In the Unix/Linux shell environment, viewing command history, editing commands, and re-executing them are essential tasks for efficient command-line usage.

Here’s how you can perform these actions:

1. Viewing Command History:

• Use the history command to display a list of previously executed commands along with their line numbers.
• By default, the history command displays the most recent 1000 commands, but you can configure this limit.

[root@uadev ~]# history |head
1 exit
2 cd /var/
3 du -h log
4 netstat -tuln
5 iftop
6 dnf install iftop
7 ps aux
8 pgrep sshd
9 pgrep colord
10 pgrep auditd
[root@uadev ~]# 

2. Re-executing Commands:

• Use the !n syntax to re-execute a specific command by its line number, where n is the line number of the command in the history.
  • Example: !123 re-executes the command with line number 123.
• Use !! to re-execute the last command.
• Use !string to re-execute the most recent command that starts with string.
  • Example: !ls re-executes the most recent command that starts with ls.

[root@uadev ~]# !8
pgrep sshd
982
1862
1889
[root@uadev ~]# 

[root@uadev ~]# !!
pgrep sshd
982
1862
1889
[root@uadev ~]# 

3. Editing Commands:

• Use the arrow keys (up and down) to navigate through the command history.
• Press Ctrl + R to search backward through the command history.
  • Start typing a part of the command you want to search for, and press Ctrl + R to search.
  • Press Ctrl + R again to find the previous matching command.

[root@uadev ~]# 
(failed reverse-i-search)`date': date

• Press Ctrl + G to cancel the search and revert to the original command line.
• Use the fc command to open the default text editor (usually vi or nano) to edit a range of commands in the history.
  • Example: fc opens the default text editor with the last command for editing.
  • After editing and saving the changes, the commands are executed sequentially.

[root@uadev ~]# fc
  ls -lrt
  whoami
  date

[root@uadev ~]# fc
ls -lrt
total 4
-rw-------. 1 root root 879 May 27 22:49 anaconda-ks.cfg
whoami
root
date
Fri Jun 7 10:04:55 PM IST 2024
[root@uadev ~]#

Tips:

• Use history | grep keyword to search for commands containing a specific keyword.
• Use history -c to clear the command history.
• To preserve command history between sessions, configure the HISTFILE environment variable in your shell configuration file (e.g., .bashrc, .zshrc).

Efficiently viewing command history, editing commands, and re-executing them are essential skills for command-line users. These techniques can help streamline your workflow, save time, and avoid repetitive typing. By mastering these commands and shortcuts, you can become more productive and effective in using the Unix/Linux shell environment.