Bash Tips: Command History

There are several ways of working with command line history in a bash shell. The history command helps you check what commands have been typed previously. The bash command line history is usually stored in the file .bash_history in a user's home directory (that is; if you use bash as your user shell-which is true for Linux (I run the 3.0.0.12 kernel). You can check which shell is your default from the /etc/passwd file. It is usually the last value after the last colon on your username line e.g: user_name:x:1000:1000:FirstName LastName,,,:/home/my_user_name:/bin/bash The command line history is controlled by HISTSIZE and HISTFILE environment variables. The default HISTSIZE is 1000 and the default HISTFILE is ~/bash_history. This is where a history of your bash activity is stored. The HISTFILESIZE variable is set to 2000 (default). You can modify the defaults by editing the file ~/.bashrc (in your home directory) or /etc/skel/.bashrc for new subsequent users. In the .bashrc file, the line shopt -s histappend explicitly requests bash to append to the HISTFILE deleting this line will likely result in the contents of bash_history being overwritten every other time bash tries to save to history the commands that you have entered. An important thing to note about the bash history is that a command is written to history before bash actually expands the command. For example, the command ls Do* will be saved in ~/.bash_history as typed by the user before the bash shell expands it to list the contents of directories with names starting with the two letters “Do”. Also, if a user logs multiple times into a host, the last shell to be closed on that host will be the one whose commands are written to history. As with Linux/UNIX, there are many ways in which one can navigate the bash command line history. These include; Assuming that after typing the history command, we get this:

1687 dhclient eth0

1688 sudo dhclient eth0

1689 ifconfig

1690 clear

1691 pgrep mysql

1692 kill 3071

1693 pgrep mysql

1694 ssh-keygen -R 192.168.1.1

1695 ssh 192.168.1.1 -l userxyz

1696 sudo ifconfig vboxnet0 192.168.1.20

1697 clear

1698 ssh 192.168.1.1 -l userxyz

1699 ssh-keygen -R 192.168.1.1

1700 ssh 192.168.1.1 -l user

1701 ifconfig

1702 ifconfig vboxnet0 down

1703 sudo ifconfig vboxnet0 down

1704 ssh 192.168.1.1 -l user

1705 pgrep mysql

1706 pgrep mysql | xargs kill

1707 pgrep mysql

1708 clear

1709 htop

1710 clear

1711 info whohas

1712 man dpkg

1713 dpkg half-installed

1714 man dpkg

1715 sudo apt-get purge mysql-query-browser

1716 sudo apt-get purge mysql-admin

1717 kill `pgrep mysql`

1718 sudo apt-get purge mysql-workbench-gpl

1719 sudo dpkg-reconfigure mysql-workbench-gpl

1720 sudo apt-get remove mysql-workbench-gpl

1721 clear

1722 VBoxManage startvm "Test"

1723 sudo bin/vm-manager start Dev

1724 ifconfig

1725 ifconfig -a

1726 VBoxManage controlvm Dev poweroff

1727 ifconfig

1728 clear

1729 ping 192.168.56.101

1730 VBoxManage list vms

1731 VBoxManage list runningvms

1732 clear

1733 env | grep -i hist

1734 export

1735 clear

1736 ls /etc/

1737 cat /etc/environment

1738 cat /etc/bash_completion

• Arrow keys: pressing the up arrow key navigates through previously entered commands. A similar action is accomplished by pressing Ctrl + P. To navigate forward, pressing the down arrow key moves through the next entered commands. This only works is one was previously navigating through earlier entered commands. Ctrl + N should work the same.
• !!: immediately executes the previously/last entered command.
  Using our history output above, !! would result in line 1738 being executed.
• !n: executes command number n. Example: Typing !1735 would result in the clear command being executed.
  
• !-n: executes command number: current command minus n command. If your last command was history, then !-1 executes that last command history.
  Example: Typing !-3 would result in 1736 being executed (listing of /etc directory-if you have read privileges to the /etc directory)
  
• history: a list of previously entered commands is displayed. The number of commands displayed depends on the configured HISTSIZE value.
• !string: execute the most recent command starting with string.
  Example: Typing !VBox would execute the line 1731 which displays a list of currently running VirtualBox virtual machines (if you have VirtualBox installed)
• !?string: execute the most recent command containing string.
  Example: Typing !?mysql would execute line 1720 (which removes mysql-workbench package)
  
• ^string1^string2: repeat the last command replacing the first occurrence of the string1 with string2. Example: Typing ^mysql^postgres would search the most recent command containing the word mysql and replace it with the word postgres, in our sample history file, this would replace line 1720: sudo apt-get remove mysql-workbench-gpl with this sudo apt-get remove postgres-workbench-gpl (if there is any such package)
  
• Ctrl+R, string: This command does a reverse search through history to find the most recent command that contains string. The command does not have to start with string. This brings a prompt (reverse-i-search:) that requests you to enter the characters of the command that you would like to execute. These characters do not have to be the entire command or starting letters of the command. Any words in the command are good enough. The more words of the command you type, the narrower the search becomes making it easier to retrieve the command that you want. Typing Ctrl + R repeats the search further backwards through history until you get to the command you want or reach the top of the bash_history file.