Security

How to effectively clear your bash history

If you're serious about security on your Linux machines, you might want to clear the bash history. Learn how to do this more effectively than with just a single command.

Image: Jack Wallen

On your Linux machines, a history of your bash commands is retained. This is great when you need to repeat a command or can't remember exactly how you executed a command in a previous session. However, this can also be seen as a security issue. What if someone gains access to your machine, opens a terminal window, and checks through your bash history to see what commands you've run?

Bash has a handy way to clear the history: issue the command history -c. There's a slight problem with that approach. Let me explain.

First off, your bash history is retained in the file ~/.bash_history. When you have a terminal open, and you issue a command, it writes the command to the history file. So issuing history -c will clear the history from that file. The problem comes about when you have multiple terminal windows open.

Say you have two terminal windows open and you issue history -c from the first one and close that window. You then move to the second terminal window, and you type the exit command to close that window. Because you had a second bash window open, even after running the history -c command in the first, that history will be retained. In other words, the history -c command only works when it is issued from the last remaining terminal window.

How do you get around that? You empty the .bash_history file either on a per-instance basis or by using a crontab job to do it regularly. If security is a serious matter for you, consider setting up the crontab job. Here's how.

SEE: Linux Foundation launches badge program to boost open source security (ZDNet)

Clearing bash history on a regular basis

Before I show how to set up the crontab job for this, know that the ~/.bash_history file can be cleared with the command:

cat /dev/null > ~/.bash_history

That will empty out the contents of the file, but keep the file in place.

Let's say you want to clear the .bash_history file for user olivia (who administers your Linux server) at 11:00 p.m. every day. You would create a cron job under the olivia account. To do that, log in as the user olivia, open a terminal window, and issue the command crontab -e. When the crontab editor opens, enter the following:

00 23 * * * cat /dev/null > ~/.bash_history

Save that file and cron will start clearing out olivia's history at 11:00 p.m. every day.

A surefire method

This is a surefire method of clearing out your bash history. Don't always rely on the history -c command, because you never know when a second (or a third) terminal is still open, ready to keep that history retained.

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About Jack Wallen

Jack Wallen is an award-winning writer for TechRepublic and Linux.com. He’s an avid promoter of open source and the voice of The Android Expert. For more news about Jack Wallen, visit his website jackwallen.com.

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