Thursday, July 14, 2011

Sed - reducing effort since '74

Sed, short for 'stream editor', is a (pretty awesome) utility created by  Lee E. McMahon of Bell Labs. I would like to take a detour here and mention that I am in real awe of Bell labs. They have given us so much, starting from C and C++ to Unix itself. Another useful utility (which I should cover here soon) made by them is cscope.

Now, coming back to 'sed', it saved me quite some effort today while working with Kate. Well, frankly, the time it took me to learn some basic tricks and perfect the command might have taken a little longer than it would have taken me to manually make the change, but I won't add that time. Why ? Because the time invested would save me plenty more in the future, whereas doing the task manually wouldn't save me anything in the future.

So what was this task ? As I have explained here, I am working on the modeline variable editor in Kate. The widget that my mentor and I had created, to be used in place of the 'QLineEdit' , had some help text. I had forgotten to wrap the text in i18n() wrappers.

The initial text was something like this :-
item-> setHelpText(" Help text goes here ") ;

Now had there been just two or three of such occurrences, I wouldn't have thought about using sed; but there were a lot more (I didn't check then, but as it turns out, about 40 changes were made using sed). Anyways, I needed the text to be modified to :-

item->setHelpText(i18n(" Help text goes here"));

With sed, that wasn't too hard.

sed 's:setHelpText(\"\([a-zA-Z0-9.() ]*\)\"):setHelpText(i18n(\"\1\")):g' <old >new

Just one line and all the changes were made. And I had with me another patch to offer to the community ;-).

Combined with Regular Expression (Regex), sed has a lot to offer.

Suggested links :

For more information on Internalization (i18n), check out the Wikipedia page.
For a tutorial on Sed, I would suggest this.
Online manual for Sed is available here.


  1. There might be a better way to accomplish this, depending on your text editor: regular expression replace. It can be done in emacs with the command "M-x replace-regexp". So the question is: which editor are you using?

  2. Talking about Bell Labs' awesomeness, they also invented the transistor *and* the laser, without which the world would've been very different today.