Charly's Column – Shell History
Charly's Column – Shell History
For admins like Charly, who try to avoid typing at all costs, the shell offers an excellent opportunity to avoid wear on your fingertips in the form of built-in 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).
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
.
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).
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.
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