For the last three and a half years, every single command I’ve run from the command line on my MacBook Pro has been logged to a set of log files.

Uncompressed, these files take up 16 MB of disk space on my laptop. But the return I’ve gotten on that small investment is immense. Being able to go back and find any command you’ve run in the past is so valuable, and it’s so easy to configure, you should definitely set it up today. I’m going to share how to do this so you can take advantage of it as well.

Bash Configuration File

You’ll need to configure an environment variable so that it’s loaded in every command line session. On my MacBook Pro, I use the .bash_profile file. On other operating systems, the .bashrc file is an option. See this blog post on .bash_profile vs .bashrc for more on the differences.

PROMPT_COMMAND

The Bash Prompt HOWTO describes the PROMPT_COMMAND environment variable as follows:

Bash provides an environment variable called PROMPT_COMMAND. The contents of this variable are executed as a regular Bash command just before Bash displays a prompt.

We’re going to set the PROMPT_COMMAND variable to be something that logs the most recent line of history to a file. To do this, add the following to your chosen Bash configuration file (.bash_profile for me):

export PROMPT_COMMAND='if [ "$(id -u)" -ne 0 ]; then echo "$(date "+%Y-%m-%d.%H:%M:%S") $(pwd) $(history 1)" >> ~/.logs/bash-history-$(date "+%Y-%m-%d").log; fi'

First, this checks to make sure we’re not root .

If that checks out, it appends a line that includes the current timestamp, the current working directory, and the last command executed to a log file that includes the current date in the filename.

Having the commands stored in separate files like this really helps when you’re trying to find a command you ran sometime last month, for example.

> grep -h logcat ~/.logs/bash-history-2016-04* 2016-04-01.10:18:03 /Users/me 66555 adb logcat 2016-04-01.10:19:56 /Users/me 66555 adb logcat 2016-04-01.11:01:36 /Users/me 66555 adb logcat 2016-04-05.09:50:25 /Users/me/git/android-project 66368 adb logcat 2016-04-05.13:42:54 /Users/me/git/android-project 66349 adb -s emulator-5554 logcat 2016-04-06.10:40:08 /Users/me/git/android-project 66390 adb logcat 2016-04-06.10:48:54 /Users/me/git/android-project 66342 adb logcat

Conclusion

It will only take a few seconds to update your PROMPT_COMMAND so that it logs every command to a file.

And the next time you’re trying to remember the command line options you used with find that one time (but can’t find in your current session’s history), you’ll be able to look it up in the log files.