Vim is my favourite text editor. It takes a bit of a learning curve to get used to its quirks but it is time well spent if you are doing lots of stuff on the command line or don’t have access to a GUI. I never used emacs so I won’t go into the editor war.
Macros in vim allow to record a sequence of commands and play them back in one go. Here is a cheat sheet adapted from vim tips:
qa start recording to register a
... enter a sequence of commands
q stop recording
@a execute the macro
@@ execute the macro again
Macros allow to automate complex sequences of vim commands and there is much more to it than this simple cheat sheet. Practical Vim dedicates a full chapter to the subject and it’s a good place to start if you want to delve into the magic world of vim macros.
This weekend I am attending XML London at University College London, a 2 day conference on XML and Semantic Web technologies. I am presenting a paper on Continuous Integration for XML and RDF Data. The paper is available here and the slides of my presentation are available here.
There are lots of GUI utilities to zip and unzip files but sometimes you only have access to the command line on a machine. Linux has different types of data compression formats (.tar, tar.gz, tar.bz2, etc.) but I find the zip utilities easier to use and more compatible with Windows systems. So how do you zip and unzip files from the command line?
First install the zip and unzip packages. If you are on a Debian-based system like Ubuntu:
sudo apt-get install -y zip unzip
or if you are on Red Hat/CentOs:
sudo yum install -y zip unzip
The most intuitive way to zip a directory and all its files is to navigate to that folder and zip everything recursively inside the current directory:
zip -r foo.zip .
To extract the zip file into a given directory (e.g. /tmp), navigate to the directory where the zip file is stored and unzip it with:
The other day I had to do an svn checkout from an Amazon Web Services (AWS) instance and, of course, my user name on SVN was different from the user name on the AWS instance which is by default ec2-user. So how to do a checkout from a Subversion repository and specify at the same time the SVN user who is performing the checkout? Subversion has a handy username parameter, here is the full command with some dummy output.
[ec2-user@ip-172-31-9-166 ~] $ svn checkout --username mysvnusername http://www.example.org/mysvnrepository
Authentication realm: <http://www.example.org:80> USVN
Password for 'mysvnusername': **********
-----------------------------------------------------------------------
ATTENTION! Your password for authentication realm:
<http://www.example.org:80> USVN
can only be stored to disk unencrypted! You are advised to configure
your system so that Subversion can store passwords encrypted, if
possible. See the documentation for details.
You can avoid future appearances of this warning by setting the value
of the 'store-plaintext-passwords' option to either 'yes' or 'no' in
'/home/ec2-user/.subversion/servers'.
-----------------------------------------------------------------------
Store password unencrypted (yes/no)? no
A mysvnrepository/file
Checked out revision 3027.
You will be asked to enter your SVN password and if you want to store your password unencrypted. I suggest to select no for the latter for security reasons but this means that you will need to provide the SVN password every time you perform an SVN command.
A colleague of mine recently asked me to install the Groovy programming language on our Red Hat 6.5 server and to make it accessible to all users. I thought it would be a very straightforward task but a quick search on the Red Hat 6.5 official repositories didn’t return any package for Groovy.
The easiest way to install Groovy manually is via gvm. I followed this procedure to do it:
Log in as root
sudo -i
2. Retrieve the gvm install script and store it in a temporary file
curl –s get.gvmtool.net > /tmp/gvm.sh
3. Make the temporary file executable
chmod +x /tmp/gvm.sh
4. Run the install script
./tmp/gvm.sh
5. Complete the installation as requested at prompt
source "/root/.gvm/bin/gvm-init.sh"
6. Check that gvm is installed (this should return the help message explaining how to use gvm)
gvm help
7. Remove the temporary install script
rm /tmp/gvm.sh
8. Install groovy via gvm
gvm install groovy
9. Select the current version of groovy as default (at the time of writing version 2.4.3) and check that groovy is installed
Recently I had to install groovy on other Linux systems and discovered that that it is now much easier using sdkman, which is the evolution of gvm. This is the procedure to follow:
Every year in March and in October the clock changes in most countries for energy saving purposes. This has the annoying effect of messing up the current time on your server if the timezone is not set properly. Servers in Europe are often set up to UTC time. For example my web hosting provider explicitly says:
Note that any times specified are executed in the timezone of the server, and not your local timezone. Accordingly, you may need to make allowance for this when selecting when to run your cron job. Our servers run in the ‘UTC’ timezone because our client base is global, and it remains constant throughout the year with no changes for daylight saving.
This is a bit annoying for cron jobs as you need to do some calculations in case you want to run them at a specific local time.
Even if you don’t have sudo rights on your server, you can at least modify the time for a given user. For example, to change the timezone for the current user to a local time (e.g. Europe/London) run the following command and add it to your bash_profile to make the change permanent:
export TZ="Europe/London"
If you have full control on your server, you can change the timezone system-wide by symlinking /etc/localtime to the appropriate file in /usr/share/zoneinfo. For example, to set the timezone system-wide to the local time in Paris:
Let’s say you have a new user needing sudo access to a Linux server. How do you grant him/her this great privilege? First, explain to the new user that sudo rights allow to do pretty much anything on a Linux machine – including screwing everything up! – and that with greater power also comes greater responsibilities. Then, follow this procedure:
1. Open the command line
2. Create a new user (e.g. newusername) and add it to group wheel (members of this group have sudo rights):
sudo useradd -G wheel newusername
3. Set up the password for the new user. First, login as root:
sudo -i
Then set the password for the new user (e.g. newusername):
passwd newusername
Finally exit from root with:
exit
4. Test the newly created user and password by logging in as user newusername:
su - newusername
5. Test that the newly created user is in group wheel:
