Friday, March 20, 2015

Automate Debian system updates using Crontab

Updating a Debian system is as easy as executing the following command as root:

# apt-get update && apt-get upgrade

If you have a sudo account, run the command like this:

$ sudo apt-get update && sudo apt-get upgrade

Instead of running the command interactively, you can automate the manual update process by running a cron job. Below, I assume you login as root.

Run the following command to create or edit your cron jobs. Note that the default text editor is opened automatically for you to enter the cron jobs.

# crontab -e

As an example, I will schedule the update to happen daily at 2am. I entered the following line as my first (failed) attempt.

00 02 * * * apt-get update 2>&1 && apt-get -y upgrade 2>&1

A typical upgrade usually prompts you to confirm a transaction before it is executed. Because the cron upgrade is non-interactive, I specify the -y parameter to tell apt-get to assume yes for all prompts.

At 2am, the above command executed, and failed with the following errors:

debconf: unable to initialize frontend: Dialog   
debconf: (TERM is not set, so the dialog frontend is not usable.)    
debconf: falling back to frontend: Readline   
debconf: unable to initialize frontend: Readline   
debconf: (This frontend requires a controlling tty.)   
debconf: falling back to frontend: Teletype  
dpkg-preconfigure:unable to re-open stdin: Fetched 49.5 MB in 17s (2,840 kB/s)  
dpkg: warning: 'ldconfig' not found in PATH or not executable  
dpkg: warning: 'start-stop-daemon' not found in PATH or not executable  
dpkg: error: 2 expected programs not found in PATH or not executable  
Note: root's PATH should usually contain /usr/local/sbin, /usr/sbin and /sbin  
E: Sub-process /usr/bin/dpkg returned an error code (2)

There were 2 problems. First, a front-end was expected, but there was none. Second, the PATH for locating commands was not set up correctly.

To correct the problems, re-run crontab -e, and insert the following lines to properly set up the run-time environment.

00 02 * * * apt-get update 2>&1 && apt-get -y upgrade 2>&1

Automating the system update process saves you time, and keep your system more up-to-date as a protection against potential cyber attacks. If you are interested in Debian system administration, please see What to do after spinning up a Debian VPS.

Wednesday, March 4, 2015

How to merge or split pdf files using convert

convert is a member of the ImageMagick software suite for image manipulation. Two of my earlier posts dealt with using convert to slice and resize an image. It is a lesser-known fact that convert also works with pdf files. I'd previously explained how to merge and split up pdf files using tools such as pdftk and gs. In this post, I'll illustrate how to do the same using the convert program.

First, you need to install convert which is packaged in the ImageMagick suite.

$ sudo apt-get install imagemagick

Merging 2 pdf files (file1 and file2) into a new file (output) is as simple as executing:

$ convert file1.pdf file2.pdf output.pdf

You can merge a subset of pages instead of the entire input files. To accomplish that, use the angle brackets to specify the target subset of pages. For example, to merge page 1 of file1 with pages 1, 2 and 4 of file2, run the following command:

$ convert file1.pdf[0] file2.pdf[0-1,3] output.pdf

Note that page numbers are zero-based. Therefore, [0] is page 1, and [0-1] are the pages ranging from page 1 to page 2.

Finally, the following example splits up input into 2 files: first2output and next2output. The former output file contains pages 1 and 2 from the original file; the latter, pages 3 and 4.

$ convert input.pdf[0-1] first2output.pdf
$ convert input.pdf[2-3] next2output.pdf

As you can see, convert is a versatile utility program to manipulate both image and pdf files.