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:

• If a user is logged in multiple times, the overwrite will ensure that only the last shell to exit will save its history.
• If your shell terminates abnormally - for example because of network problems, firewall changes or because it was killed - no history will be written.

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:

• A new login will be able to immediately scroll back through the history of existing logins. So if you wish to run the same command in two sessions, run the command and then initiate the second login and you will be able to retrieve the command immediately.
• More negatively, the history commands of simultaneous interactive shells (for a given user) will be intertwined. Therefore the history is not a guaranteed sequential list of commands as they were executed in a single shell. You may find this confusing if you review the history file as a whole, looking for sections encapsulating particular tasks rather than searching for individual commands. It's probably only an issue if you have multiple people using a single account simultaneously, which is not ideal in any case.

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

•  unset HISTFILESIZE
   HISTSIZE=10000
   PROMPT_COMMAND="history -a"
   export HISTSIZE PROMPT_COMMAND
  
   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 unsetting $HISTFILESIZE, we ensure that bash never removes lines from the history file - 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).

Finally, 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 $$

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:

•  #!/bin/bash
   umask 077
   max_lines=10000
  
   linecount=$(wc -l < ~/.bash_history)
  
   if (($linecount > $max_lines)); then
           prune_lines=$(($linecount - $max_lines))
           head -$prune_lines ~/.bash_history >> ~/.bash_history.archive \
                  && sed -e "1,${prune_lines}d"  ~/.bash_history > ~/.bash_history.tmp$$ \
                  && mv ~/.bash_history.tmp$$ ~/.bash_history
   fi

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.