If you've spent any time on the Unix command line, chances are you've come across the history command. It's that nifty tool that remembers all the commands you've typed before. But, like many things in Unix, there's more beneath the surface than meets the eye. While you might know the basics, I'm betting there's more for you to discover. This guide is designed to walk you through the nuances of the history command. By the end, not only will you understand it better, but you'll also pick up practical tips to streamline your tasks. So, even if you've used history a hundred times, stick around — you might just learn something new.

Why Use the history Command?

The history command in Unix serves as a recorder of your command-line activities. It's not just about listing old commands, but optimizing repetitive tasks, error corrections, and more. Let's deep dive into its utility.

Getting Started: Basic Commands

Initiating the history command is simple:

• View Commands: Input history to display a list of previously executed commands.
• Navigate Backward:Use the Up Arrow or CTRL + P to navigate to previous (older) commands
• Navigate forward: Use the Down Arrow or CTRL + N to navigate to next (newer) commands.
• Rerun Last Command: The !! (double exclamation mark) command lets you effortlessly redo your last action.
• Use Specific Command Number: If you see a command in your history list numbered 123, you can rerun it by typing !123.
• Clearing History: Keep your history concise by periodically using history -c.

Tips for Enhanced Usage

Here below some of my favorite history features:

• Reverse-Search for commands: Press CTRL + R and start typing a portion of your command to search in previous commands. If you hit CTRL + R again, it will show you older commands.
• Forward-Search: After you enter the Reverse-Search, you can search for newer commands by pressing CTRL + S. So by using CTRL + R and CTRL + S you will search back and forth in your history.
• Preview Commands: !123:p displays the 123rd command without immediately executing it.
• Tagging: Use comments starting with # to tag specific commands. This is a handy technique to annotate certain commands. It can be invaluable, especially when you revisit your command history and wonder why a particular command was executed, or to search for commands that are hard to remember.

For example here we are tagging our tar command with a # BACKUP tag:

tar -czvf backup.tar.gz /path/to/directory # BACKUP

You can then search for this command with:

history | grep "# BACKUP"

Or with the reverse-search as seen before by pressing CTRL + R and start typing your tag # BACKUP:

(reverse-i-search)`# BACKUP': tar -czvf backup.tar.gz /path/to/directory # BACKUP

Tweaks and Settings for Your Command History

Here are some of the settings that you can apply to personalize your history behavior.

History Size: To control the number of stored commands, add these two lines to your, .bashrc or .zshrc file:

export HISTFILESIZE=5000
export HISTSIZE=5000

If you want to better understand the difference between the two modifiers, I suggest to read the following Stack Overflow answer:

bash HISTSIZE vs. HISTFILESIZE?

What is the difference in HISTSIZE vs. HISTFILESIZE? They are used to extend bash history beyond the default 500 lines. There seems to be a lack of clarity here and in other forums about why they are

Stack Overflowarturomp

Excluding Specific Commands: HISTIGNORE for Bash or HISTORY_IGNORE for Zsh, are useful for preventing frequent or sensitive commands from being stored in the history. Just specify a list of patterns to be excluded from the command history.

For Bash: add the following line to your .bashrc file:

export HISTIGNORE="ls:clear:exit"

For Zsh: add this line to your .zshrc file:

export HISTORY_IGNORE="ls|clear|exit"

Please note the different separators: a colon : for the HISTIGNORE and a pipe | for the HISTORY_IGNORE.

Excluding Commands that start with a space: As an alternative to the HISTIGNORE / HISTORY_IGNORE above, you can use the HISTCONTROL for Bash or HIST_IGNORE_SPACE for Zsh, so that every time you add a space at the beginning of a command, it will not go to the history.

For Bash: add this line to your .bashrc:

export HISTCONTROL=ignorespace

For Zsh: add this line to your .zshrc file:

setopt HIST_IGNORE_SPACE

Exclude Duplicate commands: You can avoid saving a command to the history if it's the same as the last command you entered. In simpler terms, it prevents duplicate entries from appearing one after the other in your command history.

For Bash: add this line to your .bashrc file:

export HISTCONTROL=ignoredups

For Zsh: add this line to your .zshrc file:

setopt HIST_IGNORE_DUPS

Note that in Bash, ignoreboth is used when we want to use both ignorespace and ignoredups so, the following line

export HISTCONTROL=ignoreboth

is the same as:

export HISTCONTROL=ignorespace:ignoredups

Add Timestamps to your history: If you want to store timestamps with commands use the HISTTIMEFORMAT and HIST_STAMPS modifiers for Bash and Zsh, respectively.

For Bash: add this line in your .bashrc file:

export HISTTIMEFORMAT="%F %T: "

For Zsh: add this line to your .zshrc file:

HIST_STAMPS="%F %T: "

Change history file location: The default history file location is:

• .bash_history for Bash shells
• .zsh_history for Zsh shells
• if it's not either of these above, you can check yours with echo $HISTFILE

If you want to change the defaults, just add the following line to your .bashrc or .zshrc file so that your history will be written to a different file. In our example here below we are changing it to a file named .my_custom_history inside your home directory:

export HISTFILE=~/.my_custom_history

Apply your settings immediately: After making changes to your .bashrc or .zshrc, you'll need to source the file for the changes to take effect in your current shell session. This allows you to immediately see and use any changes you've made without having to close and reopen your terminal or start a new shell session.

Here's how you can do it:

For .bashrc:

source ~/.bashrc

or simply:

. ~/.bashrc

For .zshrc:

source ~/.zshrc

or:

. ~/.zshrc

Conclusion

The history command in Unix is more than just a recap of what you've done—it's a handy feature that can boost your productivity on the command line. With the tips from this guide, you're better equipped to use history to its fullest. Give it a try, and see how it simplifies your Unix tasks.