history

Accessing Your Command History

The simplest way to use history is to simply type the command itself. This displays a numbered list of your recent commands:

history

The output will look something like this (the numbers will vary based on your shell’s history size and your previous commands):

  1  ls -l
  2  cd /tmp
  3  sudo apt update
  4  grep "error" logfile.txt
  5  history

The numbers preceding each command are crucial; they’re used to re-execute commands.

Re-executing Commands with history

Instead of retyping a command, you can use its history number to rerun it. For example, to re-execute command number 3 ( sudo apt update in the example above):

!3

The ! symbol signifies that you are referencing a command from your history. You can also use the !! to execute the very last command.

Searching and Filtering Command History

history offers powerful search capabilities. Let’s say you want to re-run a command containing the word “grep”. You can use a pattern matching feature:

!grep*

This will execute the most recent command containing “grep”. If multiple commands match, the most recent one will be executed. You can also use more complex patterns with wildcards:

!g*p*

This would execute the most recent command starting with ‘g’ and containing ‘p’ anywhere within it.

Modifying and Re-executing Commands

You can modify commands before re-executing them. Suppose you want to rerun command 4 but replace logfile.txt with newlogfile.txt. You can do this:

!4:s/logfile.txt/newlogfile.txt

The :s/old/new syntax performs a substitution. The s stands for substitute, old is the text to be replaced, and new is the replacement text. This is incredibly useful for quickly making small changes to previous commands.

Controlling the History Size

The number of commands stored in your history is configurable. This is often controlled by the HISTSIZE and HISTFILESIZE environment variables. HISTSIZE controls how many commands are kept in memory, while HISTFILESIZE controls how many commands are saved to the history file (usually .bash_history). You can modify these variables in your shell configuration files (like .bashrc or .profile). For example, to increase the number of commands saved in memory to 1000:

export HISTSIZE=1000

You would then need to source your configuration file (e.g., source ~/.bashrc) for the change to take effect.

Viewing History with Specific Options

The history command offers other options to tailor the output:

• history 10: Show only the last 10 commands.
• history | grep "apt": Pipe the history output to grep to filter for commands containing “apt”.
• history -c: Clear the current history. Be careful with this command!

This article has covered the fundamental aspects of the history command. Exploring its capabilities will improve your command-line workflow and efficiency.