Master Bash History: Powerful ‘!’ Expansion Tricks for Faster Command Reuse

Bash keeps a record of every command you type in its history list, whose size is controlled by the HISTSIZE variable (default 500) and whose file location is defined by HISTFILE (default ~/.bash_history).

Basic "!" Expansion

The ! operator pulls commands from the history list into the current input line, allowing you to quickly repeat, edit, or reuse previous commands without retyping them.

Repeating Commands by Prefix

!l

– repeats the most recent command that starts with l (e.g., ls /root/). !ls – repeats the last command that begins with ls.

Repeating Any Matching Command

!?

followed by a string searches the entire history for the most recent command containing that string, regardless of its position. Example:

[root@localhost ~]# cat employee.json
[{"name": "John Brooks", "id": "003"}, ...]
[root@localhost ~]# !?employee
cat employee.json
[...same output...]

Repeating the Nth Command

Use !<em>n</em> where n is the command number in the history. Example: !772 re‑executes command number 772.

Repeating the Last Command

!!

– repeats the most recent command (equivalent to !-1).

Combine with sudo: sudo !! re‑runs the previous command with root privileges.

Pipe the result: !! | grep file runs the last command and filters its output.

String Substitution in the Last Command

Use the syntax !!:s^oldstring^newstring to replace the first occurrence of oldstring with newstring in the previous command. The caret ( ^) acts as the delimiter. !!:s^conf.d^conf After running ll /etc/httpd/conf.d, the above substitution changes the command to ll /etc/httpd/conf.

Practical Examples

# List files in /root and repeat with prefix
[root@localhost ~]# ls /root/
[root@localhost ~]# !l
ls /root/

# Use sudo with the previous command
$ yum update
$ sudo !!
sudo yum update

# Pipe the last command's output
$ ls
file  file1  file2
$ !! | grep file
file
file1
file2

Conclusion

Understanding and leveraging Bash’s history and ! expansion can save system administrators and developers considerable time, reducing repetitive typing, correcting mistakes on the fly, and streamlining complex command sequences.