Lethe's Ccielogs

Hello all, one of the things I love more about Linux is that you can do everything easily and sometimes saving a lot of time and typing, this is specially important when you’re preparing for a performance (and time) based exam like RHCE.

Today I want to talk about a great time saver which is the history command and feature built-in the bash shell of Linux.

I think you all know that every time you input a command into a shell Linux places that command in a special system variable which is, surprise, called HISTORY. This “buffer” is used to hold in memory a finite, but configurable, number of commands the administrator or the user has used over time just to give you an idea this is the partial output of the history command on one of my lab machines :

[ccielogs@lethe ~]# history
...
  110  ll
  111  cat newpasswd
  112  sed 's/Test/test/g' /etc/passwd > newpasswd
  113  ll
  114  cat newpasswd
  115  w
  116  who
  117  ll ; w
  118  w ; ll
  119  history
  120  history 114
  121  cat newpasswd
  122  rm newpasswd
  123  cat /etc/passwd
  124  vi /etc/passwd
  125  cat /etc/passwd
  126  clear
  127  history
...
[ccielogs@lethe ~]#

As you can see my Linux system registered a lot of the commands I’ve typed so far, this good both for reference and for taking advantage of its “do it quickly” approach as, for example, I was to launch again the command at line 112 all I have to do is to write the following :

[ccielogs@lethe ~]# !112

And automatically Linux would execute the command at line 112 (sed ‘s/Test/test/g’ /etc/passwd > newpasswd in this example), I think it’s clear how much time this can save you! Ok but what if I want to execute just the last command I’ve typed in? Yes I know you could use the up arrow to recall the last command, but an alternate way to do this is typing a double exclamation mark, that is !!, and Linux automatically will launch the last command,.

But there is more, consider the following output from the history command :

[ccielogs@lethe ~]# history
  128  pwd
  129  pwd
  130  pwd
  131  history
[root@lethe ~]# 

Do you notice anything weird here? Yes Linux, by default, won’t take care about duplicated commands into its HISTORY variable. This can be easily modified with a few simple steps. First you could simply launch the following command :

[ccielogs@lethe ~]# export HISTORY=ignoredups

Ok now every time I input a command that is already in the shell’s history it’ll get ignored. If you want to make this change available to all users, or system wide if you prefer, just modify the .bashrc file so with the same line (expect the "export” command of course) but I'll cover this in another post.

I hope you'll find this post informative and useful if so please digg it to make available to even more people. If you face any problem or have any question don't hesitate to comment or to write me with your questions that I'll be glad to answer.

Lethe.

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Daniele Catanesi, aka Lethe, is the mind behing Lethe's Ccielogs. He is a veteran of the IT industry with more than ten years of experience in the various fields. Daniele has been mainly working as IT Architect and Trainer for a lot of Enteprise customers implementing cutting edge technologies and complex solutions. Daniele currently holds various certification among which MCITP, CCNP, LPI, Citrix and working tirelessly to his RHCE and RHCA. When he is not busy working or studying for the next exam he writes on the blog trying to give administrators and students around the world tips and suggestion to help them succeed in their career and job.