ignoredups` (consecutive duplicates collapse), `ignoreboth` (both), and `erasedups` (remove all previous copies of the line). `ignorespace` is the habit people use to keep a one-off secret out of the file — a convenience, not a security control: the value still appears in the process table and in whatever the command wrote. - **`HISTIGNORE`** is a colon-separated pattern list for commands never worth recording. ## Sharing history immediately If you want a command typed in one terminal to be findable in another a second later, do the file work on every prompt instead of at exit. `PROMPT_COMMAND` holds a command bash runs before printing each primary prompt: ```bash shopt -s histappend PROMPT_COMMAND='history -a; history -n' ``` - `history -a` appends the lines added in *this* session since the last append to the file. - `history -n` reads lines appended to the file by *other* sessions into this session's list. - The stricter variant `history -a; history -c; history -r` clears the in-memory list and re-reads the whole file, giving perfectly merged history at the cost of interleaving every shell's commands into your up-arrow — most people find that worse, not better. Be aware `PROMPT_COMMAND` is a shared hook: append to it rather than assigning over it if other tooling already uses it. ## Searching what you have `Ctrl-R` runs an incremental reverse search through the in-memory list; `history` prints it with line numbers; `!123` re-runs a numbered entry and `!!` the previous one. Because all of these read the in-memory list, none of them will show another session's commands until that session's lines have been appended and read back in. ## Failure modes to expect - **A shell killed with `SIGKILL`, or a machine that loses power, writes nothing** — that whole session's history is lost. Per-prompt `history -a` is the mitigation. - **`HISTFILE` unset** in a startup file disables saving entirely; some hardened profiles do this deliberately. - **A read-only or root-owned `~/.bash_history`** (a classic result of running a shell under `sudo` without `-i`) makes every write fail. - **Huge history files** slow startup, because the whole file is parsed into memory at every shell start. ## What to put in ~/.bashrc A reasonable baseline is `shopt -s histappend`, generous `HISTSIZE`/`HISTFILESIZE`, `HISTCONTROL=ignoreboth`, `HISTTIMEFORMAT` for timestamps, and per-prompt appending if you live in many terminals at once.