Shell History

Display timestamp using HISTTIMEFORMAT

Typically when you type history from command line, it displays the command number and the command. For auditing purpose, it may be beneficial to display the timepstamp along with the command as shown below.

Search the history using Control+R

This may be your most frequently used feature of history. When you’ve already executed a very long command, you can simply search history using a keyword and re-execute the same command without having to type it fully. Press Control+R and type the keyword. In the following example, I searched for red, which displayed the previous command "cat /etc/redhat-release" in the history that contained the word red.

Sometimes you want to edit a command from history before executing it e.g. you can search for httpd, which will display "service httpd stop" from the command history, select this command and change the stop to start and re-execute it again as shown below.

Repeat previous command quickly using 4 different methods

1. Use the up arrow (↑) and press enter
2. Type !! and press enter
3. Type !-1 and press enter
4. Press Control+P and press enter

Execute a specific command from history

In the following example, If you want to repeat the command #4, you can do !4 as shown below.

Execute previous command that starts with a specific word

Type ! followed by the starting few letters of the command that you would like to re-execute. In the following example, typing !ps and enter, executed the previous command starting with ps, which is "ps aux | grep sshd".

Control the total number of lines in the history using HISTSIZE

Append the following two lines to the ~/.bash_profile and relogin to the bash shell again to see the change. In this example, only 100 commands will be stored in the bash history.

Change the history file name using HISTFILE

By default, history is stored in ~/.bash_history file. Add the following line to the ~/.bash_profile and relogin to the bash shell, to store the history command in ~/.command_history file instead of .bash_history file. This may be useful when you want to track commands executed from different terminals using different history file name.

Eliminate the continuous repeated entry from history using HISTCONTROL

In the following example, pwd was typed three times, when you do history, you can see all 3 continuous occurrences of it. To eliminate duplicates, set HISTCONTROL to ignoredups as shown below.

Erase duplicates across the whole history using HISTCONTROL

The ignoredups shown above removes duplicates only if they are consecutive commands. To eliminate duplicates across the whole history, set the HISTCONTROL to erasedups as shown below.

Force history not to remember a particular command using HISTCONTROL

When you execute a command, you can instruct history to ignore the command by setting HISTCONTROL to ignorespace AND typing a space in front of the command as shown below.

Clear all the previous history using option -c

Sometime you may want to clear all the previous history, but want to keep the history moving forward.

Subtitute words from history commands

When you are searching through history, you may want to execute a different command but use the same parameter from the command that you’ve just searched.

In the example below, the !!:$ next to the vi command gets the argument from the previous command to the current command.

In the example below, the !^ next to the vi command gets the first argument from the previous command (i.e cp command) to the current command (i.e vi command).

Substitute a specific argument for a specific command.

In the example below, !cp:2 searches for the previous command in history that starts with cp and takes the second argument of cp and substitutes it for the ls -l command as shown below.

In the example below, !cp:$ searches for the previous command in history that starts with cp and takes the last argument (in this case, which is also the second argument as shown above) of cp and substitutes it for the ls -l command as shown below.

Disable the usage of history using HISTSIZE

If you want to disable history all together and don’t want bash shell to remember the commands you’ve typed, set the HISTSIZE to 0 as shown below. # export HISTSIZE=0
# history
# [Note that history did not display anything]

Ignore specific commands from the history using HISTIGNORE

Sometimes you may not want to clutter your history with basic commands such as pwd and ls. Use HISTIGNORE to specify all the commands that you want to ignore from the history. Please note that adding ls to the HISTIGNORE ignores only ls and not ls -l. So, you have to provide the exact command that you would like to ignore from the history. # export HISTIGNORE="pwd:ls:ls -ltr:" # pwd # ls # ls -ltr # service httpd stop # history | tail -3 79 export HISTIGNORE="pwd:ls:ls -ltr:" 80 service httpd stop 81 history [Note that history did not record pwd, ls and ls -ltr]

Links

• thegeekstuff.com - 15 Examples To Master Linux Command Line History
• pendrivelinux.com - How to clear the terminal command history