Charly's Column – Shell History

There are commands that I type several dozen times a day – grep <something> /var/log/syslog is such a classic. The shell keeps a history of all my entries; thanks to the history command, I can always see in a numbered list which commands I typed last.

The history command is not a separate tool; typing which history at the command line just drops you into a black hole. Instead, history is a part of the shell, a built-in keyword. history's killer feature, for which lazy people like me are eternally grateful, is the interactive search. You enable it with Ctrl+R, changing the command-line prompt to (reverse-i-search)`':.

If you start typing now, for example, the word net, the shell will show you the last command typed containing net. When you press Ctrl+R again, the history feature shows you an increasing number of older commands that contain net (Figure 1).

Figure 1: Interactive search with Ctrl+R.

There are a number of other ways to execute commands stored in the history one more time. To repeat just the last command entered, you can do any of the following:

• Press the up arrow
• Press Ctrl+P ("previous" on keyboards without arrow keys)
• Type !!
• Type !-1

Sometimes using relative addressing backwards through history proves helpful. In the example from Figure 2, I reran the third-to-last command from the history by typing !-3. If you wanted to repeat the last command that started with echo, you would just need to type !echo.

Figure 2: Going back three commands.

You can also access the parameters from previous commands. If you just typed ls .bashrc, you can enter vim !!:$ to open .bashrc in the editor. If you have a command that requires root privileges, sudo !! does the trick. In the meantime, I defined but as an alias (Figure 3).

Figure 3: No? But! Oooh…

Occasionally, however, I find the history's length problematic, as it only stores 1,000 entries on my test system. This is not enough for me, so I added a HISTSIZE=10000 line to the /.bashrc file to multiply the history size by 10. I also added HISTCONTROL=erasedups to /.bashrc. This means that the history command, which I type several times, is only saved once – this saves space and gives a better overview.