How can I avoid losing any history lines?

This method is designed to allow you to store a complete log of all commands executed by a friendly user; it is not meant for secure auditing of commands - see securing bash against history removal.

By default, Bash updates its history only on exit, and it overwrites the existing history with the new version. This prevents you from keeping a complete history log, for two reasons:

To solve the first problem, we set the shell option histappend which causes all new history lines to be appended, and ensures that multiple logins do not overwrite each other's history.

To prevent history lines being lost if Bash terminates abnormally, we need to ensure that they are written after each command. We can use the shell builtin history -a to cause an immediate write of all new history lines, and we can automate this execution by adding it to the PROMPT_COMMAND variable. This variable contains a command to be executed before any new prompt is shown, and is therefore run after every interactive command is executed.

Note that there are two side effects of running 'history -a' after every command:

To set all this, use the following in your own ~/.bashrc file:

PROMPT_COMMAND="history -a"

shopt -s histappend

In the above we have also increased the maximum number of lines of history that will be stored in memory, and removed any limit for the history file itself. The default for these is 500 lines, which will cause you to start to lose lines fairly quickly if you are an active user. By setting HISTFILESIZE to a large value we ensure a file big enough so that it is infinite in practice - and by setting $HISTSIZE, we limit the number of these lines to be retained in memory. Unfortunately, bash will read in the full history file before truncating its memory copy to the length of $HISTSIZE - therefore if your history file grows very large, bash's startup time can grow annoyingly high. Even worse, loading a large history file then truncating it via $HISTSIZE results in bloated resource usage; bash ends up using much more memory than if the history file contained only $HISTSIZE lines. Therefore if you expect your history file to grow very large, for example above 20,000 lines, you should archive it periodically. See Archiving History Files below.

PROMPT_COMMAND may already be used in your setup, for example containing control codes to update an XTerm's display bar with your current prompt. If yours is already in use, you can append to it with: PROMPT_COMMAND="${PROMPT_COMMAND:-:} ; history -a"

You may also want to set the variables HISTIGNORE and HISTCONTROL to control what is saved, for example to remove duplicate lines - though doing so prevents you from seeing how many times a given command was run by a user, and precisely when (if HISTTIMEFORMAT is also set).

Note that because PROMPT_COMMAND executes just before a new prompt is printed, you may still lose the last command line if your shell terminates during the execution of this line. As an example, consider: this_cmd_is_never_written_to_history ; kill -9 $$

Using extended attributes

Even after you've convinced bash to record history without truncating the history file, it's still very easy to lose. If you ever start a shell in interactive mode without the shell sourcing your .bashrc for any reason (e.g. via the --rcfile option) bash will default to indiscriminately truncating the history, which could mean losing everything unless you archive and backup the file as detailed in the next sections.

Under Linux and some other OSes, this can be prevented by setting the append-only extended attribute on the history file. Subsequently, open(2) calls without the O_APPEND flag will fail and the file cannot be deleted, moved, truncated, or otherwise modified other than to append data to the end (even by the root user) until the append-only bit is unset. Usually only a root user can set or unset this attribute.

# Linux example on btrfs - setting the append-only flag with chattr(1)

ormaaj-laptop # chattr +a .bash_history
ormaaj-laptop # lsattr -a .bash_history
-----a---------- .bash_history
ormaaj-laptop # rm .bash_history
rm: cannot remove '.bash_history': Operation not permitted
ormaaj-laptop # >.bash_history
bash: .bash_history: Operation not permitted

The exact method and which attributes are supported varies by OS and file system. See this wikipedia article for details. Under (at least) some BSD-like systems and OS X the analogous command is chflags. The append-only feature is split between "user" and "system" versions presumably so non-root users can use it on their own files. Linux appears to have no equivalent way for a non-root user to set/unset append-only.

Prevent mangled history with atomic writes and lock files


Compressing History Files

The result of the above is a history file with a great many duplicate commands. Appending history causes your history file to grow by all the shell's loaded history each time.

More importantly, the main thing we care about with regards to history is being able find previously executed commands. The following script will remove all commands from the history file that are already in there, while keeping the order of the commands intact in such a way that commands you most recently executed will remain at the bottom of the file (ie. keep the last occurrence of a command, not the first).

    awk 'NR==FNR && !/^#/{lines[$0]=FNR;next} lines[$0]==FNR' "$HISTFILE" "$HISTFILE" > "$HISTFILE.compressed" &&
    mv "$HISTFILE.compressed" "$HISTFILE"

After a few months, this compressed my history file from 761474 lines to 2349. Note that this does not preserve the timestamps if you have HISTTIMEFORMAT set.

Archiving History Files

Once you have enabled these methods, you should find that your bash history becomes much more valuable, allowing you to recall any command you have executed at any time. As such, you should ensure your history file(s) are included in your regular backups.

You may also want to enable regular archiving of your history file, to prevent the full history from being loaded into memory by each new bash shell. With a history file size of 10,000 entries, bash uses approximately 5.5MB of memory on Solaris 10, with no appreciable start-up delay (with $HOME on a local disk, I assume? -- GreyCat). With a history size of 100,000 entries this has grown to 10MB with a noticeable 3-5 second delay on startup. Periodic archiving is advisable to remove the oldest log lines and to avoid wasting resources, particular if RAM is at a premium. (My largest ~/.bash_history is at 7500 entries after 1.5 months.)

This is best done via a tool that can archive just part of the file. A simple script to do this would be:

This script removes enough lines from the top of the history file to truncate its size to X lines, appending the rest to ~/.bash_history.archive. This mimics the pruning functionality of HISTFILESIZE, but archives the remainder rather than deleting it - ensuring you can always query your past history by grepping ~/.bash_history*.

Such a script could be run nightly or weekly from your personal crontab to enable periodic archiving. Note that the script does not handle multiple users and will archive the history of only the current user - extending it to run for all system users (as root) is left as an exercise for the reader.

Archiving by month

mkdir -p ~/bash_history
shopt -s histappend
PROMPT_COMMAND="history -a; history -n; $PROMPT_COMMAND"

# If your bash is older than 4.3, set these to a large number instead
# else your history files will be empty

HISTFILE=~/bash_history/$(date +%Y-%m)

h() {
    grep "$@" ~/bash_history/*

BashFAQ/088 (last edited 2016-02-04 18:47:59 by ormaaj)