Latest Updates: linux

  • Using grep to Exclude Lines Containing Certain Characters/Text

    04:37 on January 10, 2010 | 0 Comments Tweet This! | Digg This!
    Tags: , linux,

    I like to watch Apache log files using tail -f but they often get filled up with data I don’t want/need to see – like, when I access the web site.

    grep is a great tool to search for certain information in text files – but it can also exclude certain pieces of information which can be equally as useful.

    So, for example, if I want to watch a log file using tail -f, but exclude any information relating to the IP address “192.168.1.101″ then we can use the grep -v switch:

    tail -f access.log | grep -v "192.168.1.101"

    What this will do is show everything, apart from a line with that IP address in.

    Of course, this works with any other command with text being piped into grep:

    cat file.txt | grep -v "heh"

    This would output the contents of file.txt but remove any lines with “heh” in them.

    You can find out more about the various grep options in it’s man page (man grep from the command line)

     

  • Kill a GNU Screen Session from the Command Line

    17:48 on January 4, 2010 | 0 Comments Tweet This! | Digg This!
    Tags: , linux,

    This article is about GNU Screen – you can read about GNU Screen here: http://www.gnu.org/software/screen/

    There may be times when you have multiple, or even a single, screen session which you want to kill without attaching to it and ending it as you normally would.

    You can kill a screen session using the session ID or the name (if the name is unique enough).

    Warning: Both ways end all processes within the screen session, so if you have any open files/applications, make sure you don’t need/want them – they’ll be kill’d along with the session.

    Killing by session ID

    First, run screen -list to get the id of the current sessions (we use the id to kill the screen session)

    daz@scampi:~$ screen -list
There are screens on:
11493.irssi     (04/01/10 16:25:44)     (Detached)
30784.newapps   (01/01/10 19:42:38)     (Detached)

    As you can see in this example, I have two sessions, a session called “irssi” with id 11493, and a session called “newapps”, with an id of 30784.

    So, to kill the “newapps” session, I’d run:

    daz@scampi:~$ screen -X -S 30784 kill
daz@scampi:~$

    .. and the session will now be gone.  You can verify that by running screen -list again.

    Killing by session name

    If your screen session name is unique enough (for example, if you have one called “newapps” and another called “irssi” as per my example) you can kill by name:

    daz@scampi:~$ screen -X -S newapps kill
daz@scampi:~$

    The reason I say unique enough –  if you have a session called “newapps” and one called “newapps1″, you will need to use the ID to end “newapps” before “newapps1″ – you’ll simply be asked to specify the session again otherwise.

     

  • Removing packages only installed as dependancies and no longer needed (Ubuntu, Debian)

    20:00 on January 3, 2010 | 0 Comments Tweet This! | Digg This!
    Tags: linux,

    We’ve all installed packages, using apt-get, where it’s installed a lot of other packages to satisfy dependancies.

    But what if you remove that package at a later date that needed all of those dependancies?  You’ll probably find it’s left the dependancies installed.. and if you’ve not installed any other packages which also need those dependacies, they’re just taking up hard drive space.

    It’s easy to remove them, though – after all, you no longer need them so why keep them?

    The command is simple (and needs to be run as root):

    apt-get autoremove

    … and that’s it.  The man page entry for this command:

    “autoremove is used to remove packages that were automatically installed  to satisfy dependencies for some package and that are no more needed.”

    So the next time you install a package which needs lots of dependancies, and you decide you want to remove it for whatever reason, remember to run apt-get autoremove afterwards!

    Warning: You do need to be careful when running this – always check which packages it’s proposing to uninstall before confirming.  I have seen reports of random packages being removed, breaking the entire system – even though it is an officially supported feature of apt-get, and it can be very useful, use it with caution.

     

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